Silently removing key from a python dict

Question

I have a python dict and I'd like to silently remove either None and '' keys from my dictionary so I came up with something like this:

try:
    del my_dict[None]
except KeyError:
    pass

try:
    del my_dict['']
except KeyError:
   pass

As you see, it is less readable and it causes me to write duplicate code. So I want to know if there is a method in python to remove any key from a dict without throwing a key error?

Aren't you mixing *values* and *keys* here? – Martijn Pieters Jul 06 '12 at 08:20 — Martijn Pieters, Jul 06 '12 at 08:20
I am confused, why would a key not appear in `dict.keys()`? – acattle Jul 06 '12 at 08:23 — acattle, Jul 06 '12 at 08:23

score 148 · Accepted Answer · answered Jul 06 '12 at 08:23

148

You can do this:

d.pop("", None)
d.pop(None, None)

Pops dictionary with a default value that you ignore.

answered Jul 06 '12 at 08:23

Keith

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4

Be careful not to write d.pop(None) - As this will raise a KeyError as no default value was provided – andy boot Apr 23 '15 at 10:46
how would I remove keys with a `None` value? – knowbody May 07 '15 at 07:38
1

knowbody: You can do exactly as this answer above said: d.pop(None, None) – deadbeef404 Jan 27 '16 at 04:29
Ran into the gotcha mentioned by andy boot. Just for clarity, here's what the docs say: `If default is not given and key is not in the dictionary, a KeyError is raised.` – Mike B May 04 '21 at 18:49

score 15 · Answer 2 · answered Jul 06 '12 at 08:22

15

You could use the dict.pop method and ignore the result:

for key in [None, '']:
    d.pop(key, None)

answered Jul 06 '12 at 08:22

Jon Clements

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score 3 · Answer 3 · edited Dec 23 '16 at 01:14

3

The following will delete the keys, if they are present, and it won't throw an error:

for d in [None, '']:
    if d in my_dict:
        del my_dict[d]

edited Dec 23 '16 at 01:14

Ozgur Vatansever

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answered Jul 06 '12 at 08:20

Simeon Visser

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@ozgurv Dictionary lookup is not O(n), it is O(1) – jamylak Jul 06 '12 at 08:23
@ozgurv: checking whether a key is in a dict is O(1), as is deleting a key from a dict. It *does* however cause you to do two lookups rather than one. – Ben Jul 06 '12 at 08:25
@ozgurv no, it's not. Check out the Python docs on [time complexity](http://wiki.python.org/moin/TimeComplexity) for data structures. You aren't looking for keys that end up in the amortized case there either. – Jeff Tratner Jul 06 '12 at 08:25
Thank you I was not aware of that. I thought sth like: if d in my_sexy_dict.keys(). Because .keys() returns a list and I consider as searching an element in a list. – Ozgur Vatansever Jul 06 '12 at 08:25
@ozgurv `my_sexy_dict.keys()` is not O(1), because it generates a list first (or in Python a "view"). `k in my_sexy_dict` is O(1) – Jeff Tratner Jul 06 '12 at 08:27
1

@ozgurv In Python 3 `dict.keys` no longer returns a list so they are practically the same, as they should be. – jamylak Jul 06 '12 at 08:32
2

@jamylak you're definitely right, I never understood why it returns a list instead of a set. – Ozgur Vatansever Jul 06 '12 at 08:34
I think it returns a list so that if you iterate over `keys()` and `values()` you get the key-value pairs. (I know, why would you do this intead of using `items()`? I don't know.) If you want a set of the keys in dict `dd`, you can just use `set(dd)` - the default iterator over a dict is to iterate over its keys. – PaulMcG Jul 06 '12 at 08:57

Maksym Polshcha · Answer 4 · 2012-07-06T08:42:32.217

3

You can try:

d = dict((k, v) for k,v in d.items() if k is not None and k != '')

or to remove all empty-like keys

d = dict((k, v) for k,v in d.items() if k )

edited Jul 06 '12 at 08:42

answered Jul 06 '12 at 08:24

Maksym Polshcha

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No need for `[`, `]`. You are creating an unnecessary intermediary list comprehension. If you remove them, the more efficient generator will be used. – jamylak Jul 06 '12 at 08:28

Silently removing key from a python dict

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