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<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">dataclasses</code> — Data Classes</a><ul>
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 <section id="module-dataclasses">
<h1><a class="reference internal" href="#module-dataclasses" title="dataclasses: Generate special methods on user-defined classes."><code class="xref py py-mod docutils literal notranslate">dataclasses</code></a> — Data Classes<a class="headerlink" href="#module-dataclasses" title="Permalink to this headline">¶</a></h1>
Source code: <a class="reference external" href="https://github.com/python/cpython/tree/3.10/Lib/dataclasses.py">Lib/dataclasses.py</a>
<hr class="docutils" />
This module provides a decorator and functions for automatically
adding generated <a class="reference internal" href="../glossary.html#term-special-method">special method</a>s such as <code class="xref py py-meth docutils literal notranslate">__init__()</code> and
<code class="xref py py-meth docutils literal notranslate">__repr__()</code> to user-defined classes. It was originally described
in <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0557">PEP 557</a>.
The member variables to use in these generated methods are defined
using <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526">PEP 526</a> type annotations. For example, this code:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from dataclasses import dataclass

@dataclass
class InventoryItem:
 &quot;&quot;&quot;Class for keeping track of an item in inventory.&quot;&quot;&quot;
 name: str
 unit_price: float
 quantity_on_hand: int = 0

def total_cost(self) -&gt; float:
 return self.unit_price * self.quantity_on_hand
</pre></div>
</div>
will add, among other things, a <code class="xref py py-meth docutils literal notranslate">__init__()</code> that looks like:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def __init__(self, name: str, unit_price: float, quantity_on_hand: int = 0):
 self.name = name
 self.unit_price = unit_price
 self.quantity_on_hand = quantity_on_hand
</pre></div>
</div>
Note that this method is automatically added to the class: it is not
directly specified in the <code class="docutils literal notranslate">InventoryItem</code> definition shown above.
<div class="versionadded">
New in version 3.7.
</div>
<section id="module-contents">
<h2>Module contents<a class="headerlink" href="#module-contents" title="Permalink to this headline">¶</a></h2>
<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.dataclass">
&#64;dataclasses.dataclass(*, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False)<a class="headerlink" href="#dataclasses.dataclass" title="Permalink to this definition">¶</a></dt>
<dd>This function is a <a class="reference internal" href="../glossary.html#term-decorator">decorator</a> that is used to add generated
<a class="reference internal" href="../glossary.html#term-special-method">special method</a>s to classes, as described below.
The <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> decorator examines the class to find
<code class="docutils literal notranslate">field</code>s. A <code class="docutils literal notranslate">field</code> is defined as a class variable that has a
<a class="reference internal" href="../glossary.html#term-variable-annotation">type annotation</a>. With two
exceptions described below, nothing in <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a>
examines the type specified in the variable annotation.
The order of the fields in all of the generated methods is the
order in which they appear in the class definition.
The <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> decorator will add various “dunder” methods to
the class, described below. If any of the added methods already
exist in the class, the behavior depends on the parameter, as documented
below. The decorator returns the same class that it is called on; no new
class is created.
If <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> is used just as a simple decorator with no parameters,
it acts as if it has the default values documented in this
signature. That is, these three uses of <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> are
equivalent:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 ...

@dataclass()
class C:
 ...

