Master Python Slicing: From Basics to Advanced Tricks

What is slice()

The slice() function creates a slice object that can be used to specify start, stop, and step values when extracting subsections of sequence types such as lists, tuples, and strings.

Syntax

Two forms are supported:

slice(stop)

slice(start, stop[, step])

Parameters

start – the index where the slice begins (default None).

stop – the index where the slice ends (exclusive).

step – the interval between elements (default None).

Basic Example

Creating a slice that captures the first five elements:

myslice = slice(5)
print(myslice)  # slice(None, 5, None)

Applying it to a range object:

arr = range(10)
print(arr)  # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
# Using the slice
print(arr[myslice])  # [0, 1, 2, 3, 4]

Loop vs. Slice

Instead of looping to print each element, slicing can produce the same result in a single statement:

for i in range(3):
    print(arr[i])
# Output: 0 1 2

# Collecting values into a list
r = []
for i in range(3):
    r.append(arr[i])
print(r)  # [0, 1, 2]

Using Colon Syntax

Python’s slice notation sequence[start:stop] is concise and expressive:

print(arr[0:5])   # [0, 1, 2, 3, 4]
print(arr[:5])    # same as above, start omitted
print(arr[1:5])   # [1, 2, 3, 4]

Negative indices allow reverse slicing:

print(arr[-5:])      # [5, 6, 7, 8, 9]
print(arr[-5:-1])   # [5, 6, 7, 8]

Edge Cases

If the start index is greater than the stop index, the result is an empty list:

print(arr[5:4])  # []

Step Slicing

Adding a step value extracts elements at regular intervals:

# Every two elements from the first ten
print(arr[:10:2])  # [0, 2, 4, 6, 8]

# Every fifth element from a range of 100
arr = range(100)
print(arr[::5])   # [0, 5, 10, 15, ..., 95]

Slicing Other Sequence Types

Tuples, strings, and Unicode strings also support slicing:

t = (1, 2, 3, 4)
print(t[:2])       # (1, 2)

s = 'abcde'
print(s[:2])      # 'ab'

u = u'我来了'
print(u[:2])      # u'\u6211\u6765'