How can I check if a key exists in a dictionary?

Question

Let's say I have an associative array like so: {'key1': 22, 'key2': 42}.

How can I check if key1 exists in the dictionary?

In python 3 you can just use `'key1' in {'key1': 22, 'key2': 42}.keys()` refer to the `keys()` method in [Dictionary](https://docs.python.org/2/tutorial/datastructures.html#dictionaries) — Eat at Joes, Feb 10 '20 at 20:13

score 712 · Accepted Answer · edited Dec 12 '15 at 22:51

712

if key in array:
  # do something

Associative arrays are called dictionaries in Python and you can learn more about them in the stdtypes documentation.

edited Dec 12 '15 at 22:51

mkobit

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answered Oct 02 '10 at 11:01

Rafał Rawicki

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And, be sure to put the key name in quotes if it's a string. – JAL Oct 02 '10 at 11:04
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This throws a `TypeError: string indices must be integers` error for me. – starbeamrainbowlabs Apr 14 '14 at 07:51
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how do you negate this ? if not in array ? – astroanu May 10 '15 at 09:56
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@astroanu `if key not in array` – Spencer Wood Aug 06 '15 at 14:38
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Where did the word "array" come from in this answer? – Ray Salemi Mar 29 '17 at 17:19
@RaySalemi Not just "array". "associative array" is a different name for a dictionary. – Rafał Rawicki Mar 29 '17 at 23:49
@RafałRawicki, I mean the word 'array' in the source code. It was not initialized or described. – Ray Salemi Mar 30 '17 at 21:00
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As accepted answers go, this one is horrible. It in no way answers the question and is nothing more than a condescending link to the documentation. It is sad that Google ranks this so highly. – cstrutton Jul 12 '17 at 00:59
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@cstrutton Please suggest improvements. At the moment of writing this answer I didn't feel more details is necessary. – Rafał Rawicki Jul 12 '17 at 11:13
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@RafałRawicki I agree that the answer is a bit sparse and that it is unfortunate that Google ranks this answer so highly. But it isn't your fault, either. You answered the way you thought was necessary... Having said that, I don't think cstrutton's comment was meant for you but more on Google. – Ray Oct 26 '17 at 04:57
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it exactly answers the question, @cstrutton... – Philip Schiff Dec 03 '17 at 17:12
Not entirely, Philip. Combined with Spencer Wood's comment, it fully answers the question. But I have no objection to a concise answer. – cssyphus Sep 10 '18 at 19:38
I would promote ghostdog's answer which is the closest match to the question. mydict.has_key("keyname") [Python 2.X]. if keyname in mydict for Python 3.X. – ThinkCode Feb 12 '19 at 17:13

score 65 · Answer 2 · answered Oct 02 '10 at 12:50

65

If you want to retrieve the key's value if it exists, you can also use

try:
    value = a[key]
except KeyError:
    # Key is not present
    pass

If you want to retrieve a default value when the key does not exist, use value = a.get(key, default_value). If you want to set the default value at the same time in case the key does not exist, use value = a.setdefault(key, default_value).

answered Oct 02 '10 at 12:50

Marc

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It should be noted that you should only use the `try/except` case if you expect that the key will be present approaching 100% of the time. otherwise, `if in` is prettier and more efficient. +1 for mentioning the other possibilities. – aaronasterling Oct 02 '10 at 13:37
Interesting, I remember that I read somewhere in the official docs that it is better to ask for forgiveness than permission... – Yannic Hamann Jan 15 '20 at 06:36
from point of view of performance , try/expect is better if you are dealing with dict with many keys – Avo Asatryan Mar 03 '20 at 07:52

score 63 · Answer 3 · edited Jul 27 '18 at 12:55

63

Another method is has_key() (if still using Python 2.X):

>>> a={"1":"one","2":"two"}
>>> a.has_key("1")
True

edited Jul 27 '18 at 12:55

Peter Mortensen

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answered Oct 02 '10 at 11:07

ghostdog74

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`has_key` is deprecated, removed in python 3, and half as fast in python 2 – aaronasterling Oct 02 '10 at 11:10
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yes it is deprecated in 2.x and yes it is half as fast in python 2.x. – aaronasterling Oct 02 '10 at 11:44
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deprecated applies to all new code. once it's deprecated, don't use it anymore. – aaronasterling Oct 02 '10 at 12:40
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the form 'key in dict' has existed since 2.2 – Tim W. Oct 02 '10 at 13:01
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we could go on and on about this, the fact remains, its still an alternative option for <=2.5. – ghostdog74 Oct 02 '10 at 14:15
`has_key` is deprecated, use `if mykey in mydict:` – Pedro Lobito Mar 21 '18 at 15:13
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**2018**: you can use `.__contains__` instead of `has_key` (available in Python [2](https://docs.python.org/2/reference/datamodel.html#object.__contains__) and [3](https://docs.python.org/3/reference/datamodel.html#object.__contains__)) - I'm finding it useful to write one liners like `foo = mydict['foo'] if mydict.__contains__('foo') else 'bar'` – ron_g Jan 08 '19 at 09:56
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@rong even simpler you can use `foo = mydict.get('foo', 'bar')`. The `in` operator calls `__contains__` so this is just more verbose. – anthonybell Apr 05 '19 at 21:24

How can I check if a key exists in a dictionary?

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