Replace element with a list in python

Question

In python, what is the best way to replace an element in a list with the elements from another list?

For example, I have:

a = [ 1, 'replace_this', 4 ]

I want to replace replace_this with [2, 3]. After replacing it must be:

a = [ 1, 2, 3, 4 ]

Update

Of course, it is possible to do with slicing (I'm sorry that I didn't write it in the question), but the problem is that it is possible that you have more than one replace_this value in the list. In this case you need to do the replacement in a loop and this becomes unoptimal.

I think that it would be better to use itertools.chain, but I'm not sure.

I've updated with an example of using a replacement dictionary and chain — Jon Clements
– Jon Clements, Commented Feb 6, 2013 at 14:51

Jon Clements · Accepted Answer · 2013-02-06 14:50:11Z

12

You can just use slicing:

>>> a = [ 1, 'replace_this', 4 ]
>>> a[1:2] = [2, 3]
>>> a
[1, 2, 3, 4]

And as @mgilson points out - if you don't happen to know the position of the element to replace, then you can use a.index to find it... (see his comment)

update related to not using slicing

Using itertools.chain, making sure that replacements has iterables:

from itertools import chain

replacements = {
    'replace_this': [2, 3],
    4: [7, 8, 9]
}

a = [ 1, 'replace_this', 4 ]
print list(chain.from_iterable(replacements.get(el, [el]) for el in a))
# [1, 2, 3, 7, 8, 9]

edited Feb 6, 2013 at 14:50

answered Feb 6, 2013 at 14:19

Jon Clements

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2 Comments

mgilson Over a year ago

+1 -- And you can get the 1:2 via i = a.index('replace_this'); a[i:i+1] = [2,3]

Igor Chubin Over a year ago

This solution is really beautiful! Thank you very much for it

Community · Accepted Answer · 2017-05-23 11:56:23Z

1

I would go with a generator to replace the elements, and another generator to flatten it. Assuming you have a working flatten generator, such that in the link provided:

def replace(iterable, replacements):
    for element in iterable:
        try:
            yield replacements[element]
        except KeyError:
            yield element

replac_dict = {
    'replace_this': [2, 3],
    1: [-10, -9, -8]
}
flatten(replace(a, replac_dict))
# Or if you want a list
#list(flatten(replace(a, replac_dict)))

edited May 23, 2017 at 11:56

CommunityBot

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answered Feb 6, 2013 at 16:54

themiurgo

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2 Comments

Igor Chubin Over a year ago

The problem is if you have nested lists that will not work. But anyway, +1

themiurgo Over a year ago

You didn't mention you had nested lists. And, in fact, also the other solution does not work with nested lists.

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