How to check if a value exists in a dictionary (python)

Question

I have the following dictionary in python:

d = {'1': 'one', '3': 'three', '2': 'two', '5': 'five', '4': 'four'} 

I need a way to find if a value such as "one" or "two" exists in this dictionary.

For example, if I wanted to know if the index "1" existed I would simply have to type:

"1" in d 

And then python would tell me if that is true or false, however I need to do that same exact thing except to find if a value exists.

Answer 1

>>> d = {'1': 'one', '3': 'three', '2': 'two', '5': 'five', '4': 'four'} >>> 'one' in d.values() True 

Out of curiosity, some comparative timing:

>>> T(lambda : 'one' in d.itervalues()).repeat() [0.28107285499572754, 0.29107213020324707, 0.27941107749938965] >>> T(lambda : 'one' in d.values()).repeat() [0.38303399085998535, 0.37257885932922363, 0.37096405029296875] >>> T(lambda : 'one' in d.viewvalues()).repeat() [0.32004380226135254, 0.31716084480285645, 0.3171098232269287] 

EDIT: And in case you wonder why... the reason is that each of the above returns a different type of object, which may or may not be well suited for lookup operations:

>>> type(d.viewvalues()) <type 'dict_values'> >>> type(d.values()) <type 'list'> >>> type(d.itervalues()) <type 'dictionary-valueiterator'> 

EDIT2: As per request in comments...

>>> T(lambda : 'four' in d.itervalues()).repeat() [0.41178202629089355, 0.3959040641784668, 0.3970959186553955] >>> T(lambda : 'four' in d.values()).repeat() [0.4631338119506836, 0.43541407585144043, 0.4359898567199707] >>> T(lambda : 'four' in d.viewvalues()).repeat() [0.43414998054504395, 0.4213531017303467, 0.41684913635253906] 

Answer 2

In Python 3, you can use

"one" in d.values() 

to test if "one" is among the values of your dictionary.

In Python 2, it's more efficient to use

"one" in d.itervalues() 

instead.

Note that this triggers a linear scan through the values of the dictionary, short-circuiting as soon as it is found, so this is a lot less efficient than checking whether a key is present.