# Most elegant way to modify elements of nested lists in place

I have a 2D list that looks like this:

table = [['donkey', '2', '1', '0'], ['goat', '5', '3', '2']]

I want to change the last three elements to integers, but the code below feels very ugly:

for row in table:
for i in range(len(row)-1):
row[i+1] = int(row[i+1])

But I'd rather have something that looks like:

for row in table:
for col in row[1:]:
col = int(col)

I think there should be a way to write the code above, but the slice creates an iterator/new list that's separate from the original, so the references don't carry over.

Is there some way to get a more Pythonic solution?

for row in table:
row[1:] = [int(c) for c in row[1:]]

Does above look more pythonic?

Try:

>>> for row in table:
...     row[1:]=map(int,row[1:])
...
>>> table
[['donkey', 2, 1, 0], ['goat', 5, 3, 2]]

AFAIK, assigning to alistslice forces the operation to be done in place instead of creating a newlist.

I like Shekhar answer a lot.

As a general rule, when writing Python code, if you find yourself writing for i in range(len(somelist)), you're doing it wrong:

• tryenumerateif you have a single list
• tryziporitertools.izipif you have 2 or more lists you want to iterate on in parallel

In your case, the first column is different so you cannot elegantly use enumerate:

for row in table:
for i, val in enumerate(row):
if i == 0: continue
row[i] = int(val)

Your "ugly" code can be improved just by callingrangewith two arguments:

for row in table:
for i in range(1, len(row)):
row[i] = int(row[i])

This is probably the best you can do if you insist on changing the items in place without allocating new temporary lists (either by using a list comprehension,map, and/or slicing). SeeIs there an in-place equivalent to 'map' in python?

Although I don't recommend it, you can also make this code more general by introducing your own in-place map function:

def inplacemap(f, items, start=0, end=None):
"""Applies f to each item in the iterable items between the range
start and end."""
# If end was not specified, make it the length of the iterable
# We avoid setting end in the parameter list to force it to be evaluated on
# each invocation
if end is None:
end = len(items)
for i in range(start, end):
items[i] = f(items[i])

for row in table:
inplacemap(int, row, 1)

Personally, I find thislessPythonic. There is preferably only one obvious way to do it, and this isn't it.

Use list comprehensions:

table = [row[0] + [int(col) for col in row[1:]] for row in table]

This will work:

table = [[row[0]] + [int(v) for v in row[1:]] for row in table]

However you might want to think about doing the conversion at the point where the list is first created.

This accomplishes what you are looking for. It is a readable solution. You can go for similar one using listcomp too.

>>> for row in table:
...     for i, elem in enumerate(row):
...             try:
...                     int(elem)
...             except ValueError:
...                     pass
...             else:
...                     row[i] = int(elem)
...