UnboundLocalError

[Gabriel Genellina]

At Saturday 11/11/2006 02:35, Camellia wrote:

>But sorry I'm so dumb I can't say I really understand,
>what do I actually do when I define a function with its name "number"?

Don't apologize, Python is a lot more dumb than you. It obeys very 
simple rules (a good thing, so we can clearly understand them, once 
we know them).
Recalling your previous example:

> > >> def main():
> > >>    number = number()

Python first scans the source code looking for assigned-to names. 
That is, names to the left of equal signs. Those names, plus the 
function formal parameters, make the list of "local names". Any other 
names referenced are assumed to be globals, that is, living outside 
your function. (That's not entirely true but enough for now).
Notice that "number" is a local name because it's assigned to; it 
doesn't matter whether a global name "number" exists or not.
When the code is executed, the "number" to the right references the 
local name, which has not been assigned to yet - that's why you get 
an UnboundLocalError. "number" can't be a global name when used on 
the right hand side, and a local name when used on the left hand 
side. It's simple: it is local, or not, but not both.

>why does a name of a function has something to do with a variable?

Notice also that it does not matter *what* kind of object a name 
refers to: it may be a function, a class, an object instance, 
whatever. Inside a function, by example, a local name "a" may be 
bound at most to a single object at a time, it doesn't matter its 
type. A local name "a" can't refer to an integer and a class at the same time.
The left hand side of an assign statement *always* uses local names, 
except when you declare a name to be global by using the "global" keyword.
And notice also that I've never used the word variable.

>Oh wait can I do this in Python?:
><code>
>def a():
>     def b()
></code>
>
>so the b() will appear to be a local function which is the possible
>cause of my little own error because the compiler will interpret the
>number() as a local function but a global one?

Yes, you can. Python has "lexically nested scopes". The simple 
local/global rule of above is a bit more complicated: when a name is 
not local, it is searched inside the enclosing functions, then in the 
containing module's global namespace, and last in the builtin names. 
And yes, b is local to a, so it effectively hides any external b that 
could be in outer scopes.


-- 
Gabriel Genellina
Softlab SRL 

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