In Python, the special name __main__ is used for two important constructs:
the name of the top-level environment of the program, which can bechecked using the
__name__ == '__main__'expression; andthe
__main__.pyfile in Python packages.
Both of these mechanisms are related to Python modules; how users interact withthem and how they interact with each other. They are explained in detailbelow. If you’re new to Python modules, see the tutorial sectionModules for an introduction.
__name__ == '__main__'¶
When a Python module or package is imported, __name__ is set to themodule’s name. Usually, this is the name of the Python file itself without the.py extension:
>>> import configparser>>> configparser.__name__'configparser'
If the file is part of a package, __name__ will also include the parentpackage’s path:
>>> from concurrent.futures import process>>> process.__name__'concurrent.futures.process'
However, if the module is executed in the top-level code environment,its __name__ is set to the string '__main__'.
What is the “top-level code environment”?¶
__main__ 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:
the scope of an interactive prompt:
>>> __name__'__main__'
the Python module passed to the Python interpreter as a file argument:
$ python helloworld.pyHello, world!
the Python module or package passed to the Python interpreter with the-m argument:
$ python -m tarfileusage: tarfile.py [-h] [-v] (...)
Python code read by the Python interpreter from standard input:
$ echo "import this" | pythonThe Zen of Python, by Tim PetersBeautiful is better than ugly.Explicit is better than implicit....
Python code passed to the Python interpreter with the -c argument:
$ python -c "import this"The Zen of Python, by Tim PetersBeautiful is better than ugly.Explicit is better than implicit....
In each of these situations, the top-level module’s __name__ is set to'__main__'.
As a result, a module can discover whether or not it is running in thetop-level environment by checking its own __name__, which allows a commonidiom for conditionally executing code when the module is not initialized froman import statement:
if __name__ == '__main__': # Execute when the module is not initialized from an import statement. ...
See also
For a more detailed look at how __name__ is set in all situations, seethe tutorial section Modules.
Idiomatic Usage¶
Some modules contain code that is intended for script use only, like parsingcommand-line arguments or fetching data from standard input. If a modulelike this was imported from a different module, for example to unit testit, the script code would unintentionally execute as well.
This is where using the if __name__ == '__main__' code block comes inhandy. Code within this block won’t run unless the module is executed in thetop-level environment.
Putting as few statements as possible in the block below if __name__ =='__main__' can improve code clarity and correctness. Most often, a functionnamed main encapsulates the program’s primary behavior:
# echo.pyimport shleximport sysdef echo(phrase: str) -> None: """A dummy wrapper around print.""" # for demonstration purposes, you can imagine that there is some # valuable and reusable logic inside this function print(phrase)def main() -> int: """Echo the input arguments to standard output""" phrase = shlex.join(sys.argv) echo(phrase) return 0if __name__ == '__main__': sys.exit(main()) # next section explains the use of sys.exit
Note that if the module didn’t encapsulate code inside the main functionbut instead put it directly within the if __name__ == '__main__' block,the phrase variable would be global to the entire module. This iserror-prone as other functions within the module could be unintentionally usingthe global variable instead of a local name. A main function solves thisproblem.
Using a main function has the added benefit of the echo function itselfbeing isolated and importable elsewhere. When echo.py is imported, theecho and main functions will be defined, but neither of them will becalled, because __name__ != '__main__'.
Packaging Considerations¶
main functions are often used to create command-line tools by specifyingthem as entry points for console scripts. When this is done,pip inserts the function call into a template script,where the return value of main is passed into sys.exit().For example:
sys.exit(main())
Since the call to main is wrapped in sys.exit(), the expectation isthat your function will return some value acceptable as an input tosys.exit(); typically, an integer or None (which is implicitlyreturned if your function does not have a return statement).
By proactively following this convention ourselves, our module will have thesame behavior when run directly (i.e. python echo.py) as it will have ifwe later package it as a console script entry-point in a pip-installablepackage.
In particular, be careful about returning strings from your main function.sys.exit() will interpret a string argument as a failure message, soyour program will have an exit code of 1, indicating failure, and thestring will be written to sys.stderr. The echo.py example fromearlier exemplifies using the sys.exit(main()) convention.
See also
Python Packaging User Guidecontains a collection of tutorials and references on how to distribute andinstall Python packages with modern tools.
