Python: what does `if __name__ == "__main__":` do?

Question

I see this at the bottom of many scripts. What happens if I remove it?

Accepted Answer

Every module has a `__name__` variable. When you **run** a file directly (`python script.py`), `__name__` is `"__main__"`. When another file **imports** it, `__name__` is the module's name (`"script"`).

So the block means: "run this only when the file is executed directly, not when it's imported."

```python
def add(a, b):
    return a + b

if __name__ == "__main__":
    print(add(2, 3))  # runs with `python math_utils.py`, not on `import math_utils`
```

Without it, importing your module (from tests, say) would also run the script's side effects.

Python: what does `if name == "main":` do?

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