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<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">__main__</code> — Top-level code environment</a><ul>
<li><a class="reference internal" href="#name-main"><code class="docutils literal notranslate">__name__ == '__main__'</code></a><ul>
<li><a class="reference internal" href="#what-is-the-top-level-code-environment">What is the “top-level code environment”?</a></li>
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<li><a class="reference internal" href="#main-py-in-python-packages"><code class="docutils literal notranslate">__main__.py</code> in Python Packages</a><ul>
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 <section id="module-__main__">
<h1><a class="reference internal" href="#module-__main__" title="__main__: The environment where top-level code is run. Covers command-line interfaces, import-time behavior, and ``__name__ == '__main__'``."><code class="xref py py-mod docutils literal notranslate">__main__</code></a> — Top-level code environment<a class="headerlink" href="#module-__main__" title="Permalink to this headline">¶</a></h1>
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In Python, the special name <code class="docutils literal notranslate">__main__</code> is used for two important constructs:
<ol class="arabic simple">
<li>the name of the top-level environment of the program, which can be
checked using the <code class="docutils literal notranslate">__name__ == '__main__'</code> expression; and</li>
<li>the <code class="docutils literal notranslate">__main__.py</code> file in Python packages.</li>
</ol>
Both of these mechanisms are related to Python modules; how users interact with
them and how they interact with each other. They are explained in detail
below. If you’re new to Python modules, see the tutorial section
<a class="reference internal" href="../tutorial/modules.html#tut-modules">Modules</a> for an introduction.
<section id="name-main">
<h2><code class="docutils literal notranslate">__name__ == '__main__'</code><a class="headerlink" href="#name-main" title="Permalink to this headline">¶</a></h2>
When a Python module or package is imported, <code class="docutils literal notranslate">__name__</code> is set to the
module’s name. Usually, this is the name of the Python file itself without the
<code class="docutils literal notranslate">.py</code> extension:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import configparser
&gt;&gt;&gt; configparser.__name__
&#39;configparser&#39;
</pre></div>
</div>
If the file is part of a package, <code class="docutils literal notranslate">__name__</code> will also include the parent
package’s path:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from concurrent.futures import process
&gt;&gt;&gt; process.__name__
&#39;concurrent.futures.process&#39;
</pre></div>
</div>
However, if the module is executed in the top-level code environment,
its <code class="docutils literal notranslate">__name__</code> is set to the string <code class="docutils literal notranslate">'__main__'</code>.
<section id="what-is-the-top-level-code-environment">
<h3>What is the “top-level code environment”?<a class="headerlink" href="#what-is-the-top-level-code-environment" title="Permalink to this headline">¶</a></h3>
<code class="docutils literal notranslate">__main__</code> is the name of the environment where top-level code is run.
“Top-level code” is the first user-specified Python module that starts running.
It’s “top-level” because it imports all other modules that the program needs.
Sometimes “top-level code” is called an entry point to the application.
The top-level code environment can be:
<ul>
<li>the scope of an interactive prompt:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; __name__
&#39;__main__&#39;
</pre></div>
</div>
</li>
<li>the Python module passed to the Python interpreter as a file argument:
<blockquote>
<div><div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 helloworld.py
Hello, world!
</pre></div>
</div>
</div></blockquote>
</li>
<li>the Python module or package passed to the Python interpreter with the
<a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate">-m</code></a> argument:
<blockquote>
<div><div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 -m tarfile
usage: tarfile.py [-h] [-v] (...)
</pre></div>
</div>
</div></blockquote>
</li>
<li>Python code read by the Python interpreter from standard input:
<blockquote>
<div><div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ echo &quot;import this&quot; | python3
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
...
</pre></div>
</div>
</div></blockquote>
</li>
<li>Python code passed to the Python interpreter with the <a class="reference internal" href="../using/cmdline.html#cmdoption-c"><code class="xref std std-option docutils literal notranslate">-c</code></a> argument:
<blockquote>
<div><div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 -c &quot;import this&quot;
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
...
</pre></div>
</div>
</div></blockquote>
</li>
</ul>
In each of these situations, the top-level module’s <code class="docutils literal notranslate">__name__</code> is set to
<code class="docutils literal notranslate">'__main__'</code>.
As a result, a module can discover whether or not it is running in the
top-level environment by checking its own <code class="docutils literal notranslate">__name__</code>, which allows a common
idiom for conditionally executing code when the module is not initialized from
an import statement:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>if __name__ == &#39;__main__&#39;:
 # Execute when the module is not initialized from an import statement.
 ...
</pre></div>
</div>
<div class="admonition seealso">
See also
For a more detailed look at how <code class="docutils literal notranslate">__name__</code> is set in all situations, see
the tutorial section <a class="reference internal" href="../tutorial/modules.html#tut-modules">Modules</a>.
</div>
</section>
<section id="idiomatic-usage">
<h3>Idiomatic Usage<a class="headerlink" href="#idiomatic-usage" title="Permalink to this headline">¶</a></h3>
Some modules contain code that is intended for script use only, like parsing
command-line arguments or fetching data from standard input. If a module
like this was imported from a different module, for example to unit test
it, the script code would unintentionally execute as well.
This is where using the <code class="docutils literal notranslate">if __name__ == '__main__'</code> code block comes in
handy. Code within this block won’t run unless the module is executed in the
top-level environment.
Putting as few statements as possible in the block below <code class="docutils literal notranslate">if __name___ ==
'__main__'</code> can improve code clarity and correctness. Most often, a function
named <code class="docutils literal notranslate">main</code> encapsulates the program’s primary behavior:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># echo.py