@dataclass(init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False,
 match_args=True, kw_only=False, slots=False)
class C:
 ...
</pre></div>
</div>
The parameters to <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> are:
<ul>
<li><code class="docutils literal notranslate">init</code>: If true (the default), a <code class="xref py py-meth docutils literal notranslate">__init__()</code> method will be
generated.
If the class already defines <code class="xref py py-meth docutils literal notranslate">__init__()</code>, this parameter is
ignored.
</li>
<li><code class="docutils literal notranslate">repr</code>: If true (the default), a <code class="xref py py-meth docutils literal notranslate">__repr__()</code> method will be
generated. The generated repr string will have the class name and
the name and repr of each field, in the order they are defined in
the class. Fields that are marked as being excluded from the repr
are not included. For example:
<code class="docutils literal notranslate">InventoryItem(name='widget', unit_price=3.0, quantity_on_hand=10)</code>.
If the class already defines <code class="xref py py-meth docutils literal notranslate">__repr__()</code>, this parameter is
ignored.
</li>
<li><code class="docutils literal notranslate">eq</code>: If true (the default), an <code class="xref py py-meth docutils literal notranslate">__eq__()</code> method will be
generated. This method compares the class as if it were a tuple
of its fields, in order. Both instances in the comparison must
be of the identical type.
If the class already defines <code class="xref py py-meth docutils literal notranslate">__eq__()</code>, this parameter is
ignored.
</li>
<li><code class="docutils literal notranslate">order</code>: If true (the default is <code class="docutils literal notranslate">False</code>), <code class="xref py py-meth docutils literal notranslate">__lt__()</code>,
<code class="xref py py-meth docutils literal notranslate">__le__()</code>, <code class="xref py py-meth docutils literal notranslate">__gt__()</code>, and <code class="xref py py-meth docutils literal notranslate">__ge__()</code> methods will be
generated. These compare the class as if it were a tuple of its
fields, in order. Both instances in the comparison must be of the
identical type. If <code class="docutils literal notranslate">order</code> is true and <code class="docutils literal notranslate">eq</code> is false, a
<a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> is raised.
If the class already defines any of <code class="xref py py-meth docutils literal notranslate">__lt__()</code>,
<code class="xref py py-meth docutils literal notranslate">__le__()</code>, <code class="xref py py-meth docutils literal notranslate">__gt__()</code>, or <code class="xref py py-meth docutils literal notranslate">__ge__()</code>, then
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised.
</li>
<li><code class="docutils literal notranslate">unsafe_hash</code>: If <code class="docutils literal notranslate">False</code> (the default), a <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method
is generated according to how <code class="docutils literal notranslate">eq</code> and <code class="docutils literal notranslate">frozen</code> are set.
<code class="xref py py-meth docutils literal notranslate">__hash__()</code> is used by built-in <a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-meth docutils literal notranslate">hash()</code></a>, and when objects are
added to hashed collections such as dictionaries and sets. Having a
<code class="xref py py-meth docutils literal notranslate">__hash__()</code> implies that instances of the class are immutable.
Mutability is a complicated property that depends on the programmer’s
intent, the existence and behavior of <code class="xref py py-meth docutils literal notranslate">__eq__()</code>, and the values of
the <code class="docutils literal notranslate">eq</code> and <code class="docutils literal notranslate">frozen</code> flags in the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> decorator.
By default, <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> will not implicitly add a <code class="xref py py-meth docutils literal notranslate">__hash__()</code>
method unless it is safe to do so. Neither will it add or change an
existing explicitly defined <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method. Setting the class
attribute <code class="docutils literal notranslate">__hash__ = None</code> has a specific meaning to Python, as
described in the <code class="xref py py-meth docutils literal notranslate">__hash__()</code> documentation.
If <code class="xref py py-meth docutils literal notranslate">__hash__()</code> is not explicitly defined, or if it is set to <code class="docutils literal notranslate">None</code>,
then <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> may add an implicit <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method.
Although not recommended, you can force <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> to create a
<code class="xref py py-meth docutils literal notranslate">__hash__()</code> method with <code class="docutils literal notranslate">unsafe_hash=True</code>. This might be the case
if your class is logically immutable but can nonetheless be mutated.
This is a specialized use case and should be considered carefully.
Here are the rules governing implicit creation of a <code class="xref py py-meth docutils literal notranslate">__hash__()</code>
method. Note that you cannot both have an explicit <code class="xref py py-meth docutils literal notranslate">__hash__()</code>
method in your dataclass and set <code class="docutils literal notranslate">unsafe_hash=True</code>; this will result
in a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
If <code class="docutils literal notranslate">eq</code> and <code class="docutils literal notranslate">frozen</code> are both true, by default <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> will
generate a <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method for you. If <code class="docutils literal notranslate">eq</code> is true and
<code class="docutils literal notranslate">frozen</code> is false, <code class="xref py py-meth docutils literal notranslate">__hash__()</code> will be set to <code class="docutils literal notranslate">None</code>, marking it
unhashable (which it is, since it is mutable). If <code class="docutils literal notranslate">eq</code> is false,
<code class="xref py py-meth docutils literal notranslate">__hash__()</code> will be left untouched meaning the <code class="xref py py-meth docutils literal notranslate">__hash__()</code>
method of the superclass will be used (if the superclass is
<a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>, this means it will fall back to id-based hashing).
</li>
<li><code class="docutils literal notranslate">frozen</code>: If true (the default is <code class="docutils literal notranslate">False</code>), assigning to fields will
generate an exception. This emulates read-only frozen instances. If
<code class="xref py py-meth docutils literal notranslate">__setattr__()</code> or <code class="xref py py-meth docutils literal notranslate">__delattr__()</code> is defined in the class, then
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> is raised. See the discussion below.</li>
<li><code class="docutils literal notranslate">match_args</code>: If true (the default is <code class="docutils literal notranslate">True</code>), the
<code class="docutils literal notranslate">__match_args__</code> tuple will be created from the list of
parameters to the generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method (even if
<code class="xref py py-meth docutils literal notranslate">__init__()</code> is not generated, see above). If false, or if
<code class="docutils literal notranslate">__match_args__</code> is already defined in the class, then
<code class="docutils literal notranslate">__match_args__</code> will not be generated.</li>
</ul>
<blockquote>
<div><div class="versionadded">
New in version 3.10.
</div>
</div></blockquote>
<ul class="simple">
<li><code class="docutils literal notranslate">kw_only</code>: If true (the default value is <code class="docutils literal notranslate">False</code>), then all
fields will be marked as keyword-only. If a field is marked as
keyword-only, then the only affect is that the <code class="xref py py-meth docutils literal notranslate">__init__()</code>
parameter generated from a keyword-only field must be specified
with a keyword when <code class="xref py py-meth docutils literal notranslate">__init__()</code> is called. There is no
effect on any other aspect of dataclasses. See the
<a class="reference internal" href="../glossary.html#term-parameter">parameter</a> glossary entry for details. Also see the
<a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a> section.</li>
</ul>
<blockquote>
<div><div class="versionadded">
New in version 3.10.
</div>
</div></blockquote>
<ul class="simple">
<li><code class="docutils literal notranslate">slots</code>: If true (the default is <code class="docutils literal notranslate">False</code>), <code class="xref py py-attr docutils literal notranslate">__slots__</code> attribute
will be generated and new class will be returned instead of the original one.
If <code class="xref py py-attr docutils literal notranslate">__slots__</code> is already defined in the class, then <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>
is raised.</li>
</ul>
<blockquote>
<div><div class="versionadded">
New in version 3.10.
</div>
</div></blockquote>
<code class="docutils literal notranslate">field</code>s may optionally specify a default value, using normal
Python syntax:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 a: int # &#39;a&#39; has no default value
 b: int = 0 # assign a default value for &#39;b&#39;
</pre></div>
</div>
In this example, both <code class="docutils literal notranslate">a</code> and <code class="docutils literal notranslate">b</code> will be included in the added
<code class="xref py py-meth docutils literal notranslate">__init__()</code> method, which will be defined as:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def __init__(self, a: int, b: int = 0):
</pre></div>
</div>
<a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> will be raised if a field without a default value
follows a field with a default value. This is true whether this
occurs in a single class, or as a result of class inheritance.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.field">
dataclasses.field(*, default=MISSING, default_factory=MISSING, init=True, repr=True, hash=None, compare=True, metadata=None, kw_only=MISSING)<a class="headerlink" href="#dataclasses.field" title="Permalink to this definition">¶</a></dt>
<dd>For common and simple use cases, no other functionality is
required. There are, however, some dataclass features that
require additional per-field information. To satisfy this need for
additional information, you can replace the default field value
with a call to the provided <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> function. For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 mylist: list[int] = field(default_factory=list)

