Comparing Two Dictionaries Key Values and Returning the Value If Match

Question

I'm a beginner to Python, but I've been trying this syntax and I cannot figure it out -- which was been really baffling.

crucial = {'eggs': '','ham': '','cheese': ''}
dishes = {'eggs': 2, 'sausage': 1, 'bacon': 1, 'spam': 500}

if crucial.keys() in dishes.keys():
    print dishes[value]

What I want to do is -- if crucial has a key (in this case, eggs) in the dishes, it will return 2. It seems simple enough, but I believe I must be messing some type of syntax somewhere. If someone could guide me a little, that would be greatly appreciated.

The real dictionaries I'm comparing with is about 150 keys long, but I'm hoping this code is simple enough.

Answer 1

You need to iterate over the keys in crucial and compare each one against the dishes keys. So a directly modified version of your code.

for key in crucial.keys():
    if key in dishes.keys():
        print dishes[key]

It could be better stated as (no need to specify .keys):

for key in crucial:
    if key in dishes:
        print dishes[key]

Answer 2

If you're using Python 3, the keys method of dictionaries follows the set interface. That means you can do an intersection of the keys of the two dictionaries using the & operator.

for key in crucial.keys() & dishes.keys():
    print(dishes[key])

Or if you need a list of the values:

result = [dishes[key] for key in crucial.keys() & dishes.keys()]

In Python 2 you could manage the same thing by explicitly creating sets (perhaps from the iterkeys generator), but probably it would be better to simply do a loop over the keys, like several of the other answer suggest.

Here's a variation on the loop that I don't think I've seen anyone else do. The outer loop gets both the keys and values from the dishes dict, so you don't need to separately look up the value by key.

for key, value in dishes.iteritems(): # use items() in Python 3
    if key in crucial:
        print value