How to retrieve from python dict where key is only partially known?

Question

I have a dict that has string-type keys whose exact values I can't know (because they're generated dynamically elsewhere). However, I know that that the key I want contains a particular substring, and that a single key with this substring is definitely in the dict.

What's the best, or "most pythonic" way to retrieve the value for this key?

I thought of two strategies, but both irk me:

for k,v in some_dict.items():
    if 'substring' in k:
        value = v
        break

-- OR --

value = [v for (k,v) in some_dict.items() if 'substring' in k][0]

The first method is bulky and somewhat ugly, while the second is cleaner, but the extra step of indexing into the list comprehension (the [0]) irks me. Is there a better way to express the second version, or a more concise way to write the first?

Answer 1

There is an option to write the second version with the performance attributes of the first one.

Use a generator expression instead of list comprehension:

value = next(v for (k,v) in some_dict.iteritems() if 'substring' in k)

The expression inside the parenthesis will return an iterator which you will then ask to provide the next, i.e. first element. No further elements are processed.

Answer 2

How about this:

value = (v for (k,v) in some_dict.iteritems() if 'substring' in k).next()

It will stop immediately when it finds the first match.

But it still has O(n) complexity, where n is the number of key-value pairs. You need something like a suffix list or a suffix tree to speed up searching.