if __name__ == '__main__'

Syntax

if __name__ == '__main__':\n    # code that runs only when executed directly

Examples

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

if __name__ == "__main__":
    print(greet("World"))
    print("This only runs when the file is executed directly")

# calculator.py
def add(a, b):
    return a + b

if __name__ == "__main__":
    # Only runs with: python calculator.py
    print("Running calculator.py directly")
    print(add(2, 3))

# In another file:
# import calculator
# calculator.add(2, 3)  # works fine
# but "Running calculator.py directly" is NOT printed

def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False
    return True

if __name__ == "__main__":
    for num in [2, 4, 7, 9, 13]:
        print(num, is_prime(num))

Best practices

• Put your file's main execution logic inside the if __name__ == '__main__': guard so the file can be safely imported elsewhere without side effects
• Use this pattern for any script you might also want to reuse as a library of functions in another project
• Keep the guarded block focused on orchestration (calling functions) rather than defining new functions - definitions should be at module level
• This is one of Python's most common idioms - expect to see it in nearly every well-structured standalone Python script

At a glance

Purpose

Scripting and general-purpose applications

File extension

.py

Runs in

Python interpreter

Usually used with

Python standard library and packages

Specifications & further reading

• Python language reference

Related Python documentation