c = C()
c.mylist += [1, 2, 3]
</pre></div>
</div>
As shown above, the <a class="reference internal" href="#dataclasses.MISSING" title="dataclasses.MISSING"><code class="xref py py-const docutils literal notranslate">MISSING</code></a> value is a sentinel object used to
detect if some parameters are provided by the user. This sentinel is
used because <code class="docutils literal notranslate">None</code> is a valid value for some parameters with
a distinct meaning. No code should directly use the <a class="reference internal" href="#dataclasses.MISSING" title="dataclasses.MISSING"><code class="xref py py-const docutils literal notranslate">MISSING</code></a> value.
The parameters to <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> are:
<ul>
<li><code class="docutils literal notranslate">default</code>: If provided, this will be the default value for this
field. This is needed because the <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-meth docutils literal notranslate">field()</code></a> call itself
replaces the normal position of the default value.</li>
<li><code class="docutils literal notranslate">default_factory</code>: If provided, it must be a zero-argument
callable that will be called when a default value is needed for
this field. Among other purposes, this can be used to specify
fields with mutable default values, as discussed below. It is an
error to specify both <code class="docutils literal notranslate">default</code> and <code class="docutils literal notranslate">default_factory</code>.</li>
<li><code class="docutils literal notranslate">init</code>: If true (the default), this field is included as a
parameter to the generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method.</li>
<li><code class="docutils literal notranslate">repr</code>: If true (the default), this field is included in the
string returned by the generated <code class="xref py py-meth docutils literal notranslate">__repr__()</code> method.</li>
<li><code class="docutils literal notranslate">hash</code>: This can be a bool or <code class="docutils literal notranslate">None</code>. If true, this field is
included in the generated <code class="xref py py-meth docutils literal notranslate">__hash__()</code> method. If <code class="docutils literal notranslate">None</code> (the
default), use the value of <code class="docutils literal notranslate">compare</code>: this would normally be
the expected behavior. A field should be considered in the hash
if it’s used for comparisons. Setting this value to anything
other than <code class="docutils literal notranslate">None</code> is discouraged.
One possible reason to set <code class="docutils literal notranslate">hash=False</code> but <code class="docutils literal notranslate">compare=True</code>
would be if a field is expensive to compute a hash value for,
that field is needed for equality testing, and there are other
fields that contribute to the type’s hash value. Even if a field
is excluded from the hash, it will still be used for comparisons.
</li>
<li><code class="docutils literal notranslate">compare</code>: If true (the default), this field is included in the
generated equality and comparison methods (<code class="xref py py-meth docutils literal notranslate">__eq__()</code>,
<code class="xref py py-meth docutils literal notranslate">__gt__()</code>, et al.).</li>
<li><code class="docutils literal notranslate">metadata</code>: This can be a mapping or None. None is treated as
an empty dict. This value is wrapped in
<a class="reference internal" href="types.html#types.MappingProxyType" title="types.MappingProxyType"><code class="xref py py-func docutils literal notranslate">MappingProxyType()</code></a> to make it read-only, and exposed
on the <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> object. It is not used at all by Data
Classes, and is provided as a third-party extension mechanism.
Multiple third-parties can each have their own key, to use as a
namespace in the metadata.</li>
<li><code class="docutils literal notranslate">kw_only</code>: If true, this field will be marked as keyword-only.
This is used when the generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method’s
parameters are computed.</li>
</ul>
<blockquote>
<div><div class="versionadded">
New in version 3.10.
</div>
</div></blockquote>
If the default value of a field is specified by a call to
<a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a>, then the class attribute for this field will be
replaced by the specified <code class="docutils literal notranslate">default</code> value. If no <code class="docutils literal notranslate">default</code> is
provided, then the class attribute will be deleted. The intent is
that after the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> decorator runs, the class
attributes will all contain the default values for the fields, just
as if the default value itself were specified. For example,
after:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 x: int
 y: int = field(repr=False)
 z: int = field(repr=False, default=10)
 t: int = 20
</pre></div>
</div>
The class attribute <code class="docutils literal notranslate">C.z</code> will be <code class="docutils literal notranslate">10</code>, the class attribute
<code class="docutils literal notranslate">C.t</code> will be <code class="docutils literal notranslate">20</code>, and the class attributes <code class="docutils literal notranslate">C.x</code> and
<code class="docutils literal notranslate">C.y</code> will not be set.
</dd></dl>

