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

>>> 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]

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.

Python dictionary has get(key) function

>>> d.get(key)

For Example,

>>> d = {'1': 'one', '3': 'three', '2': 'two', '5': 'five', '4': 'four'}
>>> d.get('3')
'three'
>>> d.get('10')
None

If your key does'nt exist, will return None value.

foo = d[key] # raise error if key doesn't exist
foo = d.get(key) # return None if key doesn't exist

Content relevant to versions less than 3.0 and greater than 5.0.

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