How to explain the first colon in Python slice syntax list[::-1] [duplicate]

Question

I recently read a code snippets about how to reverse a sequence

>> l = [1,2,3,4,5,6]
>> print l[::-1]

Output

>> [6,5,4,3,2,1]

How to explain the first colon in bracket?

Answer 1

The colons with no values given means resort to default values. The default values for the start index when the step is negative is len(l), and the end index is -len(l)-1. So, the reverse slicing can be written as

l[len(l):-len(l)-1:-1]

which is of the form.

l[start:end:step]

Removing the default values, we can use it in a shorter notation as l[::-1].

It might be useful to go through this question on Python's Slice Notation.

Answer 2

some_list[start:end:step]

When you ommit any of the slicing operands they take on default values. Default values for start, end and step: start - the beginning of an indexated iterable which is always of an index 0 when step is positive, end - the ending index of an indexated iterable which is always its length (following the same convention as range) when step is positive, step - the default step is always one.

When you're using a minus sign on step omitting other operands you're basically saying "return a reversed list".

EDIT: Funny, but

[1,2,3,4,5,6][5:-7:-1]

returns the same result as

[1,2,3,4,5,6][::-1]

in Python3. Can anyone comment on to why? That means that default values of start and end actually rest upon the step operand (more specifically its sign).