List of list, converting all strings to int, Python 3

Question

I am trying to convert all elements of the small lists in the big list to integers, so it should look like this:

current list:
list = [[\'1\',\'2\',\'3\'],[\         


        

Answer 1

In short, you are not mutating lst:

for e in lst:
    for i in e:
        # do stuff with i

is the equivalent of

for e in lst:
    for n in range(len(e)):
        i = e[n]  # i and e[n] are assigned to the identical object
        # do stuff with i

Now, whether the "stuff" you are doing to i is reflected in the original data, depends on whether it is a mutation of the object, e.g.

i.attr = 'value'  # mutation of the object is reflected both in i and e[n]

However, string types (str, bytes, unicode) and int are immutable in Python and variable assignment is not a mutation, but a rebinding operation.

i = int(i)  
# i is now assigned to a new different object
# e[n] is still assigned to the original string

So, you can make your code work:

for e in lst:
    for n in range(len(e)):
        e[n] = int(e[n])

or use a shorter comprehension notation:

 new_lst = [[int(x) for x in sub] for sub in lst]

Note, however, that the former mutates the existing list object lst, while the latter creates a new object new_lst leaving the original unchanged. Which one you choose will depend on the needs of your program.