python – Delete a dictionary item if the key exists

The Question :

304 people think this question is useful

Is there any other way to delete an item in a dictionary only if the given key exists, other than:

if key in mydict:
    del mydict[key]

The scenario is that I’m given a collection of keys to be removed from a given dictionary, but I am not certain if all of them exist in the dictionary. Just in case I miss a more efficient solution.

The Question Comments :
  • @user1929959 Nope, that question wants a copy of the dictionary, and doesn’t care about hte case that the key is not in the input.

The Answer 1

651 people think this answer is useful

You can use dict.pop:

 mydict.pop("key", None)

Note that if the second argument, i.e. None is not given, KeyError is raised if the key is not in the dictionary. Providing the second argument prevents the conditional exception.

The Answer 2

18 people think this answer is useful

There is also:

try:
    del mydict[key]
except KeyError:
    pass

This only does 1 lookup instead of 2. However, except clauses are expensive, so if you end up hitting the except clause frequently, this will probably be less efficient than what you already have.

The Answer 3

5 people think this answer is useful

Approach: calculate keys to remove, mutate dict

Let’s call keys the list/iterator of keys that you are given to remove. I’d do this:

keys_to_remove = set(keys).intersection(set(mydict.keys()))
for key in keys_to_remove:
    del mydict[key]

You calculate up front all the affected items and operate on them.

Approach: calculate keys to keep, make new dict with those keys

I prefer to create a new dictionary over mutating an existing one, so I would probably also consider this:

keys_to_keep = set(mydict.keys()) - set(keys)
new_dict = {k: v for k, v in mydict.iteritems() if k in keys_to_keep}

or:

keys_to_keep = set(mydict.keys()) - set(keys)
new_dict = {k: mydict[k] for k in keys_to_keep}

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