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4.2. Slicing Strings

In this section, we'll discuss how strings can be sliced to produce substrings.

Understanding syntax

String slicing is used to select some characters from a string to form a new smaller string (substring).

The syntax for string slicing is: 

string[start:end:step]

  • start is the index of character to start slicing from
  • end is the index of character to slice until (exclusive)
  • step is the increment in start for each subsequent character

Note that each of three arguments may be omitted and would be given a default value automatically.

Zero based index

Keep in mind, as mentioned in previous section, indexes always start from 0.

Basic Usage

string[start:end] returns a substring of string starting from character at start index until character at end - 1 index.

string = 'Hello World'
print(string[6:11])

World

Note that in string[start:end], step defaults to 1.

Exclusive upper index

The upper index, end, in string[start:end] is exclusive. This means string[6:11] starts from 6th index up until 10th index and doesn't include 11th index.

Substring from first character

When index before first colon is omitted, it defaults to 0.

string[:end] returns a substring of string starting from first character until character on the end - 1 index.

This is equivalent to string[0:end] (start given the default value)

string = 'Hello World'
print(string[:6])

Hello

Substring till last character

When index after first colon is omitted, it defaults to 0.

string[start:] returns a substring of string starting from start index until last character.

This is equivalent to string[start:length_of_string] (end given the default value of length of string).

string = 'Hello World'
print(string[6:])

World

The step parameter

step defines the increment in start for each subsequent character in the string.

string[start:end:step] starts from start index and for each next index, adds step to the start until end is reached.

For example, if our string is Hello World and we slice the string using string[0:11:2],

  • Start from 0 (start) index so first character in substring is H
  • Add 2 (step) to 0 (start) and second character is at index 2, l
  • Add 2 (step) to 2 (previous index) and third character is at index 4, o
  • Add 2 (step) to 4 (previous index) and third character is at index 6, W
  • Repeat until 11 (end) is reached so final character would be at index 10, d
string = 'Hello World'
print(string[0:11:2])

HloWrd

string[0::2] also produces the same result where end is given the default value of 11 (length of string).

Note

step cannot be zero but could be negative (to move in reverse order).

Negative indexes

Negative indexes can be used while slicing to extract characters from end of string.

string = 'Hello World'
print(string[-10:-6])

ello