<dl class="py class">
<dt class="sig sig-object py" id="dataclasses.Field">
class dataclasses.Field<a class="headerlink" href="#dataclasses.Field" title="Permalink to this definition">¶</a></dt>
<dd><a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> objects describe each defined field. These objects
are created internally, and are returned by the <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a>
module-level method (see below). Users should never instantiate a
<a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> object directly. Its documented attributes are:
<blockquote>
<div><ul class="simple">
<li><code class="docutils literal notranslate">name</code>: The name of the field.</li>
<li><code class="docutils literal notranslate">type</code>: The type of the field.</li>
<li><code class="docutils literal notranslate">default</code>, <code class="docutils literal notranslate">default_factory</code>, <code class="docutils literal notranslate">init</code>, <code class="docutils literal notranslate">repr</code>, <code class="docutils literal notranslate">hash</code>,
<code class="docutils literal notranslate">compare</code>, <code class="docutils literal notranslate">metadata</code>, and <code class="docutils literal notranslate">kw_only</code> have the identical
meaning and values as they do in the <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> function.</li>
</ul>
</div></blockquote>
Other attributes may exist, but they are private and must not be
inspected or relied on.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.fields">
dataclasses.fields(class_or_instance)<a class="headerlink" href="#dataclasses.fields" title="Permalink to this definition">¶</a></dt>
<dd>Returns a tuple of <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> objects that define the fields for this
dataclass. Accepts either a dataclass, or an instance of a dataclass.
Raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if not passed a dataclass or instance of one.
Does not return pseudo-fields which are <code class="docutils literal notranslate">ClassVar</code> or <code class="docutils literal notranslate">InitVar</code>.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.asdict">
dataclasses.asdict(obj, *, dict_factory=dict)<a class="headerlink" href="#dataclasses.asdict" title="Permalink to this definition">¶</a></dt>
<dd>Converts the dataclass <code class="docutils literal notranslate">obj</code> to a dict (by using the
factory function <code class="docutils literal notranslate">dict_factory</code>). Each dataclass is converted
to a dict of its fields, as <code class="docutils literal notranslate">name: value</code> pairs. dataclasses, dicts,
lists, and tuples are recursed into. Other objects are copied with
<a class="reference internal" href="copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a>.
Example of using <a class="reference internal" href="#dataclasses.asdict" title="dataclasses.asdict"><code class="xref py py-func docutils literal notranslate">asdict()</code></a> on nested dataclasses:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class Point:
 x: int
 y: int

@dataclass
class C:
 mylist: list[Point]

p = Point(10, 20)
assert asdict(p) == {&#39;x&#39;: 10, &#39;y&#39;: 20}

c = C([Point(0, 0), Point(10, 4)])
assert asdict(c) == {&#39;mylist&#39;: [{&#39;x&#39;: 0, &#39;y&#39;: 0}, {&#39;x&#39;: 10, &#39;y&#39;: 4}]}
</pre></div>
</div>
To create a shallow copy, the following workaround may be used:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>dict((field.name, getattr(obj, field.name)) for field in fields(obj))
</pre></div>
</div>
<a class="reference internal" href="#dataclasses.asdict" title="dataclasses.asdict"><code class="xref py py-func docutils literal notranslate">asdict()</code></a> raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if <code class="docutils literal notranslate">obj</code> is not a dataclass
instance.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.astuple">
dataclasses.astuple(obj, *, tuple_factory=tuple)<a class="headerlink" href="#dataclasses.astuple" title="Permalink to this definition">¶</a></dt>
<dd>Converts the dataclass <code class="docutils literal notranslate">obj</code> to a tuple (by using the
factory function <code class="docutils literal notranslate">tuple_factory</code>). Each dataclass is converted
to a tuple of its field values. dataclasses, dicts, lists, and
tuples are recursed into. Other objects are copied with
<a class="reference internal" href="copy.html#copy.deepcopy" title="copy.deepcopy"><code class="xref py py-func docutils literal notranslate">copy.deepcopy()</code></a>.
Continuing from the previous example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>assert astuple(p) == (10, 20)
assert astuple(c) == ([(0, 0), (10, 4)],)
</pre></div>
</div>
To create a shallow copy, the following workaround may be used:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>tuple(getattr(obj, field.name) for field in dataclasses.fields(obj))
</pre></div>
</div>
<a class="reference internal" href="#dataclasses.astuple" title="dataclasses.astuple"><code class="xref py py-func docutils literal notranslate">astuple()</code></a> raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if <code class="docutils literal notranslate">obj</code> is not a dataclass
instance.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.make_dataclass">
dataclasses.make_dataclass(cls_name, fields, *, bases=(), namespace=None, init=True, repr=True, eq=True, order=False, unsafe_hash=False, frozen=False, match_args=True, kw_only=False, slots=False)<a class="headerlink" href="#dataclasses.make_dataclass" title="Permalink to this definition">¶</a></dt>
<dd>Creates a new dataclass with name <code class="docutils literal notranslate">cls_name</code>, fields as defined
in <code class="docutils literal notranslate">fields</code>, base classes as given in <code class="docutils literal notranslate">bases</code>, and initialized
with a namespace as given in <code class="docutils literal notranslate">namespace</code>. <code class="docutils literal notranslate">fields</code> is an
iterable whose elements are each either <code class="docutils literal notranslate">name</code>, <code class="docutils literal notranslate">(name, type)</code>,
or <code class="docutils literal notranslate">(name, type, Field)</code>. If just <code class="docutils literal notranslate">name</code> is supplied,
<code class="docutils literal notranslate">typing.Any</code> is used for <code class="docutils literal notranslate">type</code>. The values of <code class="docutils literal notranslate">init</code>,
<code class="docutils literal notranslate">repr</code>, <code class="docutils literal notranslate">eq</code>, <code class="docutils literal notranslate">order</code>, <code class="docutils literal notranslate">unsafe_hash</code>, <code class="docutils literal notranslate">frozen</code>,
<code class="docutils literal notranslate">match_args</code>, <code class="docutils literal notranslate">kw_only</code>, and <code class="docutils literal notranslate">slots</code> have the same meaning as
they do in <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a>.
This function is not strictly required, because any Python
mechanism for creating a new class with <code class="docutils literal notranslate">__annotations__</code> can
then apply the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> function to convert that class to
a dataclass. This function is provided as a convenience. For
example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>C = make_dataclass(&#39;C&#39;,
 [(&#39;x&#39;, int),
 &#39;y&#39;,
 (&#39;z&#39;, int, field(default=5))],
 namespace={&#39;add_one&#39;: lambda self: self.x + 1})
</pre></div>
</div>
Is equivalent to:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 x: int
 y: &#39;typing.Any&#39;
 z: int = 5

