itertools.groupby() in Python

The itertools.groupby() function in Python is part of the itertools module and is used for grouping consecutive elements of an iterable that have the same value. It generates a group of consecutive elements from an iterable and returns them as tuples containing a key and a group.

Example:

import itertools 

L = [("a", 1), ("a", 2), ("b", 3), ("b", 4)] 

# Key function 
key_func = lambda x: x[0] 

for key, group in itertools.groupby(L, key_func): 
	print(key + " :", list(group)) 

a : [('a', 1), ('a', 2)]
b : [('b', 3), ('b', 4)]

Explanation: In this code, itertools.groupby() groups the list of tuples based on the first element of each tuple. The key function (key_func) extracts the first element of each tuple (x[0]). The groupby() function then categorizes the tuples into groups with the same first element, and each group is displayed in the output.

Syntax

itertools.groupby(iterable, key=None)

Parameters

• iterable: This is the iterable (like a list, tuple, string, etc.) whose elements are to be grouped.
• key (optional): A function to compute a key value for each element. This is used for grouping, and by default, it is None. When None, the elements are grouped based on their actual value.

Return Value

The groupby() method returns an iterator of consecutive (key, group) pairs. Each group is itself an iterator, containing consecutive elements from the iterable that share the same key.

Examples of itertools.groupby()

1. Grouping Consecutive Even and Odd Numbers

This example demonstrates how to use itertools.groupby() to group consecutive numbers into even and odd categories.

import itertools

# List of numbers
a = [2, 4, 6, 7, 8, 10, 11, 12, 13]

# Grouping consecutive even and odd numbers
g_data = itertools.groupby(a, key=lambda x: 'Even' if x % 2 == 0 else 'Odd')

# Display grouped elements
for key, group in g_data:
    print(f"Group: {key} -> {list(group)}")

Group: Even -> [2, 4, 6]
Group: Odd -> [7]
Group: Even -> [8, 10]
Group: Odd -> [11]
Group: Even -> [12]
Group: Odd -> [13]

Explanation: This example groups numbers into consecutive even and odd numbers. The groupby() function checks the parity of each number and forms groups accordingly. The output shows the even and odd groups.

2. Grouping Strings Based on Length

In this example, itertools.groupby() is used to group strings based on their length, categorizing words into groups of equal length.

import itertools

# List of strings
l = ['apple', 'banana', 'cherry', 'date', 'fig', 'grape']

# Grouping strings by their length
g_data = itertools.groupby(l, key=lambda x: len(x))

# Display grouped elements
for key, group in g_data:
    print(f"Length: {key} -> {list(group)}")

Length: 5 -> ['apple']
Length: 6 -> ['banana', 'cherry']
Length: 4 -> ['date']
Length: 3 -> ['fig']
Length: 5 -> ['grape']

Explanation: Here, words are grouped by their length using the groupby() function. It categorizes the words into groups of the same length, and the output displays these groups.