Binding frustration

[Rob Hunter]

So I thought I had come to peace with binding in Python, but then this
happened to me:

I was trying to do things the Python way (as opposed to the Scheme way)
as was suggested to me, and so here's a shortened version of my program:

def getGenres(title ): #it takes a movie title and returns a list of
genres that the movie falls into

result = [] # my accumulator

def inGenre(g): # g is a genre
if <here I test if "title" is of genre g (using a simple Python
dictionary I have collected from a mini web crawl)>:
result = result + [g] # if title is of genre g, then add
it to the accumulator

# and then I do a number of inGenre tests:
inGenre('Comedy ')
inGenre('Sci-fi')
inGenre('Suspen se')
...
return result

So what's wrong with this program? Well, Python tells me:

UnboundLocalErr or: local variable 'result' referenced before assignment

It seems my only choice is to move result to the global environment,
but if that's not a kludge, I don't know what is. So why doesn't this
work? Python lambdas are able to use "free" variables in this way.
Why not a def? And more importantly, how should I get around this?

Thanks all,

Rob

[JCM]

Re: Binding frustration

Rob Hunter <rob@cs.brown.e du> wrote:
....
[color=blue]
> def getGenres(title ): #it takes a movie title and returns a list of
> genres that the movie falls into[/color]
[color=blue]
> result = [] # my accumulator[/color]
[color=blue]
> def inGenre(g): # g is a genre
> if <here I test if "title" is of genre g (using a simple Python
> dictionary I have collected from a mini web crawl)>:
> result = result + [g] # if title is of genre g, then add[/color]

Here you're attempting to rebind a variable which lives in an
enclosing scope. This isn't possible in Python (except for globals).
Use result.append(g ) instead and it should work.

[Terry Reedy]

Re: Binding frustration


"Rob Hunter" <rob@cs.brown.e du> wrote in message
news:mailman.10 63920805.24865. python-list@python.org ...[color=blue]
> def inGenre(g): # g is a genre
> if <here I test if "title" is of genre g (using a simple[/color]
Python[color=blue]
> dictionary I have collected from a mini web crawl)>:
> result = result + [g] # if title is of genre g, then[/color]
add[color=blue]
> it to the accumulator
>
> UnboundLocalErr or: local variable 'result' referenced before[/color]
assignment

When, within a function, you assign to a variable that has not been
declared global, then you implicitly declare that variable to be local
to the function -- in this case, inGenre(). But local var 'result'
has not previously been assigned a value within inGenre. Hence the
error message. As JCM said, try result.append(g ).

Terry J. Reedy