[Tutor] variable existence q

[Mark Lawrence]

On 15/08/2015 22:11, Peter Otten wrote:
> Clayton Kirkwood wrote:
>
>> 10 top_directory = "/users/Clayton/Pictures"
>>
>>      def override_defaults():
>> 56     return( top_directory, filetypes, target_directory )
>>
>> 80 top_directory, filetypes, target_directory = override_defaults()
>>
>>
>>    File "C:/Users/Clayton/python/find picture duplicates/find picture
>> duplicates", line 80, in <module>
>>      top_directory, filetypes, target_directory = override_defaults()
>>    File "C:/Users/Clayton/python/find picture duplicates/find picture
>> duplicates", line 56, in override_defaults
>>      return( top_directory, filetypes, target_directory )
>> UnboundLocalError: local variable 'top_directory' referenced before
>> assignment
>>
>> I am facing the above error:
>> 10 occurs first
>> 80 then runs
>> 56 appears to not work, the function logically does nothing
>> I thought that variables in the main were visible to defined functions in
>> the same file, as long as the assignment occurs physically before use.
>
> I don't think it's relevant here, but generally speaking the order in the
> file doesn't matter, only the order of execution matters. For example
>
>>>> def f(): return x
> ...
>>>> x = 42
>>>>
>>>> print(f())
> 42
>
> Even though the assignment to x occurs physically after the function
> definition, as the function is invoked after that assignment you don't get a
> NameError.
>
>> When debugging, inside of override_defaults sees the correct value.
>> What am I not seeing?
>
> There must be an assignment to top_directory inside override_defaults().
> This assignment turns top_directory into a local variable:
>
>>>> def f():
> ...     if False: x = 42 # this turns x into a local name
> ...     return x
> ...
>>>> x = 42
>>>> f()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<stdin>", line 3, in f
> UnboundLocalError: local variable 'x' referenced before assignment
>>>> x # the global x is defined, but not visible inside the function
> 42
>
> Wether a name is local to the function or global is determined statically by
> the compiler. This is different from class definitions. Compare:
>
>>>> x = 42
>>>> class A: x = x
> ...
>>>> A.x
> 42
>>>> def f(): x = x
> ...
>>>> f()
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
>    File "<stdin>", line 1, in f
> UnboundLocalError: local variable 'x' referenced before assignment
>

Your explanation doesn't make any sense to me.  I'd have thought that 
having assigned top_directory at line 10, but then trying to reassign it 
at line 80, means that the function now knows nothing about it, hence 
the error.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence