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How to slice sequences in Python

Learn slicing with its variations and examples.

Slicing of sequences in Python is a crucial and easy to learn concept. In this article we will see different types of slicing and understand them with examples.

About Sequences

Sequence data structures are iterable and the elements of a sequence can be accessed via their index except set and frozenset.

Slicing relies on indexing to specify the portion of sequence to extract data from.

As slicing is allowed only for indexable sequences the following data structures are eligible:

  • list
  • tuple
  • bytearray
  • string
  • range
  • byte sequences

The Slice Notation:

my_list[start: end: step]

Alternatively, slice() can be used 

my_list[slice(start, end, step)]

Here, start, end and step are integers. start defines the index to start slicing from and continue till index end - 1(end index is excluded).

There are multiple variations of using slice notation:

  • [:, end]: Select portion from sequence start till end - 1
  • [start: ]: Select portion from start till the end of sequence
  • [:]: Create a copy of sequence

Examples:

1. With start and end

colors = ["red", "green", "blue", "yellow", "cyan"]
print(colors[1: 3])

Output: 

['green', 'blue']

2. With end only

colors = ["red", "green", "blue", "yellow", "cyan"]
print(colors[: 4])

Output: 

['red', 'green', 'blue', 'yellow']

3. With start only

colors = ["red", "green", "blue", "yellow", "cyan"]
print(colors[2:])

Output: 

['blue', 'yellow', 'cyan']

The step in Slicing

step defines the number of index to move forward while slicing an object. If step is not specified, default value is taken as 1 which means move without skipping any index.

Example: 

colors = ["red", "green", "blue", "yellow", "cyan"]
print(colors[: 4: 2])

Output: 

['red', 'blue']

With a string: 

alphabets = "abcdefghijklmnopqrstuvwxyz"
print(alphabets[::2])

Output: 

acegikmoqsuwy

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