Joel Goldstick wrote:
> On Mon, Nov 10, 2014 at 6:39 AM, Wolfgang Maier
> <wolfgang.ma...@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
--
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