[python] Variables declared outside function

The local names for a function are decided when the function is defined:

>>> x = 1
>>> def inc():
...     x += 5
...     
>>> inc.__code__.co_varnames
('x',)

In this case, x exists in the local namespace. Execution of x += 5 requires a pre-existing value for x (for integers, it's like x = x + 5), and this fails at function call time because the local name is unbound - which is precisely why the exception UnboundLocalError is named as such.

Compare the other version, where x is not a local variable, so it can be resolved at the global scope instead:

>>> def incg():
...    print(x)
...    
>>> incg.__code__.co_varnames
()

Similar question in faq: http://docs.python.org/faq/programming.html#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value