macOS Grr! Python 3 NameError: global name is not defined

[wrldwzrd89]

def makeApplication():
    global app
    app = Application.Application()
    
def getApplication():
    global app
    return app

if __name__ == "__main__":
    global app
    makeApplication()
    del makeApplication
    # Start GUI
    app.mainloop()

When I try to use the above code, the program runs just fine, until something outside this module calls getApplication() - then it gives me an error message:

Exception in Tkinter callback
Traceback (most recent call last):
(lots of stuff removed for brevity)
   return app
NameError: global name 'app' is not defined

I cannot, for the life of me, figure out why Python's complaining.

 

[lee1210]

Where is the global declaration of app? That is, app = ... unindented?

-Lee

 

[wrldwzrd89]

Where is the global declaration of app? That is, app = ... unindented?

-Lee

There isn't one, because if I create a global one, every time this module gets imported in order to fetch the app variable, a new app gets created - which I don't want.

 

[AlmostThere]

Has Python 3 changed this behaviour?

Statements are only executed the first time a module is imported.

http://docs.python.org/tutorial/modules.html#more-on-modules

 

[wrldwzrd89]

Has Python 3 changed this behaviour?

Statements are only executed the first time a module is imported.

http://docs.python.org/tutorial/modules.html#more-on-modules

According to the Py3K docs (http://docs.python.org/py3k/tutorial/modules.html#more-on-modules) it hasn't, which is strange... why, then, am I getting the behavior I am?

 

[AlmostThere]

Don't you just want something like this:

# appmodule.py
from applicationmodule import Application # wherever Application lives...

app = Application.Application()

# if you are executing this module, then run the mainloop
if __name__ == "__main__":
    app.mainloop()

# anothermodule.py

# Application.Application() is called only once
# however many times app is imported
import appmodule 

# if this module is being run, call mainloop
if __name__ == "__main__":
    appmodule.app.mainloop()

global is used for scope resolution in Python (the rules have changed slightly from 2.x to 3.x and, disclaimer, I haven't used 3.x, YMMV)

if you really want a function, you could do this:

# appmodule.py

def makeApplication():
    global app
    app = Application.Application()

which will put app into the appmodule namespace after you call makeApplication()

# anothermodule.py

import appmodule 

try:
    print appmodule.app # appmodule.app doesn't exist
except AttributeError:
    print "AttributeError raised"
    pass

appmodule.makeApplication() # call Application.Application()
appmodule.app.mainloop()  # appmodule.app is now available

and if you want, you can start deleting the module function, but I get the feeling the first option is simplest ...
btw, are you trying to work around a Singleton?

 

[wrldwzrd89]

Yes, I am creating a Singleton sort of thing... and thank you, I'm sure that will help.

 

[AlmostThere]

OK. I only mention the Singleton thing as there is a good thread on Stack Overflow, in case you haven't seen it.

http://stackoverflow.com/questions/...le-elegant-way-to-define-singletons-in-python

Design patterns are less needed / more part of the language than in languages like Java or C++ (IMHO). I like the simplicity python offers without some of the design headaches