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Python Python Collections (2016, retired 2019) Slices Slice Functions

no idea whats wrong here

Not really sure how I can combine both lists into a single variable

def first_4(x):
    new_x = x.copy()
    return new_x[:4]

def first_and_last_4(x):
    new_x = x[:4]
    other_x = x.copy()
    other_x = [-4:]
    return new_x

1 Answer

Eric M
Eric M
11,545 Points

Hi Theodore,

Your logic makes sense, but extend doesn't seem to work here, perhaps we're not always getting lists as our iterable so we can't use list methods?

There are two things we can do though, one's really easy. Just add the slices together using the addition operator +.

rtr_x = new_x + other_x should be a fine return value.

You also don't need to do use x.copy() as slice already creates a new object, it doesn't mutate what you're slicing. This is true when you're assigning a slice to a variable at least (when the slice is on the right hand side of the assignment operator =).

When you're assigning values to a slice it works a little differently. Slice can give you an insertion point (or replacement range) in an existing iterable when it's on the left hand side of the assignment operator. We get an insertion point when we give it a range with nothing in it.

For instance, this code to insert the first slice into the start of the second using only slices and assignment is able to pass the challenge:

def first_and_last_4(itr):
    a = itr[:4]
    b = itr[-4:]
    b[:0] = a
    return b