How can I use list comprehensions to process a nested list?

I have this nested list:

l = [['40', '20', '10', '30'], ['20', '20', '20', '20', '20', '30', '20'], ['30', '20', '30', '50', '10', '30', '20', '20', '20'], ['100', '100'], ['100', '100', '100', '100', '100'], ['100', '100', '100', '100']]

I want to convert each element in l to float. I have this code:

newList = []
for x in l:
    for y in x:
        newList.append(float(y))

How can I solve the problem with a nested list comprehension instead?

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_{See also: How can I get a flat result from a list comprehension instead of a nested list?}

Answer

Here is how you would do this with a nested list comprehension:

[[float(y) for y in x] for x in l]

This would give you a list of lists, similar to what you started with except with floats instead of strings.

If you want one flat list, then you would use

[float(y) for x in l for y in x]

Note the loop order – for x in l comes first in this one.