Perform a string operation for every element in a Python list

Question

I have a list of strings in Python - elements. I would like to edit each element in elements. See the code below (it doesn't work, but you'll get the idea):

for element in elements:
    element = "%" + element + "%"

Is there a way to do this?

score 121 · Accepted Answer · answered Aug 19 '11 at 20:16

121

elements = ['%{0}%'.format(element) for element in elements]

answered Aug 19 '11 at 20:16

JBernardo

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@cfarm54 Yes, the [docs](http://docs.python.org/library/string.html#format-string-syntax) are pretty good – JBernardo Aug 19 '11 at 20:36
2

Sorry not the .format piece, the syntax for putting the code in brackets. – locoboy Aug 19 '11 at 20:42
@cfarm54 oh, they're called [list comprehensions](http://docs.python.org/tutorial/datastructures.html#list-comprehensions) – JBernardo Aug 19 '11 at 21:16
6

Wouldn't this copy the entire array into a new location? Is there a way to modify it in place? – Akhilesh Sep 18 '15 at 19:22

score 23 · Answer 2 · answered Aug 19 '11 at 20:16

23

You can use list comprehension:

elements = ["%" + e + "%" for e in elements]

answered Aug 19 '11 at 20:16

Achim

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score 8 · Answer 3 · answered Feb 09 '21 at 08:11

8

Python 3.6+ version (f-strings):

elements = [f'%{e}%' for e in elements]

answered Feb 09 '21 at 08:11

ZygD

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score 8 · Answer 4 · answered Aug 19 '11 at 20:16

8

You can use list comprehensions:

elements = ["%{}%".format(element) for element in elements]

answered Aug 19 '11 at 20:16

leoluk

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steveha · Answer 5 · 2014-10-14T19:15:51.953

There are basically two ways you can do what you want: either edit the list you have, or else create a new list that has the changes you want. All the answers currently up there show how to use a list comprehension (or a map()) to build the new list, and I agree that is probably the way to go.

The other possible way would be to iterate over the list and edit it in place. You might do this if the list were big and you only needed to change a few.

for i, e in enumerate(elements):
    if want_to_change_this_element(e):
        elements[i] = "%{}%".format(e)

But as I said, I recommend you use one of the list comprehension answers.

score 3 · Answer 6 · answered Aug 19 '11 at 20:21

3

elements = map(lambda e : "%" + e + "%", elements)

answered Aug 19 '11 at 20:21

bpgergo

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score 1 · Answer 7 · answered Dec 17 '20 at 21:18

Here some more examples

char = ['g:', 'l:', 'q:']

Using Replace

for i in range(len(char)):
    char[i] = char[i].replace(':', '')

Using strip

for i in range(len(char)):
    char[i] = char[i].strip(':')

Using a function

def list_cleaner(list_):
    return [char_.strip(':') for char_ in list_]


new_char = list_cleaner(char)
print(new_char)

Using Generator function(adviced if you have a large piece of data)

def generator_cleaner(list_):
    yield from (char_.strip(':') for char_ in list_)

# Prints all results in once
gen_char = list(generator_cleaner(char))

# Prints as much as you need, in this case only 8 chars
# I increase a bit the list so it makes more sense
char = ['g:', 'l:', 'q:', 'g:', 'l:', 'q:', 'g:', 'l:', 'q:']

# let's call our Generator Function
gen_char_ = generator_cleaner(char)

# We call only 8 chars
gen_char_ = [next(gen_char_) for _ in range(8)]
print(gen_char_)

Perform a string operation for every element in a Python list

7 Answers7

Using Replace

Using strip

Using a function

Using Generator function(adviced if you have a large piece of data)

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