CODEWITHSUNDEEP
pythondictionary
Luke Taylor
Luke Taylor

Reputation: 9561

Check for the presence of a key/value pair in a Python dictionary

What is the shortest way to check for the existence of a certain key/value pair in a dictionary if I don't know that the key exists?

So far, I've come up with:

if 'key' in my_dict and my_dict['key'] == 'value':
    do_something()

This is really long with longer variable names or longer key/value names, such as:

if 'X-Powered-By' in self.request.headers and self.request.headers['X-Powered-By'] == 'NodeBB':
    do_something()

What's a shorter way to check for the presence of a key and a corresponding value?

Upvotes: 2

Views: 1370

Answers (4)

Nikhil VJ
Nikhil VJ

Reputation: 6112

If you're just looking for existence of the key

if not 'key' in my_dict:
    my_dict['key'] = 'some default value'

Upvotes: 0

DevLounge
DevLounge

Reputation: 8437

Ok, my suggestion, to make your code more readable:

headers = self.request.headers
if headers.get('X-Powered-By') == 'NodeBB':
    do_something()

This could be another short code but definitely not efficient as a dict.get:

if ('X-Powered-By', 'NodeBB') in self.request.headers.items():
    do_something()

Upvotes: -1

user2390182
user2390182

Reputation: 73450

Actually, none of the answers captures the full problem. If the value that is being queried for happens to be None or whatever default value one provides, the get()-based solutions fail. The following might be the most generally applicable solution, not relying on defaults, truly checking the existence of a key (unlike get()), and not over-'except'-ing KeyErrors (unlike the other try-except answer) while still using O(1) dict lookup (unlike items() approach):

try:
    assert my_dict[key] == value:
except (KeyError, AssertionError):
    do_sth_else()  # or: pass
else:
    do_something()

Upvotes: 2

Davor Lucic
Davor Lucic

Reputation: 29390

You can fetch the value, and compare it right away:

# default return value is None if key is not found
if mydict.get("key") == "somevalue"

or

# Or specify your own default value
if mydict.get("key", False) == "somevalue"

Check out Python dict.get docs.

Upvotes: 8

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