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Main Function

Understanding if __name__ == '__main__':

In Python, if __name__ == '__main__': is used to determine whether a script is run directly or imported as a module. This helps manage code execution depending on the context.

How It Works

  1. Direct Execution: When you run a script directly (e.g., python my_script.py), __name__ is set to '__main__'. The code inside the if __name__ == '__main__': block will run.

  2. Imported as a Module: When you import the script into another script (e.g., import my_script), __name__ is set to the module's name (e.g., 'my_script'). The code inside the if __name__ == '__main__': block will not execute.

Examples

  1. Basic Example
# script1.py
def say_hello():
    print("Hello!")

if __name__ == '__main__':
    say_hello()

Running python script1.py will output "Hello!" because say_hello() is called directly.

  1. Testing Functions
# script2.py
def add(a, b):
    return a + b

if __name__ == '__main__':
    result = add(2, 3)
    print(f"Result: {result}")

Running python script2.py will print "Result: 5". When imported, add() can be used without printing the result.

  1. Module Import Example
# script3.py
def greet(name):
    print(f"Hi, {name}!")

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

# script4.py
import script3

script3.greet("Bob")

Running python script4.py will print "Hi, Bob!" but not "Hi, Alice!" since greet("Alice") in script3 only runs when script3.py is executed directly.

  1. Using Command-Line Arguments
# script5.py
import sys

def main():
    if len(sys.argv) > 1:
        print(f"Arguments: {sys.argv[1:]}")
    else:
        print("No arguments provided")

if __name__ == '__main__':
    main()

Running python script5.py arg1 arg2 will print "Arguments: ['arg1', 'arg2']". When imported, main() won't run automatically.

Takeaways

  • Direct Execution: Code in if __name__ == '__main__': runs only when the script is executed directly.
  • Module Import: Code in this block does not run when the script is imported as a module.
  • Use Cases: Ideal for testing code, running scripts, or executing code that should not run on import.

Using if __name__ == '__main__': helps keep code modular and avoids unintended executions when scripts are imported as modules.