import shlex
import sys

def echo(phrase: str) -&gt; None:
 &quot;&quot;&quot;A dummy wrapper around print.&quot;&quot;&quot;
 # for demonstration purposes, you can imagine that there is some
 # valuable and reusable logic inside this function
 print(phrase)

def main() -&gt; int:
 &quot;&quot;&quot;Echo the input arguments to standard output&quot;&quot;&quot;
 phrase = shlex.join(sys.argv)
 echo(phrase)
 return 0

if __name__ == &#39;__main__&#39;:
 sys.exit(main()) # next section explains the use of sys.exit
</pre></div>
</div>
Note that if the module didn’t encapsulate code inside the <code class="docutils literal notranslate">main</code> function
but instead put it directly within the <code class="docutils literal notranslate">if __name__ == '__main__'</code> block,
the <code class="docutils literal notranslate">phrase</code> variable would be global to the entire module. This is
error-prone as other functions within the module could be unintentionally using
the global variable instead of a local name. A <code class="docutils literal notranslate">main</code> function solves this
problem.
Using a <code class="docutils literal notranslate">main</code> function has the added benefit of the <code class="docutils literal notranslate">echo</code> function itself
being isolated and importable elsewhere. When <code class="docutils literal notranslate">echo.py</code> is imported, the
<code class="docutils literal notranslate">echo</code> and <code class="docutils literal notranslate">main</code> functions will be defined, but neither of them will be
called, because <code class="docutils literal notranslate">__name__ != '__main__'</code>.
</section>
<section id="packaging-considerations">
<h3>Packaging Considerations<a class="headerlink" href="#packaging-considerations" title="Permalink to this headline">¶</a></h3>
<code class="docutils literal notranslate">main</code> functions are often used to create command-line tools by specifying
them as entry points for console scripts. When this is done,
<a class="reference external" href="https://pip.pypa.io/">pip</a> inserts the function call into a template script,
where the return value of <code class="docutils literal notranslate">main</code> is passed into <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate">sys.exit()</code></a>.
For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>sys.exit(main())
</pre></div>
</div>
Since the call to <code class="docutils literal notranslate">main</code> is wrapped in <a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate">sys.exit()</code></a>, the expectation is
that your function will return some value acceptable as an input to
<a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate">sys.exit()</code></a>; typically, an integer or <code class="docutils literal notranslate">None</code> (which is implicitly
returned if your function does not have a return statement).
By proactively following this convention ourselves, our module will have the
same behavior when run directly (i.e. <code class="docutils literal notranslate">python3 echo.py</code>) as it will have if
we later package it as a console script entry-point in a pip-installable
package.
In particular, be careful about returning strings from your <code class="docutils literal notranslate">main</code> function.
<a class="reference internal" href="sys.html#sys.exit" title="sys.exit"><code class="xref py py-func docutils literal notranslate">sys.exit()</code></a> will interpret a string argument as a failure message, so
your program will have an exit code of <code class="docutils literal notranslate">1</code>, indicating failure, and the
string will be written to <a class="reference internal" href="sys.html#sys.stderr" title="sys.stderr"><code class="xref py py-data docutils literal notranslate">sys.stderr</code></a>. The <code class="docutils literal notranslate">echo.py</code> example from
earlier exemplifies using the <code class="docutils literal notranslate">sys.exit(main())</code> convention.
<div class="admonition seealso">
See also
<a class="reference external" href="https://packaging.python.org/">Python Packaging User Guide</a>
contains a collection of tutorials and references on how to distribute and
install Python packages with modern tools.
</div>
</section>
</section>
<section id="main-py-in-python-packages">
<h2><code class="docutils literal notranslate">__main__.py</code> in Python Packages<a class="headerlink" href="#main-py-in-python-packages" title="Permalink to this headline">¶</a></h2>
If you are not familiar with Python packages, see section <a class="reference internal" href="../tutorial/modules.html#tut-packages">Packages</a>
of the tutorial. Most commonly, the <code class="docutils literal notranslate">__main__.py</code> file is used to provide
a command-line interface for a package. Consider the following hypothetical
package, “bandclass”:
<div class="highlight-text notranslate"><div class="highlight"><pre>bandclass
 ├── __init__.py
 ├── __main__.py
 └── student.py
</pre></div>
</div>
<code class="docutils literal notranslate">__main__.py</code> will be executed when the package itself is invoked
directly from the command line using the <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate">-m</code></a> flag. For example:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 -m bandclass
</pre></div>
</div>
This command will cause <code class="docutils literal notranslate">__main__.py</code> to run. How you utilize this mechanism
will depend on the nature of the package you are writing, but in this
hypothetical case, it might make sense to allow the teacher to search for
students:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># bandclass/__main__.py