def add_one(self):
 return self.x + 1
</pre></div>
</div>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.replace">
dataclasses.replace(obj, /, **changes)<a class="headerlink" href="#dataclasses.replace" title="Permalink to this definition">¶</a></dt>
<dd>Creates a new object of the same type as <code class="docutils literal notranslate">obj</code>, replacing
fields with values from <code class="docutils literal notranslate">changes</code>. If <code class="docutils literal notranslate">obj</code> is not a Data
Class, raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>. If values in <code class="docutils literal notranslate">changes</code> do not
specify fields, raises <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a>.
The newly returned object is created by calling the <code class="xref py py-meth docutils literal notranslate">__init__()</code>
method of the dataclass. This ensures that
<code class="xref py py-meth docutils literal notranslate">__post_init__()</code>, if present, is also called.
Init-only variables without default values, if any exist, must be
specified on the call to <a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate">replace()</code></a> so that they can be passed to
<code class="xref py py-meth docutils literal notranslate">__init__()</code> and <code class="xref py py-meth docutils literal notranslate">__post_init__()</code>.
It is an error for <code class="docutils literal notranslate">changes</code> to contain any fields that are
defined as having <code class="docutils literal notranslate">init=False</code>. A <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> will be raised
in this case.
Be forewarned about how <code class="docutils literal notranslate">init=False</code> fields work during a call to
<a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate">replace()</code></a>. They are not copied from the source object, but
rather are initialized in <code class="xref py py-meth docutils literal notranslate">__post_init__()</code>, if they’re
initialized at all. It is expected that <code class="docutils literal notranslate">init=False</code> fields will
be rarely and judiciously used. If they are used, it might be wise
to have alternate class constructors, or perhaps a custom
<code class="docutils literal notranslate">replace()</code> (or similarly named) method which handles instance
copying.
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="dataclasses.is_dataclass">
dataclasses.is_dataclass(obj)<a class="headerlink" href="#dataclasses.is_dataclass" title="Permalink to this definition">¶</a></dt>
<dd>Return <code class="docutils literal notranslate">True</code> if its parameter is a dataclass or an instance of one,
otherwise return <code class="docutils literal notranslate">False</code>.
If you need to know if a class is an instance of a dataclass (and
not a dataclass itself), then add a further check for <code class="docutils literal notranslate">not
isinstance(obj, type)</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def is_dataclass_instance(obj):
 return is_dataclass(obj) and not isinstance(obj, type)
</pre></div>
</div>
</dd></dl>

<dl class="py data">
<dt class="sig sig-object py" id="dataclasses.MISSING">
dataclasses.MISSING<a class="headerlink" href="#dataclasses.MISSING" title="Permalink to this definition">¶</a></dt>
<dd>A sentinel value signifying a missing default or default_factory.
</dd></dl>

