Check if a specific Key and a value exist in a dictionary

Question

I am trying to determine if a specific key and value pair exist in a dictionary; however, if I use the contains or has-key method, it only checks for the key. I need it to c

Answer 1

Use

if key in d and d[key] == value:

Or (only in Python 3)

if (key, value) in d.items():

In Python 3 d.items() returns a Dictionary view object, which supports fast membership testing. In Python 2 d.items() returns a list, which is both slow to create and slow to to test membership. Python 2.7 is a special case where you can use d.viewitems() and get the same thing that you get with d.items() in Python 3.

Edit: In a comment you indicate that for performance reasons you prefer checkKeyValuePairExistence over key in d and d[key] == value. Below are some timings showing that checkKeyValuePairExistence is always slower (by about 2x on my system when the key-value pair is present 16x when it is not). I also tested larger and smaller dictionaries and found little variation in the timings.

>>> import random
>>> from timeit import timeit
>>> def checkKeyValuePairExistence(dic, key, value):
...     try:
...         return dic[key] == value
...     except KeyError:
...         return False
...
>>> d = {random.randint(0, 100000):random.randint(0, 100000) for i in range(1000)}
>>> setup = 'from __main__ import k, d, v, checkKeyValuePairExistence'
>>> test_try_except = 'checkKeyValuePairExistence(d, k, v)'
>>> test_k_in_d_and = 'k in d and d[k] == v'
>>> k, v = random.choice(d.items()) # to test if found
>>> timeit(test_try_except, setup=setup)
0.1984054392365806
>>> timeit(test_k_in_d_and, setup=setup)
0.10442071140778353
>>> k = -1 # test if not found
>>> timeit(test_try_except, setup=setup)
1.2896073903002616
>>> timeit(test_k_in_d_and, setup=setup)
0.07827843747497809 

Answer 2

Why not just do this:

a = {1:'a', 2:'b'}
b = (1, 'a')
print b in a.iteritems() # prints True

Answer 3

How about this function:

def checkKeyValuePairExistence(dic, key, value):
    try:
        return dic[key] == value
    except KeyError:
        return False

If you are using another type of dictionary other then the one python offers (I'm sorry, I couldnt understand from your post if you are using it or not) then let me know and i'll try to give your another solution