__main__.py in Python Packages¶
If you are not familiar with Python packages, see section Packagesof the tutorial. Most commonly, the __main__.py file is used to providea command-line interface for a package. Consider the following hypotheticalpackage, “bandclass”:
bandclass ├── __init__.py ├── __main__.py └── student.py
__main__.py will be executed when the package itself is invokeddirectly from the command line using the -m flag. For example:
$ python -m bandclass
This command will cause __main__.py to run. How you utilize this mechanismwill depend on the nature of the package you are writing, but in thishypothetical case, it might make sense to allow the teacher to search forstudents:
# bandclass/__main__.pyimport sysfrom .student import search_studentsstudent_name = sys.argv[1] if len(sys.argv) >= 2 else ''print(f'Found student: {search_students(student_name)}')
Note that from .student import search_students is an example of a relativeimport. This import style can be used when referencing modules within apackage. For more details, see Intra-package References in theModules section of the tutorial.
Idiomatic Usage¶
The content of __main__.py typically isn’t fenced with anif __name__ == '__main__' block. Instead, those files are keptshort and import functions to execute from other modules. Those other modules can then beeasily unit-tested and are properly reusable.
If used, an if __name__ == '__main__' block will still work as expectedfor a __main__.py file within a package, because its __name__attribute will include the package’s path if imported:
>>> import asyncio.__main__>>> asyncio.__main__.__name__'asyncio.__main__'
This won’t work for __main__.py files in the root directory of a.zip file though. Hence, for consistency, a minimal __main__.pywithout a __name__ check is preferred.
See also
See venv for an example of a package with a minimal __main__.pyin the standard library. It doesn’t contain a if __name__ == '__main__'block. You can invoke it with python -m venv [directory].
See runpy for more details on the -m flag to theinterpreter executable.
See zipapp for how to run applications packaged as .zip files. Inthis case Python looks for a __main__.py file in the root directory ofthe archive.
import __main__¶
Regardless of which module a Python program was started with, other modulesrunning within that same program can import the top-level environment’s scope(namespace) by importing the __main__ module. This doesn’t importa __main__.py file but rather whichever module that received the specialname '__main__'.
Here is an example module that consumes the __main__ namespace:
# namely.pyimport __main__def did_user_define_their_name(): return 'my_name' in dir(__main__)def print_user_name(): if not did_user_define_their_name(): raise ValueError('Define the variable `my_name`!') if '__file__' in dir(__main__): print(__main__.my_name, "found in file", __main__.__file__) else: print(__main__.my_name)
Example usage of this module could be as follows:
# start.pyimport sysfrom namely import print_user_name# my_name = "Dinsdale"def main(): try: print_user_name() except ValueError as ve: return str(ve)if __name__ == "__main__": sys.exit(main())
Now, if we started our program, the result would look like this:
$ python start.pyDefine the variable `my_name`!
The exit code of the program would be 1, indicating an error. Uncommenting theline with my_name = "Dinsdale" fixes the program and now it exits withstatus code 0, indicating success:
$ python start.pyDinsdale found in file /path/to/start.py
Note that importing __main__ doesn’t cause any issues with unintentionallyrunning top-level code meant for script use which is put in theif __name__ == "__main__" block of the start module. Why does this work?
Python inserts an empty __main__ module in sys.modules atinterpreter startup, and populates it by running top-level code. In our examplethis is the start module which runs line by line and imports namely.In turn, namely imports __main__ (which is really start). That’s animport cycle! Fortunately, since the partially populated __main__module is present in sys.modules, Python passes that to namely.See Special considerations for __main__ in theimport system’s reference for details on how this works.
The Python REPL is another example of a “top-level environment”, so anythingdefined in the REPL becomes part of the __main__ scope:
>>> import namely>>> namely.did_user_define_their_name()False>>> namely.print_user_name()Traceback (most recent call last):...ValueError: Define the variable `my_name`!>>> my_name = 'Jabberwocky'>>> namely.did_user_define_their_name()True>>> namely.print_user_name()Jabberwocky
Note that in this case the __main__ scope doesn’t contain a __file__attribute as it’s interactive.
The __main__ scope is used in the implementation of pdb andrlcompleter.
