Slicing

start and stop

A slice takes a range of characters. The start is included, the stop is not:

word = "programming"
print(word[0:3])
print(word[3:7])

pro
gram

That "stop is not included" rule looks fussy and pays for itself: the length of word[a:b] is always b - a, and word[:n] plus word[n:] is always the whole thing.

word = "programming"
print(len(word[2:7]))
print(word[:4] + word[4:] == word)

5
True

Leaving an end off

Omit the start to begin at the beginning, and the stop to run to the end:

word = "programming"
print(word[:3])
print(word[3:])
print(word[:])

pro
gramming
programming

Negative positions

Negative numbers count from the right, in a slice just as in an index:

filename = "report.pdf"
print(filename[-3:])
print(filename[:-4])

pdf
report

The step

A third number says how big a stride to take:

letters = "abcdefgh"
print(letters[::2])
print(letters[1::2])

aceg
bdfh

A negative step walks backwards, which is the shortest way to reverse a string:

print("stressed"[::-1])

desserts

Slices never fail

Indexing past the end raises IndexError. Slicing past the end just gives you what is there, which makes slices safe on input you have not measured:

word = "abc"
print(word[0:99])
print(word[10:20])
print(repr(word[10:20]))

abc

''

The second line looks blank because it is an empty string.

Slicing works on any sequence

The same syntax applies to lists and tuples, which is why it is worth learning properly once:

numbers = [0, 1, 2, 3, 4, 5]
print(numbers[1:4])
print(numbers[::-1])

[1, 2, 3]
[5, 4, 3, 2, 1, 0]