On Thursday, April 25, 2013 11:27:37 PM UTC+2, Alastair Thompson wrote:
> I am completely baffled by the behavior of this code with regards to the
> evaluation order of namespaces when assigning the class attributes. Both
> classes are nested within a function I called whywhywhy.
[snip weird namespace problem]
Hi,
I am not a namespace expert, but I think the following example shows the same
problem in an easier way without any classes, and gives a more helpful error
message:
In [1]: a = 123
In [2]: def f():
...: print a
...: b = 456
In [3]: f()
123
In [4]: def g():
...: print a
...: a = 456
In [5]: g()
---------------------------------------------------------------------------
UnboundLocalError Traceback (most recent call last)
/home/xxx/<ipython-input-5-d65ffd94a45c> in <module>()
----> 1 g()
/home/xxx/<ipython-input-4-c304da696fc2> in g()
1 def g():
----> 2 print a
3 a = 456
4
UnboundLocalError: local variable 'a' referenced before assignment
My guess is that in f(), the compiler sees no definition of a, so it assumes is
must come from the outer namespace. In g(), however, it sees on the second line
that a is uses as an assignment target, so it makes the variable a local. When
it is executed, it rightfully complains that the local variable is not yet
defined. A smarter compiler might prevent this problem, but then again a
smarter programmer would not have local and global variables with the same name.
In your example, something similar is probably happening, since you assign
something to third inside example2, thereby making it 'local'. Since you are
dealing with classes, the error message is probably not so clear (but whywhywhy
would you define a class inside a function?) Does that make sense?
HTH,
Bas
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