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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: A syntax question
Date: Mon, 10 Nov 2014 16:10:17 +0100
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Joel Goldstick wrote:

> On Mon, Nov 10, 2014 at 6:39 AM, Wolfgang Maier
> <wolfgang.maier@biologie.uni-freiburg.de> wrote:
>> You may want to read:
>>
>> https://docs.python.org/3/faq/programming.html?highlight=global#why-am-i-getting-an-unboundlocalerror-when-the-variable-has-a-value
>>
>> from the Python docs Programming FAQ section.
>> It explains your problem pretty well.
>>
>> As others have hinted at, always provide concrete Python error messages
>> and tracebacks instead of vague descriptions.
>>
>> Best,
>> Wolfgang
>>
>>
>>
>> On 11/10/2014 12:07 PM, Mok-Kong Shen wrote:
>>>
>>>
>>> I don't understand the following phenomenon. Could someone kindly
>>> explain it? Thanks in advance.
>>>
>>> M. K. Shen
>>>
>>> -------------------------------------------------
>>>
>>> count=5
>>>
>>> def test():
>>>    print(count)
>>>    if count==5:
>>>      count+=0  ### Error message if this line is active, otherwise ok.
>>>      print(count)
>>>    return
>>>
>>> test()
>>
>>
>> --
>> https://mail.python.org/mailman/listinfo/python-list
> 
> Your problem is that count is not local.  You are reading count from
> an outer scope.  When you try to increment count in your function, it
> can't because it doesn't exist.
> Don't use globals.

That's what most would expect, but the error is already triggered by the 
first

print(count)

Python decides at compile-time that count is a local variable if there is an 
assignment ("name binding") to count anywhere in the function's scope --  
even if the corresponding code will never be executed:

>>> x = 42
>>> def test():
...     print(x)
...     if 0: x = 42
... 
>>> test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in test
UnboundLocalError: local variable 'x' referenced before assignment

This is different from the class body where the global namespace is tried 
when a lookup in the local namespace fails:

>>> x = 42
>>> class A:
...     print(x)
...     x += 1
... 
42
>>> x
42
>>> A.x
43

Historical ;) note: In Python 2 you could trigger a similar behaviour with 
exec:

>>> def f(a):
...     if a: exec "x = 42"
...     print x
... 
>>> x = "global"
>>> f(True)
42
>>> f(False)
global