Question

What is the best way to remove an item from a dictionary? Here's a simple approach:

for key, item in some_dict.items():    if item is item_to_remove:        del some_dict[key]

Are there better ways? Is there anything wrong with mutating (deleting items) from the dictionary while iterating it?

The underline reason for prohibiting mutating dict while iterating it is because internally there is an order for iteration, if you mutate the keys, the order would be undermined, which results unknown behavior. — Jul 26 '15 at 19:21

score 52 · Accepted Answer · 2011-03-27 05:51:10Z

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Be aware that you're currently testing for object identity (is only returns True if both operands are represented by the same object in memory - this is not always the case with two object that compare equal with ==). If you are doing this on purpose, then you could rewrite your code as

some_dict = {key: value for key, value in some_dict.items()              if value is not value_to_remove}

But this may not do what you want:

>>> some_dict = {1: "Hello", 2: "Goodbye", 3: "You say yes", 4: "I say no"}>>> value_to_remove = "You say yes">>> some_dict = {key: value for key, value in some_dict.items() if value is not value_to_remove}>>> some_dict{1: 'Hello', 2: 'Goodbye', 3: 'You say yes', 4: 'I say no'}>>> some_dict = {key: value for key, value in some_dict.items() if value != value_to_remove}>>> some_dict{1: 'Hello', 2: 'Goodbye', 4: 'I say no'}

So you probably want != instead of is not.

answered

Mar 27 '11 at 5:51

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Is that a dictionary compression? When were they added? –

Mar 27 '11 at 5:52

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you could use some_dict.iteritems() here and put for and if statements on separate lines for readability –

Mar 27 '11 at 5:57

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I believe dictionary comprehensions were added in Python 2.7. –

Mar 27 '11 at 5:58

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@J.F. Sebastian: I'm on Python 3, and iteritems is now items. In Python 2.7, iteritems() is indeed better. –

Mar 27 '11 at 6:00

@Buttons840 they are called or . as the PEP says they were as backported 3.x feats. alternatively you can feed dict() with an appropriate generator expression, which is 2.4. meta: can for finding stuff out. –

May 29 '14 at 11:43

9,113 2 28 47 · Answer 2 · 2011-03-27 05:55:31Z

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>>> dic = {'a':1, 'b':2}>>> dic{'a': 1, 'b': 2}>>> dic.pop('c', 0)0>>> dic.pop('a', 0)1>>> dic{'b': 2}

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Mar 27 '11 at 5:55

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score 28 · Answer 3 · 2011-03-27 08:18:39Z

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a = {'name': 'your_name','class': 4}if 'name' in a: del a['name']

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Mar 27 '11 at 8:18

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303 2 8 · Answer 4 · 2013-02-07 03:04:54Z

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A simple comparison between del and pop():

import timeitcode = """results = {'A': 1, 'B': 2, 'C': 3}del results['A']del results['B']"""print timeit.timeit(code, number=100000)code = """results = {'A': 1, 'B': 2, 'C': 3}results.pop('A')results.pop('B')"""print timeit.timeit(code, number=100000)

result:

0.03296678571430.0451040902256

So, del is faster than pop().

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Feb 7 '13 at 3:04

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However, the performance difference is not great, and if you want to avoid raising an exception, you can provide a second argument to pop() (as @n-1-1 does above) - which is not an option for the deloperator. –

Jul 11 '14 at 7:46

Ancillary to the question, but I'd also been struggling to understand timeit. Thank you for this clear example. –

May 1 '15 at 1:02

1,060 5 21 · Answer 5 · 2011-03-27 05:52:23Z

items() returns a list, and it is that list you are iterating, so mutating the dict in the loop doesn't matter here. If you were using iteritems() instead, mutating the dict in the loop , and likewise for viewitems() in Python 2.7.

I can't think of a better way to remove items from a dict by value.

user97370 · Answer 6 · 2011-03-27 09:34:35Z

I'd build a list of keys that need removing, then remove them. It's simple, efficient and avoids any problem about simultaneously iterating over and mutating the dict.

keys_to_remove = [key for key, value in some_dict.iteritems()                  if value == value_to_remove]for key in keys_to_remove:    del some_dict[key]

score 0 · Answer 7

There is nothing wrong with deleting items from the dictionary while iterating, as you've proposed. Be careful about multiple threads using the same dictionary at the same time, which may result in a KeyError or other problems.

Of course, see the docs at

9 Answers