Python - intersection between a list and keys of a dictionary

Question

I have a list that looks like this:

l1 = [\'200:200\', \'90:728\']

I have a dictionary that looks like this:

d1 = {\'200:20         


        

Answer 1

Define efficient. Anyway here's what I would do. If it was too slow I'd probably move it to Cython.

s1 = set(l1)
s2 = set(d1.keys())
s3 = s1 & s2
# now you can access d1 using only keys in s3, or construct a new dict if you like
d2 = dict([(k,d1[k]) for k in s3])

Answer 2

In 3.x, this can be as simple as:

>>> {k: d1[k] for k in (d1.keys() & l1)}
{'200:200': {'foo': 'bar'}}

Under 2.7, you can use dict.viewkeys() to recreate this functionality:

>>> {k: d1[k] for k in (d1.viewkeys() & l1)}
{'200:200': {'foo': 'bar'}}

Under older versions of 2.x, it's a tad more verbose:

>>> {k: d1[k] for k in (set(d1).intersection(l1))}
{'200:200': {'foo': 'bar'}}

Answer 3

You can use a list comprehension in the dict constructor:

result = dict([(k,d1[k]) for k in l1 if k in d1])

If you're worried about removing duplicate keys, make l1 into a set first:

result = dict([(k,d1[k]) for k in set(l1) if k in d1])

Answer 4

Not sure about each solution performance, but I would do:

{k: v for k, v in d1.items() if k in l1}

Answer 5

If memory allocation and deallocation is making this process take too long, itertools to the rescue.

import itertools
result = {dict_key:d1[dict_key] for dict_key in itertools.ifilter(lambda list_item: list_item in d1, l1) }

This doesn't unnecessarily allocate memory for a whole new collection, and l1 could easily be an iterator instead of a list.

Answer 6

You can use the following code:

keys = set(l1).intersection(set(d1.keys()))
result = {k:d1[k] for k in keys}

EDIT: As commenters suggest you can replace the first line with, in Python 2.x:

keys = set(l1).intersection(d1)

And in Python 3.x:

keys = d1.keys() & l1