Question

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What is the difference between slice assignment that slices the whole list and direct assignment?

I don’t have money for school so I am teaching myself some Python whilst working shifts at a tollbooth on the highway (long nights with few customers). (BTW: Coursera should be translated to all languages…)

I have read here that if I have a list l:

l = ['a', '', 'b']

and I want to filter out empty strings like so:

l = [c for c in l if c]

or like so:

l = filter(lambda x: x, l)

it is advisable to do this instead:

l[:] = ... # either method 1 or method 2 above

not to “lose” the reference to the first l, especially in case other variables were pointing to it.

My question:

• Why is it that l[:] denotes “the contents of l” in this case, allowing specifically reassignment to the “same” l, when elsewhere I think of it as a “same size slice”, conveniently creating a copy of l for me?

• Did I misunderstand how to use the l[:] for same-list-reassignments completely?

I thought that if I had an l and I asked for a l[:], the latter was an actual copy of the original l?

Reference: “Learning Python” -> There are a variety of
ways to copy a list, including using the built-in list function and the standard library
copy module. Perhaps the most common way is to slice from start to finish

Thanks!

Answer 1

There’s a difference in Python between getting a slice of a list and setting a slice of a list. Indeed, they are actually separate operations (__getitem__ and __setitem__, respectively). So, what’s true for the get case may not be true for set.

In the former case, l[:] means to get a copy of the list (it generates a brand new list which has all the same contents as the old list). In the latter case, l[:] = ... means to set the contents of the list to something else.