gistfile1.txt

In [13]: def counter_factory():
   ....:     count = 0
   ....:     def _counter():
   ....:         count += 1
   ....:     return _counter
   ....: 

In [14]: f = counter_factory()

In [15]: f()
---------------------------------------------------------------------------
UnboundLocalError                         Traceback (most recent call last)
<ipython-input-15-0ec059b9bfe1> in <module>()
----> 1 f()

<ipython-input-13-e9a70e60b268> in _counter()
      2     count = 0
      3     def _counter():
----> 4         count += 1
      5     return _counter

UnboundLocalError: local variable 'count' referenced before assignment