__main__
— 顶层代码环境
¶
在 Python 中，特殊名称
__main__
用于 2 重要构造：
程序顶层环境的名称，可以被校验使用
__name__ == '__main__'
表达式; 和
__main__.py
文件在 Python 包中。
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 模块 for an introduction.
__name__ == '__main__'
¶
When a Python module or package is imported,
__name__
is set to the module’s name. Usually, this is the name of the Python file itself without the
.py
extension:
>>> import configparser >>> configparser.__name__ 'configparser'
若文件是包的一部分，
__name__
will also include the parent package’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__
被设为字符串
'__main__'
.
__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
入口点
to the application.
顶层代码环境可以是：
交互提示的作用域：
>>> __name__ '__main__'
the Python module passed to the Python interpreter as a file argument:
$ python3 helloworld.py Hello, world!
the Python module or package passed to the Python interpreter with the
-m
自变量：
$ python3 -m tarfile usage: tarfile.py [-h] [-v] (...)
Python code read by the Python interpreter from standard input:
$ echo "import this" | python3 The Zen of Python, by Tim Peters Beautiful is better than ugly. Explicit is better than implicit. ...
Python code passed to the Python interpreter with the
-c
自变量：
$ python3 -c "import this" The Zen of Python, by Tim Peters Beautiful is better than ugly. Explicit is better than implicit. ...
In each of these situations, the top-level module’s
__name__
被设为
'__main__'
.
As a result, a module can discover whether or not it is running in the top-level environment by checking its own
__name__
, which allows a common idiom for conditionally executing code when the module is not initialized from an import statement:
if __name__ == '__main__': # Execute when the module is not initialized from an import statement. ...
另请参阅
对于更多详细，查看如何
__name__
is set in all situations, see the tutorial section
模块
.
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
if __name__ == '__main__'
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
if __name___ ==
'__main__'
can improve code clarity and correctness. Most often, a function named
main
encapsulates the program’s primary behavior:
# echo.py import shlex import sys def 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 0 if __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
function but instead put it directly within the
if __name__ == '__main__'
block, the
phrase
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
main
function solves this problem.
使用
main
function has the added benefit of the
echo
function itself being isolated and importable elsewhere. When
echo.py
is imported, the
echo
and
main
functions will be defined, but neither of them will be called, because
__name__ != '__main__'
.
main
functions are often used to create command-line tools by specifying them 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()
。例如：
sys.exit(main())
由于调用
main
is wrapped in
sys.exit()
, the expectation is that your function will return some value acceptable as an input to
sys.exit()
; typically, an integer or
None
(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.
python3 echo.py
) 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
main
函数。
sys.exit()
will interpret a string argument as a failure message, so your program will have an exit code of
1
, indicating failure, and the string will be written to
sys.stderr
。
echo.py
example from earlier exemplifies using the
sys.exit(main())
convention.
另请参阅
Python 打包用户指南 contains a collection of tutorials and references on how to distribute and install Python packages with modern tools.
__main__.py
在 Python 包中
¶
If you are not familiar with Python packages, see section
包
of the tutorial. Most commonly, the
__main__.py
file is used to provide a command-line interface for a package. Consider the following hypothetical package, “bandclass”:
bandclass ├── __init__.py ├── __main__.py └── student.py
__main__.py
will be executed when the package itself is invoked directly from the command line using the
-m
flag. For example:
$ python3 -m bandclass
This command will cause
__main__.py
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:
# bandclass/__main__.py import sys from .student import search_students student_name = sys.argv[2] if len(sys.argv) >= 2 else '' print(f'Found student: {search_students(student_name)}')
注意，
from .student import search_students
is an example of a relative import. This import style can be used when referencing modules within a package. For more details, see
包内引用
在
模块
section of the tutorial.
The contents of
__main__.py
typically isn’t fenced with
if __name__ == '__main__'
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
if __name__ == '__main__'
block will still work as expected for 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, minimal
__main__.py
like the
venv
one mentioned below are preferred.
另请参阅
见
venv
for an example of a package with a minimal
__main__.py
in the standard library. It doesn’t contain a
if __name__ == '__main__'
block. You can invoke it with
python3 -m venv [directory]
.
见
runpy
for more details on the
-m
flag to the interpreter executable.
见
zipapp
for how to run applications packaged as
.zip
files. In this case Python looks for a
__main__.py
file in the root directory of the archive.
import __main__
¶
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 (
namespace
) by importing the
__main__
module. This doesn’t import a
__main__.py
file but rather whichever module that received the special name
'__main__'
.
Here is an example module that consumes the
__main__
namespace:
# namely.py import __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.py import sys from 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:
$ python3 start.py Define the variable `my_name`!
The exit code of the program would be 1, indicating an error. Uncommenting the line with
my_name = "Dinsdale"
fixes the program and now it exits with status code 0, indicating success:
$ python3 start.py Dinsdale found in file /path/to/start.py
Note that importing
__main__
doesn’t cause any issues with unintentionally running top-level code meant for script use which is put in the
if __name__ == "__main__"
block of the
start
module. Why does this work?
Python inserts an empty
__main__
module in
sys.modules
at interpreter startup, and populates it by running top-level code. In our example this is the
start
module which runs line by line and imports
namely
. In turn,
namely
imports
__main__
(which is really
start
). That’s an import cycle! Fortunately, since the partially populated
__main__
module is present in
sys.modules
, Python passes that to
namely
。见
__main__ 的特殊注意事项
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
__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
注意，在此情况下
__main__
作用域未包含
__file__
attribute as it’s interactive.
__main__
scope is used in the implementation of
pdb
and
rlcompleter
.