當(dāng)Python腳本中的__name__ == "__main__"條件成立時，表示該文件被直接運(yùn)行而非被導(dǎo)入。 1. __name__是Python的內(nèi)置變量，直接運(yùn)行腳本時其值為"__main__"，被導(dǎo)入時則為模塊名；2. 使用此結(jié)構(gòu)可控制僅在直接運(yùn)行時執(zhí)行特定代碼，如測試或演示邏輯；3. 結(jié)構(gòu)上應(yīng)先定義函數(shù)，再將需單獨運(yùn)行的代碼置於該條件塊內(nèi)；4. 優(yōu)點包括避免導(dǎo)入時副作用、增強(qiáng)模塊復(fù)用性、便於添加測試代碼；5. 注意縮進(jìn)正確、避免相對導(dǎo)入問題、保持主塊簡潔。此機(jī)制有助於編寫靈活且乾淨(jìng)的代碼。

You'll often see the if __name__ == "__main__": line in Python scripts, and it's super useful for controlling whether code runs when the file is executed directly or just imported. It basically checks if the script is being run as the main program rather than being imported into another script.

Here's how to use this pattern effectively:

What does __name__ mean?

In Python, every module has a built-in variable called __name__ . When you run a script directly (like typing python script.py ), Python sets __name__ to "__main__" . But if that same script is imported into another script ( import script ), then __name__ gets set to the name of the file instead — like "script" .

So this line:

 if __name__ == "__main__":

…is checking whether the current file is being run directly. If yes, it executes whatever's indented under that condition.

This is really handy when you want to write reusable modules that can also behave like standalone programs.

How to structure your script with it

A common way to use this is by defining functions first, maybe even ones that could be reused elsewhere, and putting the part that should only run when the file is executed directly inside the if __name__ == "__main__": block.

Here's a simple example:

 def greet(name):
    print(f"Hello, {name}!")

if __name__ == "__main__":
    greet("Alice")

• If you run this directly: python script.py , it will print Hello, Alice! .
• If you import it from somewhere else: import script , nothing will happen automatically — which is good, because maybe you just wanted access to the greet() function.

This keeps things clean and avoids unwanted behavior when importing.

Why you'd want to use it

There are a few practical reasons people use this construct:

• Avoid side effects on import : You don't want code to run just because it was imported.
• Make reusable modules : You can share functions without running extra code unless needed.
• Add test/demo blocks : Sometimes you'll include a demo section or unit tests inside the __main__ block so you can test functionality quickly.

For instance, imagine writing a math utility file. You define a bunch of functions, but you also want a quick way to test them out. Just put a little test section inside the __main__ block. That way, anyone who imports your file doesn't get all the test output unless they specifically want it.

A few gotchas and tips

• The code under if __name__ == "__main__": must be indented properly — otherwise, it won't run when expected.
• You can have multiple lines inside that block — even call other functions or read input.
• Be careful about relative imports if you're working in packages — sometimes the behavior can surprise you.
• Don't overdo it — keep the __main__ block short and focused. It's not meant for large logic sections.

One thing that trips people up is thinking that anything after that block will always run, no matter what. But actually, everything outside that conditional will run whether the file is imported or executed directly.

So if you have:

 print("Top of the file")

if __name__ == "__main__":
    print("Inside main block")

The first print will always show up, but the second one only shows up when the file is run directly.

基本上就這些。 It's a simple concept but super powerful once you understand how and why to use it.

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