<dl class="py data">
<dt class="sig sig-object py" id="dataclasses.KW_ONLY">
dataclasses.KW_ONLY<a class="headerlink" href="#dataclasses.KW_ONLY" title="Permalink to this definition">¶</a></dt>
<dd>A sentinel value used as a type annotation. Any fields after a
pseudo-field with the type of <a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a> are marked as
keyword-only fields. Note that a pseudo-field of type
<a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a> is otherwise completely ignored. This includes the
name of such a field. By convention, a name of <code class="docutils literal notranslate">_</code> is used for a
<a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a> field. Keyword-only fields signify
<code class="xref py py-meth docutils literal notranslate">__init__()</code> parameters that must be specified as keywords when
the class is instantiated.
In this example, the fields <code class="docutils literal notranslate">y</code> and <code class="docutils literal notranslate">z</code> will be marked as keyword-only fields:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class Point:
 x: float
 _: KW_ONLY
 y: float
 z: float

p = Point(0, y=1.5, z=2.0)
</pre></div>
</div>
In a single dataclass, it is an error to specify more than one
field whose type is <a class="reference internal" href="#dataclasses.KW_ONLY" title="dataclasses.KW_ONLY"><code class="xref py py-const docutils literal notranslate">KW_ONLY</code></a>.
<div class="versionadded">
New in version 3.10.
</div>
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="dataclasses.FrozenInstanceError">
exception dataclasses.FrozenInstanceError<a class="headerlink" href="#dataclasses.FrozenInstanceError" title="Permalink to this definition">¶</a></dt>
<dd>Raised when an implicitly defined <code class="xref py py-meth docutils literal notranslate">__setattr__()</code> or
<code class="xref py py-meth docutils literal notranslate">__delattr__()</code> is called on a dataclass which was defined with
<code class="docutils literal notranslate">frozen=True</code>. It is a subclass of <a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a>.
</dd></dl>

</section>
<section id="post-init-processing">
<h2>Post-init processing<a class="headerlink" href="#post-init-processing" title="Permalink to this headline">¶</a></h2>
The generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> code will call a method named
<code class="xref py py-meth docutils literal notranslate">__post_init__()</code>, if <code class="xref py py-meth docutils literal notranslate">__post_init__()</code> is defined on the
class. It will normally be called as <code class="docutils literal notranslate">self.__post_init__()</code>.
However, if any <code class="docutils literal notranslate">InitVar</code> fields are defined, they will also be
passed to <code class="xref py py-meth docutils literal notranslate">__post_init__()</code> in the order they were defined in the
class. If no <code class="xref py py-meth docutils literal notranslate">__init__()</code> method is generated, then
<code class="xref py py-meth docutils literal notranslate">__post_init__()</code> will not automatically be called.
Among other uses, this allows for initializing field values that
depend on one or more other fields. For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 a: float
 b: float
 c: float = field(init=False)

def __post_init__(self):
 self.c = self.a + self.b
</pre></div>
</div>
The <code class="xref py py-meth docutils literal notranslate">__init__()</code> method generated by <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> does not call base
class <code class="xref py py-meth docutils literal notranslate">__init__()</code> methods. If the base class has an <code class="xref py py-meth docutils literal notranslate">__init__()</code> method
that has to be called, it is common to call this method in a
<code class="xref py py-meth docutils literal notranslate">__post_init__()</code> method:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class Rectangle:
 height: float
 width: float

@dataclass
class Square(Rectangle):
 side: float

def __post_init__(self):
 super().__init__(self.side, self.side)
</pre></div>
</div>
Note, however, that in general the dataclass-generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> methods
don’t need to be called, since the derived dataclass will take care of
initializing all fields of any base class that is a dataclass itself.
See the section below on init-only variables for ways to pass
parameters to <code class="xref py py-meth docutils literal notranslate">__post_init__()</code>. Also see the warning about how
<a class="reference internal" href="#dataclasses.replace" title="dataclasses.replace"><code class="xref py py-func docutils literal notranslate">replace()</code></a> handles <code class="docutils literal notranslate">init=False</code> fields.
</section>
<section id="class-variables">
<h2>Class variables<a class="headerlink" href="#class-variables" title="Permalink to this headline">¶</a></h2>
One of the few places where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> actually inspects the type
of a field is to determine if a field is a class variable as defined
in <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526">PEP 526</a>. It does this by checking if the type of the field is
<code class="docutils literal notranslate">typing.ClassVar</code>. If a field is a <code class="docutils literal notranslate">ClassVar</code>, it is excluded
from consideration as a field and is ignored by the dataclass
mechanisms. Such <code class="docutils literal notranslate">ClassVar</code> pseudo-fields are not returned by the
module-level <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> function.
</section>
<section id="init-only-variables">
<h2>Init-only variables<a class="headerlink" href="#init-only-variables" title="Permalink to this headline">¶</a></h2>
Another place where <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> inspects a type annotation is to
determine if a field is an init-only variable. It does this by seeing
if the type of a field is of type <code class="docutils literal notranslate">dataclasses.InitVar</code>. If a field
is an <code class="docutils literal notranslate">InitVar</code>, it is considered a pseudo-field called an init-only
field. As it is not a true field, it is not returned by the
module-level <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> function. Init-only fields are added as
parameters to the generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method, and are passed to
the optional <code class="xref py py-meth docutils literal notranslate">__post_init__()</code> method. They are not otherwise used
by dataclasses.
For example, suppose a field will be initialized from a database, if a
value is not provided when creating the class:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class C:
 i: int
 j: int | None = None
 database: InitVar[DatabaseType | None] = None

def __post_init__(self, database):
 if self.j is None and database is not None:
 self.j = database.lookup(&#39;j&#39;)

