How to convert / restore a string with [] in python

Thing you want to do

I would like to convert a string defined as'[1,2,3,4]' to a list.

>>> a = [1, 2, 3, 4]
>>> a = str(a)
>>> print(a)
[1, 2, 3, 4]
>>> print(type(a))
<class 'str'>

It is difficult to understand from the standard output, but the programmatically it is the character string'[1, 2, 3, 4]'.


Method 1: Use slices

Use slices to list ranges other than "[]".

>>> b = list(map(int, a[1:-1].split(", ")))
>>> print(b)
[1, 2, 3, 4]  # <class 'list'>

Method 2: Use eval ()

eval () is a function that executes the code in a string. For example, the following execution is possible.

>>> eval("print('hello')")

You can see that the print statement in the string is being executed. If you use eval (), it will be treated as an expression, not as a string.

Conversion using eval

>>> a = [1, 2, 3, 4]
>>> a = str(a)
>>> print(a)
[1, 2, 3, 4]  # <class 'str'>

>>> b = eval(a)
>>> print(b)
[1, 2, 3, 4]  # <class 'list'>


The above two methods are summarized. It may not be used very often, but it was a good opportunity to learn the difference between str and repr. Please, try it!


Although eval () was introduced in Method 2, it was pointed out that there is a security problem. (Thank you!) There is no problem with this method, but it seems that ast.literal_eval () is recommended because it can execute arbitrary scripts when accepting input from the outside.

Reference: [Why is it dangerous to use the eval function in web forms etc.? ](Https:// 83% BC% E3% 83% A0% E3% 81% AA% E3% 81% A9% E3% 81% A7% E4% BD% BF% E3% 81% 86% E3% 81% A8% E3% 81% AA% E3% 81% 9C% E5% 8D% B1% E9% 99% BA% E3% 81% AA% E3% 81% AE% E3% 81% 8B)

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