name == "main"¶

Python modules can be executed in two different ways:

1. run directly as a script
2. imported as a module

The variable __name__ helps distinguish these cases.

flowchart TD
    A[Python file]
    A --> B{run directly?}
    B -->|yes| C[__name__ = "__main__"]
    B -->|no| D[__name__ = module name]
````

---

## 1. The **name** Variable

Every Python module has a special variable called `__name__`.

When a file is run directly:

```python
print(__name__)

Output:

__main__

When the file is imported, __name__ becomes the module name.

2. The Main Guard Pattern¶

The most common pattern is:

if __name__ == "__main__":
    main()

Example:

def main():
    print("Running program")

if __name__ == "__main__":
    main()

3. Why This Is Useful¶

The main guard prevents code from running during import.

Example:

# tools.py

print("module loaded")

If imported, the print statement runs immediately.

The main guard avoids this problem.

4. Typical Program Structure¶

def main():
    print("program logic")

if __name__ == "__main__":
    main()

This pattern separates:

• reusable code
• script execution

5. Example¶

File math_tools.py

def square(x):
    return x * x

def main():
    print(square(5))

if __name__ == "__main__":
    main()

Running the file:

25

Importing it:

import math_tools

No output occurs.

6. Summary¶

Key ideas:

• __name__ identifies how a module is executed
• __name__ == "__main__" is true when a file runs directly
• the main guard prevents unintended execution during import
• this pattern is common in Python programs

The main guard makes modules reusable while still allowing standalone execution.