c = C(10, database=my_database)
</pre></div>
</div>
In this case, <a class="reference internal" href="#dataclasses.fields" title="dataclasses.fields"><code class="xref py py-func docutils literal notranslate">fields()</code></a> will return <a class="reference internal" href="#dataclasses.Field" title="dataclasses.Field"><code class="xref py py-class docutils literal notranslate">Field</code></a> objects for <code class="docutils literal notranslate">i</code> and
<code class="docutils literal notranslate">j</code>, but not for <code class="docutils literal notranslate">database</code>.
</section>
<section id="frozen-instances">
<h2>Frozen instances<a class="headerlink" href="#frozen-instances" title="Permalink to this headline">¶</a></h2>
It is not possible to create truly immutable Python objects. However,
by passing <code class="docutils literal notranslate">frozen=True</code> to the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-meth docutils literal notranslate">dataclass()</code></a> decorator you can
emulate immutability. In that case, dataclasses will add
<code class="xref py py-meth docutils literal notranslate">__setattr__()</code> and <code class="xref py py-meth docutils literal notranslate">__delattr__()</code> methods to the class. These
methods will raise a <a class="reference internal" href="#dataclasses.FrozenInstanceError" title="dataclasses.FrozenInstanceError"><code class="xref py py-exc docutils literal notranslate">FrozenInstanceError</code></a> when invoked.
There is a tiny performance penalty when using <code class="docutils literal notranslate">frozen=True</code>:
<code class="xref py py-meth docutils literal notranslate">__init__()</code> cannot use simple assignment to initialize fields, and
must use <a class="reference internal" href="../reference/datamodel.html#object.__setattr__" title="object.__setattr__"><code class="xref py py-meth docutils literal notranslate">object.__setattr__()</code></a>.
</section>
<section id="inheritance">
<h2>Inheritance<a class="headerlink" href="#inheritance" title="Permalink to this headline">¶</a></h2>
When the dataclass is being created by the <a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-meth docutils literal notranslate">dataclass()</code></a> decorator,
it looks through all of the class’s base classes in reverse MRO (that
is, starting at <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>) and, for each dataclass that it finds,
adds the fields from that base class to an ordered mapping of fields.
After all of the base class fields are added, it adds its own fields
to the ordered mapping. All of the generated methods will use this
combined, calculated ordered mapping of fields. Because the fields
are in insertion order, derived classes override base classes. An
example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class Base:
 x: Any = 15.0
 y: int = 0

@dataclass
class C(Base):
 z: int = 10
 x: int = 15
</pre></div>
</div>
The final list of fields is, in order, <code class="docutils literal notranslate">x</code>, <code class="docutils literal notranslate">y</code>, <code class="docutils literal notranslate">z</code>. The final
type of <code class="docutils literal notranslate">x</code> is <code class="docutils literal notranslate">int</code>, as specified in class <code class="docutils literal notranslate">C</code>.
The generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method for <code class="docutils literal notranslate">C</code> will look like:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def __init__(self, x: int = 15, y: int = 0, z: int = 10):
</pre></div>
</div>
</section>
<section id="re-ordering-of-keyword-only-parameters-in-init">
<h2>Re-ordering of keyword-only parameters in <code class="xref py py-meth docutils literal notranslate">__init__()</code><a class="headerlink" href="#re-ordering-of-keyword-only-parameters-in-init" title="Permalink to this headline">¶</a></h2>
After the parameters needed for <code class="xref py py-meth docutils literal notranslate">__init__()</code> are computed, any
keyword-only parameters are moved to come after all regular
(non-keyword-only) parameters. This is a requirement of how
keyword-only parameters are implemented in Python: they must come
after non-keyword-only parameters.
In this example, <code class="docutils literal notranslate">Base.y</code>, <code class="docutils literal notranslate">Base.w</code>, and <code class="docutils literal notranslate">D.t</code> are keyword-only
fields, and <code class="docutils literal notranslate">Base.x</code> and <code class="docutils literal notranslate">D.z</code> are regular fields:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class Base:
 x: Any = 15.0
 _: KW_ONLY
 y: int = 0
 w: int = 1

@dataclass
class D(Base):
 z: int = 10
 t: int = field(kw_only=True, default=0)
</pre></div>
</div>
The generated <code class="xref py py-meth docutils literal notranslate">__init__()</code> method for <code class="docutils literal notranslate">D</code> will look like:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def __init__(self, x: Any = 15.0, z: int = 10, *, y: int = 0, w: int = 1, t: int = 0):
</pre></div>
</div>
Note that the parameters have been re-ordered from how they appear in
the list of fields: parameters derived from regular fields are
followed by parameters derived from keyword-only fields.
The relative ordering of keyword-only parameters is maintained in the
re-ordered <code class="xref py py-meth docutils literal notranslate">__init__()</code> parameter list.
</section>
<section id="default-factory-functions">
<h2>Default factory functions<a class="headerlink" href="#default-factory-functions" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div>If a <a class="reference internal" href="#dataclasses.field" title="dataclasses.field"><code class="xref py py-func docutils literal notranslate">field()</code></a> specifies a <code class="docutils literal notranslate">default_factory</code>, it is called with
zero arguments when a default value for the field is needed. For
example, to create a new instance of a list, use:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>mylist: list = field(default_factory=list)
</pre></div>
</div>
If a field is excluded from <code class="xref py py-meth docutils literal notranslate">__init__()</code> (using <code class="docutils literal notranslate">init=False</code>)
and the field also specifies <code class="docutils literal notranslate">default_factory</code>, then the default
factory function will always be called from the generated
<code class="xref py py-meth docutils literal notranslate">__init__()</code> function. This happens because there is no other
way to give the field an initial value.
</div></blockquote>
</section>
<section id="mutable-default-values">
<h2>Mutable default values<a class="headerlink" href="#mutable-default-values" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div>Python stores default member variable values in class attributes.
Consider this example, not using dataclasses:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class C:
 x = []
 def add(self, element):
 self.x.append(element)

