How to cast string back into a list

Question

I have a list:

ab = [1, 2, a, b, c]

I did:

strab = str(ab).

So strab is now a string.

I want to cast that string back into a list.

How can I do that?

@aaronman: because that will make a list comprised of each character in the string. It won't recover the original list. — DSM, Jul 21 '13 at 02:35
@aaronman: Because `list(str([1, 2, 3]))` doesn't give `[1, 2, 3]`, it gives `['[', '1', ',', ' ', '2', ',', ' ', '3', ']']` — David Robinson, Jul 21 '13 at 02:35
@DavidRobinson Thats exactly what I intended it to do, I assumed it was a list of chars — aaronman, Jul 21 '13 at 02:36
@aaronman Even if it's a list of chars, you get additional `[`s, spaces and `,`s mixed in. — David Robinson, Jul 21 '13 at 02:37
@aaronman `list(any_iterable)` gives you a list of the elements contained as they would come out in a `for` loop. — Ryan Haining, Jul 21 '13 at 04:58
@aaronman and `for c in strab: print c` gives you every character in the string, which, as David Robinson has said, gives you the brackets, commas, and spaces. — Ryan Haining, Jul 21 '13 at 04:59
@aaronman then don't answer python questions with such snide comments. — Ryan Haining, Jul 21 '13 at 05:01
See also http://stackoverflow.com/questions/1436703/difference-between-str-and-repr-in-python — Ben Jackson, Jul 21 '13 at 05:57

score 19 · Accepted Answer · edited May 23 '17 at 11:44

The easiest and safest way would be to use ast.literal_eval():

import ast

ab = [1, 2, 'a', 'b', 'c']    # a list
strab = str(ab)               # the string representation of a list
strab
=> "[1, 2, 'a', 'b', 'c']"

lst = ast.literal_eval(strab) # convert string representation back to list
lst
=> [1, 2, 'a', 'b', 'c']

ab == lst                     # sanity check: are they equal?
=> True                       # of course they are!

Notice that calling eval() also works, but it's not safe and you should not use it:

eval(strab)
=> [1, 2, 'a', 'b', 'c']

score 6 · Answer 2 · answered Jul 21 '13 at 02:34

6

Use the ast package:

import ast
lst = ast.literal_eval(strab)

answered Jul 21 '13 at 02:34

David Robinson

77,383
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score 1 · Answer 3 · answered Oct 27 '17 at 12:45

In the context of setting a numpy array element with a sequence, you can use the built-in join to bypass setting it to a string:

str_list_obj = '-'.join(list_obj)

and afterwards when needed split the string sequence again with the same connector (provided it does not appear in the list's strings):

og_list_obj = str_list_obj.split("-")

How to cast string back into a list

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