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Python Conditional Check

Posted by Hanwen Zhang
Hanwen Zhang
20,084 Points

When we run the python second_app.py, what the "app" works here and why print_hello() from the app.py?

#app.py
def print_hello():
  print("hello from app.")

print(__name__)

if __name__ == '__main__':
  print_hello()



#second_app.py
import app
print("hello from second_app.")
app.print_hello()

#console
python app.py
__main__
hello from app.


python second_app.py
app                 - what is this for???
hello from second_app.
hello from app.                - why it is it here?

what the "app" works on the python second_app.py? since the app.py has a conditional statement

if __name__ == '__main__':
  print_hello()

why print_hello() from the app.py from the python second_app.py?

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

Hey Hanwen Zhang, good question! The print(__name__) is being used to show how the variable __name__ changes depending if the current module is the top level (called directly using python), or if the current module was imported into another module.

When running python app.py, the __name__ is changed to "__main__" to indicate the file was called directly. Therefore, any code within the if __name__ == "__main__" block will be executed. A call is made to the local function print_hello() to cause something to happen. Otherwise, only the print(__name__) statement would execute.

When running python second_app.py, the following occurs:

  • The first statement imports app.py, which causes all statements in app.py to be executed and the functions to be put in the app namespace.
    • The function print_hello would be reference as app.print_hello() within second_app code.
    • The print statement would be executed. Since app.py was imported, __name__ keeps the module name "app".
    • All other execution during import is block by the if __name__ == "__main__" statement
  • The statement `print("hello from second_app.") is executed.
  • The statement app.print_hello() runs, referencing the function in the imported file.
    • This causes print("hello from app.") to be executed.

Basically, these two files demonstrate how the value of __name__ within a module changes if a module is directly executed and how the value retains the module name if a module is imported.

Post back if you need more help. Good luck!!

Hanwen Zhang
Hanwen Zhang
20,084 Points

Hi Chris, thank you! You explained it very well, I understood now.