o1 = C()
o2 = C()
o1.add(1)
o2.add(2)
assert o1.x == [1, 2]
assert o1.x is o2.x
</pre></div>
</div>
Note that the two instances of class <code class="docutils literal notranslate">C</code> share the same class
variable <code class="docutils literal notranslate">x</code>, as expected.
Using dataclasses, if this code was valid:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class D:
 x: List = []
 def add(self, element):
 self.x += element
</pre></div>
</div>
it would generate code similar to:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class D:
 x = []
 def __init__(self, x=x):
 self.x = x
 def add(self, element):
 self.x += element

assert D().x is D().x
</pre></div>
</div>
This has the same issue as the original example using class <code class="docutils literal notranslate">C</code>.
That is, two instances of class <code class="docutils literal notranslate">D</code> that do not specify a value
for <code class="docutils literal notranslate">x</code> when creating a class instance will share the same copy
of <code class="docutils literal notranslate">x</code>. Because dataclasses just use normal Python class
creation they also share this behavior. There is no general way
for Data Classes to detect this condition. Instead, the
<a class="reference internal" href="#dataclasses.dataclass" title="dataclasses.dataclass"><code class="xref py py-func docutils literal notranslate">dataclass()</code></a> decorator will raise a <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> if it
detects a default parameter of type <code class="docutils literal notranslate">list</code>, <code class="docutils literal notranslate">dict</code>, or <code class="docutils literal notranslate">set</code>.
This is a partial solution, but it does protect against many common
errors.
Using default factory functions is a way to create new instances of
mutable types as default values for fields:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@dataclass
class D:
 x: list = field(default_factory=list)

assert D().x is not D().x
</pre></div>
</div>
</div></blockquote>
</section>
<section id="descriptor-typed-fields">
<h2>Descriptor-typed fields<a class="headerlink" href="#descriptor-typed-fields" title="Permalink to this headline">¶</a></h2>
Fields that are assigned <a class="reference internal" href="../reference/datamodel.html#descriptors">descriptor objects</a> as their
default value have the following special behaviors:
<ul class="simple">
<li>The value for the field passed to the dataclass’s <code class="docutils literal notranslate">__init__</code> method is
passed to the descriptor’s <code class="docutils literal notranslate">__set__</code> method rather than overwriting the
descriptor object.</li>
<li>Similarly, when getting or setting the field, the descriptor’s
<code class="docutils literal notranslate">__get__</code> or <code class="docutils literal notranslate">__set__</code> method is called rather than returning or
overwriting the descriptor object.</li>
<li>To determine whether a field contains a default value, <code class="docutils literal notranslate">dataclasses</code>
will call the descriptor’s <code class="docutils literal notranslate">__get__</code> method using its class access
form (i.e. <code class="docutils literal notranslate">descriptor.__get__(obj=None, type=cls)</code>. If the
descriptor returns a value in this case, it will be used as the
field’s default. On the other hand, if the descriptor raises
<a class="reference internal" href="exceptions.html#AttributeError" title="AttributeError"><code class="xref py py-exc docutils literal notranslate">AttributeError</code></a> in this situation, no default value will be
provided for the field.</li>
</ul>
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class IntConversionDescriptor:
 def __init__(self, *, default):
 self._default = default

def __set_name__(self, owner, name):
 self._name = &quot;_&quot; + name

def __get__(self, obj, type):
 if obj is None:
 return self._default

return getattr(obj, self._name, self._default)

def __set__(self, obj, value):
 setattr(obj, self._name, int(value))

@dataclass
class InventoryItem:
 quantity_on_hand: IntConversionDescriptor = IntConversionDescriptor(default=100)

i = InventoryItem()
print(i.quantity_on_hand) # 100
i.quantity_on_hand = 2.5 # calls __set__ with 2.5
print(i.quantity_on_hand) # 2
</pre></div>
</div>
Note that if a field is annotated with a descriptor type, but is not assigned
a descriptor object as its default value, the field will act like a normal
field.
</section>
</section>

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 <ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">dataclasses</code> — Data Classes</a><ul>
<li><a class="reference internal" href="#module-contents">Module contents</a></li>
<li><a class="reference internal" href="#post-init-processing">Post-init processing</a></li>
<li><a class="reference internal" href="#class-variables">Class variables</a></li>
<li><a class="reference internal" href="#init-only-variables">Init-only variables</a></li>
<li><a class="reference internal" href="#frozen-instances">Frozen instances</a></li>
<li><a class="reference internal" href="#inheritance">Inheritance</a></li>
<li><a class="reference internal" href="#re-ordering-of-keyword-only-parameters-in-init">Re-ordering of keyword-only parameters in <code class="xref py py-meth docutils literal notranslate">__init__()</code></a></li>
<li><a class="reference internal" href="#default-factory-functions">Default factory functions</a></li>
<li><a class="reference internal" href="#mutable-default-values">Mutable default values</a></li>
<li><a class="reference internal" href="#descriptor-typed-fields">Descriptor-typed fields</a></li>
</ul>
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