How to convert string in list of lists to float in place?

Question

my_list=[['A','B','C','0.0'],['D','E','F','1.0'],['G','H','I','0'],['J','K','L','M']

I've tried doing

new_list=[[float(x) for x in i if x.isnumeric()] for i in my_list]

Output

[[0.0],[1.0],[0],[]]

Expected output

[['A','B','C',0.0],['D','E','F',1.0],['G','H','I',0],['J','K','L','M']

I don't know how to turn the final values into a float and modify the values in place, I've tried appending. I've tried the answers to other questions too but they created a new list, but I want to change the original list.

EDIT Changed my list to include decimals. The below answers worked if the numbers (string) were '0' but not decimals '0.0' or '1.0'.

Answer 1

You need to keep x if x is not numeric. You are close:

>>> my_list = [['A','B','C','0'], ['D','E','F','1'], ['G','H','I','J']]
>>> new_list = [[float(x) if x.isnumeric() else x for x in i] for i in my_list]
>>> new_list
[['A', 'B', 'C', 0.0], ['D', 'E', 'F', 1.0], ['G', 'H', 'I', 'J']]

Note the if x.isnumeric() else x inside the list comprehension, and that it's at the beginning of the comprehension, not the end (since it's a ternary conditional)

---

As you've noticed, this is creating a new list, not modifying the original. If you want to keep the original list, a basic for-loop would be better:

for i, sublist in enumerate(my_list):
    for j, x in enumerate(sublist):
        try:
            my_list[i][j] = float(x)
        except ValueError:
            pass

[['A', 'B', 'C', 0.0], ['D', 'E', 'F', 1.0], ['G', 'H', 'I', 'J']]

Answer 2

Assign to my_list[:] to modify it inplace. Use a ternary if instead of the comprehensions if filter to keep all items.

#       v modify list instead of replacing it
my_list[:] = [[float(x) if x.isnumeric() else x for x in i] for i in my_list]
#                       ^ ternary if provides a value for each element

---

If the nested lists must be modified in-place, consider using a loop to explicitly access each sub-list. If memory consumption is a goal, use a generator expression ((...)) instead of a list comprehension ([...]) for the new elements.

for sub_list in my_list:
    sub_list[:] = (float(x) if x.isnumeric() else x for x in sub_list)

To convert all float values, not just those that are purely digits, use try-except. str.isnumeric will for example reject '12.3'.

def try_float(literal: str):
    try:
        return float(literal)
    except ValueError:
        return literal

for sub_list in my_list:
    sub_list[:] = (try_float(x) for x in sub_list)