Question

I ran across this case of UnboundLocalError recently, which seems strange:

import pprint  def main():     if 'pprint' in globals(): print 'pprint is in globals()'     pprint.pprint('Spam')     from pprint import pprint     pprint('Eggs')  if __name__ == '__main__': main() 

Which produces:

pprint is in globals() Traceback (most recent call last):   File 'weird.py', line 9, in <module>     if __name__ == '__main__': main()   File 'weird.py', line 5, in main     pprint.pprint('Spam') UnboundLocalError: local variable 'pprint' referenced before assignment 

pprint is clearly bound in globals, and is going to be bound in locals in the following statement. Can someone offer an explanation of why it isn’t happy resolving pprint to the binding in globals here?

Edit: Thanks to the good responses I can clarify my question with relevant terminology:

At compile time the identifier pprint is marked as local to the frame. Does the execution model have no distinction where within the frame the local identifier is bound? Can it say, ‘refer to the global binding up until this bytecode instruction, at which point it has been rebound to a local binding,’ or does the execution model not account for this?

Answer 1

Looks like Python sees the from pprint import pprint line and marks pprint as a name local to main() before executing any code. Since Python thinks pprint ought to be a local variable, referencing it with pprint.pprint() before ‘assigning’ it with the from..import statement, it throws that error.

That’s as much sense as I can make of that.

The moral, of course, is to always put those import statements at the top of the scope.