import sys
from .student import search_students

student_name = sys.argv[2] if len(sys.argv) &gt;= 2 else &#39;&#39;
print(f&#39;Found student: {search_students(student_name)}&#39;)
</pre></div>
</div>
Note that <code class="docutils literal notranslate">from .student import search_students</code> is an example of a relative
import. This import style can be used when referencing modules within a
package. For more details, see <a class="reference internal" href="../tutorial/modules.html#intra-package-references">Intra-package References</a> in the
<a class="reference internal" href="../tutorial/modules.html#tut-modules">Modules</a> section of the tutorial.
<section id="id1">
<h3>Idiomatic Usage<a class="headerlink" href="#id1" title="Permalink to this headline">¶</a></h3>
The contents of <code class="docutils literal notranslate">__main__.py</code> typically isn’t fenced with
<code class="docutils literal notranslate">if __name__ == '__main__'</code> blocks. Instead, those files are kept short,
functions to execute from other modules. Those other modules can then be
easily unit-tested and are properly reusable.
If used, an <code class="docutils literal notranslate">if __name__ == '__main__'</code> block will still work as expected
for a <code class="docutils literal notranslate">__main__.py</code> file within a package, because its <code class="docutils literal notranslate">__name__</code>
attribute will include the package’s path if imported:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import asyncio.__main__
&gt;&gt;&gt; asyncio.__main__.__name__
&#39;asyncio.__main__&#39;
</pre></div>
</div>
This won’t work for <code class="docutils literal notranslate">__main__.py</code> files in the root directory of a .zip file
though. Hence, for consistency, minimal <code class="docutils literal notranslate">__main__.py</code> like the <a class="reference internal" href="venv.html#module-venv" title="venv: Creation of virtual environments."><code class="xref py py-mod docutils literal notranslate">venv</code></a>
one mentioned below are preferred.
<div class="admonition seealso">
See also
See <a class="reference internal" href="venv.html#module-venv" title="venv: Creation of virtual environments."><code class="xref py py-mod docutils literal notranslate">venv</code></a> for an example of a package with a minimal <code class="docutils literal notranslate">__main__.py</code>
in the standard library. It doesn’t contain a <code class="docutils literal notranslate">if __name__ == '__main__'</code>
block. You can invoke it with <code class="docutils literal notranslate">python -m venv [directory]</code>.
See <a class="reference internal" href="runpy.html#module-runpy" title="runpy: Locate and run Python modules without importing them first."><code class="xref py py-mod docutils literal notranslate">runpy</code></a> for more details on the <a class="reference internal" href="../using/cmdline.html#cmdoption-m"><code class="xref std std-option docutils literal notranslate">-m</code></a> flag to the
interpreter executable.
See <a class="reference internal" href="zipapp.html#module-zipapp" title="zipapp: Manage executable Python zip archives"><code class="xref py py-mod docutils literal notranslate">zipapp</code></a> for how to run applications packaged as .zip files. In
this case Python looks for a <code class="docutils literal notranslate">__main__.py</code> file in the root directory of
the archive.
</div>
</section>
</section>
<section id="import-main">
<h2><code class="docutils literal notranslate">import __main__</code><a class="headerlink" href="#import-main" title="Permalink to this headline">¶</a></h2>
Regardless of which module a Python program was started with, other modules
running within that same program can import the top-level environment’s scope
(<a class="reference internal" href="../glossary.html#term-namespace">namespace</a>) by importing the <code class="docutils literal notranslate">__main__</code> module. This doesn’t import
a <code class="docutils literal notranslate">__main__.py</code> file but rather whichever module that received the special
name <code class="docutils literal notranslate">'__main__'</code>.
Here is an example module that consumes the <code class="docutils literal notranslate">__main__</code> namespace:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># namely.py

import __main__

def did_user_define_their_name():
 return &#39;my_name&#39; in dir(__main__)

