How to remove specific element from sets inside a list using list comprehension

Question

I think this may be related to set being mutable.

Basically, I can remove an element from a set using set.discard(element). However, set.discard(element) itself returns None. But I’d like to get a copy of the updated set. For example, if I have a list of sets, how can I get an updated copy conveniently using list comprehension operations?

Sample code:

test = [{'', 'a'}, {'b', ''}]
print [x.discard('') for x in test]
print test

will return

[None, None]
[set(['a']), set(['b'])]

Asked By: Zhen Sun

||

Source

Answer 1

You can use set difference operator, like this

test, empty = [{'', 'a'}, {'b', ''}], {''}
print [x - empty for x in test]
# [set(['a']), set(['b'])]

Answered By: thefourtheye

Answer 2

Whenever you feel constrained by a method that only works in-place, you can use the behavior of or/and to achieve the semantics that you want.

[x.discard('') or x for x in test]

This technique is occasionally useful for achieving things in a lambda (or other situations where you are restricted to a single expression) that are otherwise impossible. Whether it’s the most “readable” or “pythonic” is debatable 🙂

Answered By: roippi

Answer 3

This?(duplicate of @thefourtheye answer)

set subtraction operation returns set data.

test = [{'', 'a'}, {'b', ''}]
print [x - {''} for x in test]
print test

Output:

[set(['a']), set(['b'])]
[set(['a', '']), set(['', 'b'])]

Answered By: Kei Minagawa

Answer 4

>>> s = set( ['a' , 'b', 'c' , 'd' ] )
>>> print(s)
set(['a', 'c', 'b', 'd'])
>>>
>>> s -= {'c'}
>>> print(s)
set(['a', 'b', 'd'])
>>>
>>> s -= {'a'}
>>> print(s)
set(['b', 'd'])

Answered By: jxramos

How to remove specific element from sets inside a list using list comprehension

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