Python 中 if _ _name_ _ == '_ _main_ _' 到底是什么意思？

Understanding the __name__ Variable in Python

In Python, when a script is executed directly, the special variable __name__ is set to __main__. This is demonstrated by running separate Python scripts, named a.py and b.py, where each will have its __name__ variable set to __main__ when run directly.

Interaction Between Two Python Files

When one Python file imports a function from another, the __name__ variable reflects the name of the file being imported. For instance, if a.py imports a function from b.py, and a.py is run directly, __name__ in the function from b.py will be b, indicating that the code is being run within the context of a.py and not as a stand-alone script.

Extension to Three Python Files

The concept extends to multiple files. If a.py imports functions from both b.py and c.py, and is executed directly, the __name__ variables within the imported functions will output b and c respectively. This shows that the context of the execution is within a.py.

Preventing Unintended Code Execution

Without precautions, importing a Python file might result in unintended code execution, such as print statements outside of function definitions. This behavior is demonstrated when a print statement in b.py executes upon being imported by a.py.

Using if __name__ == '__main__'

To prevent such unintended execution, the guard if __name__ == '__main__': is used. Code within this conditional block will only execute when the Python file is run directly, not when it's imported by another script. This is shown by modifying b.py to include the guard, which prevents the print statement from executing when a.py, which imports b.py, is run.

Conclusion

The use of __name__ == '__main__' is crucial for controlling script execution in Python, especially when writing code that is intended to be imported as a module. The original content is published by the "数据STUDIO" public WeChat account, which covers a range of Python-centric data science topics, from beginner to advanced levels.