Understanding the __name__ Variable in Python

While learning Python, I discovered the __name__ variable and its significance in script execution. Here's what I learned:

What is __name__?

__name__ is a special variable in Python used internally by the interpreter. Its value depends on how the script is executed:

• When the script is run directly: __name__ is set to "__main__".
• **When the script is imported as a module, __name__ is set to the module's name.

This allows developers to control the behaviour of a script when executed directly versus when imported.

Code Example

one.py

# one.py
def func():
    print("func() ran in one.py")

print("top-level print inside of one.py")

if __name__ == "__main__":
    print("one.py is being run directly")
else:
    print(f"one.py is being imported into another module {__name__}")

two.py

# two.py
import one

print("top-level in two.py")

one.func()

if __name__ == "__main__":
    print("two.py is being run directly")

three.py

import two
import one

print("top-level in three.py")

if __name__ == "__main__":
	print("three.py is being run directly")

Output

Running one.py directly:

top-level print inside of one.py  
one.py is being run directly  

Running two.py:

top-level print inside of one.py  
one.py is being imported into another module one  
top-level in two.py  
func() ran in one.py  
two.py is being run directly  

Running three.py

top-level print inside of one.py
one.py is being imported into another module one
top-level in two.py
func() ran in one.py
top-level in three.py
three.py is being run directly

Observations

Behaviour of Code Execution:

1. When running one.py directly:

   • All code at 0 indentation is executed.
   • The block if __name__ == "__main__": runs because __name__ is "__main__".

2. When running two.py:

   • The 0 indentation code in one.py runs because it's imported into two.py.
   • Since __name__ in one.py is the module name ("one"), the else block is executed.
   • Afterwards, the 0 indentation code in two.py executes, including calling func() from one.py.
   • Finally, the if __name__ == "__main__": block in two.py runs because it is the main program.

3. When running three.py:

   • The 0 indentation code in one.py runs because it’s imported via two.py.
   • Since __name__ in one.py is "one", the else block executes.
   • The 0 indentation code in two.py executes next, including calling func() from one.py.
   • The 0 indentation code in three.py executes last.
   • The if __name__ == "__main__": block in three.py runs.

Key Takeaways

• The __name__ variable helps differentiate between direct execution and import behaviour.
• The if __name__ == "__main__": block ensures that code only runs when the script is executed directly.