Question

Possible Duplicate:
Flatten (an irregular) list of lists in Python

I’m trying to use the nltk library in python, and more specifically the wordnet corpus, to extract all the words in a broad semantic category like ‘animal’. I’ve managed to write a function that goes down through all the categories and extracts the words in them, but what I end up with is a huge jumble of lists within lists. The lists aren’t of any predictable length or depth, they look like this:

['pet', 'pest', 'mate', 'young', 'stunt', 'giant', ['hen', 'dam', 'filly'], ['head', 'stray', 'dog', ['puppy', 'toy', 'spitz', 'pooch', 'doggy', 'cur', 'mutt', 'pug', 'corgi', ['Peke'], ['chow'], ['feist', 'fice'], ['hound', ['Lhasa', 'cairn']], ['boxer', 'husky']], ['tabby', 'tabby', 'queen', 'Manx', 'tom', 'kitty', 'puss', 'pussy', ['gib']]]

What I want is to be able to grab each of those strings out of that , and return a single, unnested list. Any advice?

Answer 1

In general, when you have to deal with arbitrary levels of nesting, a recursive solution is a good fit. Lists within lists, parsing HTML (tags within tags), working with filesystems (directories within directories), etc.

I haven’t tested this code extensively, but I believe it should do what you want:

ll = [ 1, 2, 3, [4, 5, [6, 7, 8]]]

def flatten(input_list):
    output_list = []
    for element in input_list:
        if type(element) == list:
            output_list.extend(flatten(element))
        else:
            output_list.append(element)
    return output_list

print (flatten(ll)) #prints [1, 2, 3, 4, 5, 6, 7, 8]

In general recursion is very easy to think about and the solutions tend to be very elegant (like above) but for really, really deeply nested things – think thousands of levels deep – you can run into problems like stack overflow.

Generally this isn’t a problem, but I believe a recursive function can always* be converted to a loop (it just doesn’t look as nice.)

• Note: I am not crash-hot on my compsci theory here. Someone can add details or correct me if I’m wrong.