def print_user_name():
 if not did_user_define_their_name():
 raise ValueError(&#39;Define the variable `my_name`!&#39;)

if &#39;__file__&#39; in dir(__main__):
 print(__main__.my_name, &quot;found in file&quot;, __main__.__file__)
 else:
 print(__main__.my_name)
</pre></div>
</div>
Example usage of this module could be as follows:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># start.py

import sys

from namely import print_user_name

# my_name = &quot;Dinsdale&quot;

def main():
 try:
 print_user_name()
 except ValueError as ve:
 return str(ve)

if __name__ == &quot;__main__&quot;:
 sys.exit(main())
</pre></div>
</div>
Now, if we started our program, the result would look like this:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 start.py
Define the variable `my_name`!
</pre></div>
</div>
The exit code of the program would be 1, indicating an error. Uncommenting the
line with <code class="docutils literal notranslate">my_name = &quot;Dinsdale&quot;</code> fixes the program and now it exits with
status code 0, indicating success:
<div class="highlight-shell-session notranslate"><div class="highlight"><pre>$ python3 start.py
Dinsdale found in file /path/to/start.py
</pre></div>
</div>
Note that importing <code class="docutils literal notranslate">__main__</code> doesn’t cause any issues with unintentionally
running top-level code meant for script use which is put in the
<code class="docutils literal notranslate">if __name__ == &quot;__main__&quot;</code> block of the <code class="docutils literal notranslate">start</code> module. Why does this work?
Python inserts an empty <code class="docutils literal notranslate">__main__</code> module in <a class="reference internal" href="sys.html#sys.modules" title="sys.modules"><code class="xref py py-attr docutils literal notranslate">sys.modules</code></a> at
interpreter startup, and populates it by running top-level code. In our example
this is the <code class="docutils literal notranslate">start</code> module which runs line by line and imports <code class="docutils literal notranslate">namely</code>.
In turn, <code class="docutils literal notranslate">namely</code> imports <code class="docutils literal notranslate">__main__</code> (which is really <code class="docutils literal notranslate">start</code>). That’s an
import cycle! Fortunately, since the partially populated <code class="docutils literal notranslate">__main__</code>
module is present in <a class="reference internal" href="sys.html#sys.modules" title="sys.modules"><code class="xref py py-attr docutils literal notranslate">sys.modules</code></a>, Python passes that to <code class="docutils literal notranslate">namely</code>.
See <a class="reference internal" href="../reference/import.html#import-dunder-main">Special considerations for __main__</a> in the
import system’s reference for details on how this works.
The Python REPL is another example of a “top-level environment”, so anything
defined in the REPL becomes part of the <code class="docutils literal notranslate">__main__</code> scope:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; import namely
&gt;&gt;&gt; namely.did_user_define_their_name()
False
&gt;&gt;&gt; namely.print_user_name()
Traceback (most recent call last):
...
ValueError: Define the variable `my_name`!
&gt;&gt;&gt; my_name = &#39;Jabberwocky&#39;
&gt;&gt;&gt; namely.did_user_define_their_name()
True
&gt;&gt;&gt; namely.print_user_name()
Jabberwocky
</pre></div>
</div>
Note that in this case the <code class="docutils literal notranslate">__main__</code> scope doesn’t contain a <code class="docutils literal notranslate">__file__</code>
attribute as it’s interactive.
The <code class="docutils literal notranslate">__main__</code> scope is used in the implementation of <a class="reference internal" href="pdb.html#module-pdb" title="pdb: The Python debugger for interactive interpreters."><code class="xref py py-mod docutils literal notranslate">pdb</code></a> and
<a class="reference internal" href="rlcompleter.html#module-rlcompleter" title="rlcompleter: Python identifier completion, suitable for the GNU readline library."><code class="xref py py-mod docutils literal notranslate">rlcompleter</code></a>.
</section>
</section>

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 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">__main__</code> — Top-level code environment</a><ul>
<li><a class="reference internal" href="#name-main"><code class="docutils literal notranslate">__name__ == '__main__'</code></a><ul>
<li><a class="reference internal" href="#what-is-the-top-level-code-environment">What is the “top-level code environment”?</a></li>
<li><a class="reference internal" href="#idiomatic-usage">Idiomatic Usage</a></li>
<li><a class="reference internal" href="#packaging-considerations">Packaging Considerations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#main-py-in-python-packages"><code class="docutils literal notranslate">__main__.py</code> in Python Packages</a><ul>
<li><a class="reference internal" href="#id1">Idiomatic Usage</a></li>
</ul>
</li>
<li><a class="reference internal" href="#import-main"><code class="docutils literal notranslate">import __main__</code></a></li>
</ul>
</li>
</ul>

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