Question

Given the following code:

a = 0
def foo():
  # global a
  a += 1
foo()

When run, Python complains: UnboundLocalError: local variable ‘a’ referenced before assignment

However, when it’s a dictionary…

a = {}
def foo():
  a['bar'] = 0
foo()

The thing runs just fine…

Anyone know why we can reference a in the 2nd chunk of code, but not the 1st?

Answer 1

The difference is that in the first example you are assigning to a which creates a new local name a that hides the global a.

In the second example you are not making an assignment to a so the global a is used.

This is covered in the documentation.

A special quirk of Python is that – if no global statement is in effect – assignments to names always go into the innermost scope.