# [] (slicing)¶

## Syntax¶

sequence [start:stop[:step]]

start
Optional. Starting index of the slice. Defaults to 0.
stop
Optional. The last index of the slice or the number of items to get. Defaults to len(sequence).
step
Optional. Extended slice syntax. Step value of the slice. Defaults to 1.

## Return Value¶

The same as selected.

## Time Complexity¶

O(k) for slice retrieval

O(n) for deletion

O(n+k) for slice assignment

## Remarks¶

Consider the following ASCII graph showing the contents of the “ABCD” string:

```>>> +---+---+---+---+
>>> |-4 |-3 |-2 |-1 |  <= negative indexes
>>> +---+---+---+---+
>>> | A | B | C | D |  <= sequence elements
>>> +---+---+---+---+
>>> | 0 | 1 | 2 | 3 |  <= positive indexes
>>> +---+---+---+---+
>>> |<- 0:3:1 ->|      <= extent of the slice: "ABCD"[0:3:1]
```

Consider the following example:

## Example 1¶

```>>> "ABCD"[0:2]
'AB'
```

It can be read as: get every single one item between indexes 0 and 2 (exclusive).

The next example shows usage of the step argument:

## Example 2¶

```>>> "ABCD"[0:4:2]
'AC'
```

That can be interpreted as: get every second element between indexes 0 and 4.

Usage of start, stop and step operators is optional:

## Example 3¶

```>>> "ABCD"[1:]
'BCD'
>>> "ABCD"[:3]
'ABC'
>>> "ABCD"[1:3]
'BC'
>>> "ABCD"[1:3:]
'BC'
>>> "ABCD"[::2]
'AC'
>>> "ABCD"[::]
'ABCD'
>>> "ABCD"[:]
'ABCD'
```

Negative step argument can be used to reverse the sequence:

## Example 4¶

```>>> "ABCD"[::-1]
'DCBA'
>>> [0, 1, 2, 3][::-1]
[3, 2, 1, 0]
```

## Example 5¶

```>>> # slices can be used to replace multiple items
>>> l = [0, 1, 2, 3]
>>> l[:2] = ("AB", "CD")
>>> l
['AB', 'CD', 2, 3]
```

## Example 6¶

```>>> l = [0, 1, 2, 3]
>>> l[1:2] = (7, 8, 9, 10)
>>> l
[0, 7, 8, 9, 10, 2, 3]
```

## Example 7¶

```>>> # when using extended slice syntax both chunks must match
>>> l = [0, 1, 2, 3]
>>> l[::2] = "ABCD"
Traceback (most recent call last):
File "<interactive input>", line 1, in <module>
ValueError: attempt to assign sequence of size 4 to extended slice of size 2
```

## Example 8¶

```>>> # deleting items
>>> l = [0, 1, 2, 3]
>>> del l[::2]
>>> l
[1, 3]
```