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Python heapq.merge() Method - GeeksforGeeks

Python heapq.merge() Method

Last Updated : 23 Jul, 2025

The heapq.merge() method in Python is part of the heapq module, which is used for heap-related operations. This method allows you to merge multiple sorted input iterables into a single sorted output iterable, efficiently using the heap data structure. It is particularly useful when working with sorted data and helps in merging sorted sequences without needing to sort the entire data again.

Example: Merging Two Sorted Lists

Python


 import heapq

# Two sorted lists
a = [1, 3, 5, 7]
b = [2, 4, 6, 8]

# Merge the lists
merged = heapq.merge(a, b)

print(list(merged))

Output

[1, 2, 3, 4, 5, 6, 7, 8]

Explanation:

The heapq.merge() method merges the two sorted lists list1 and list2 into one sorted list, yielding the output [1, 2, 3, 4, 5, 6, 7, 8].

Syntax of merge() method

heapq.merge(*iterables, key=None, reverse=False)

Parameters

*iterables: Variable number of sorted iterable objects (e.g., lists, tuples, or other sequences).
key: An optional function to determine the sort order (similar to sorted()).
reverse: A boolean flag; if True, the merge happens in descending order (default is False for ascending order).

Return Value

The method returns an iterator that yields the elements from the sorted sequences in ascending order by default (or descending if reverse=True), combining the elements of all input iterables.

Examples of merge() method 1. Merging Multiple Sorted Iterables Python


 import heapq

# Three sorted lists
a = [1, 3, 5, 7]
b = [2, 4, 6, 8]
c = [0, 9, 10]

# Merge the lists
merged = heapq.merge(a, b, c)

print(list(merged))

Output

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Explanation:

The method merges three sorted lists into one sorted list, resulting in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

2. Merging with a Custom Key Function

You can use the key parameter to customize the sorting of the elements. For example, sorting strings by their length.

Python


 import heapq

# Two lists of strings
a = ["apple", "banana", "kiwi"]
b = ["cherry", "grape", "mango"]

# Merge the lists by string length
merged = heapq.merge(a, b, key=len)

print(list(merged))

Output

['apple', 'banana', 'kiwi', 'cherry', 'grape', 'mango']

Explanation:

The key=len argument sorts the strings based on their length. The merged result is sorted by the length of each string.

3. Merging in Reverse Order

If you want to merge the iterables in descending order, you can use the reverse parameter.

Python


 import heapq

# Two sorted lists
a = [1, 3, 5, 7]
b = [2, 4, 6, 8]

# Merge the lists in reverse (descending) order
merged = heapq.merge(a, b, reverse=True)

print(list(merged))

Output

[2, 4, 6, 8, 1, 3, 5, 7]

Explanation:

The reverse=True argument causes the merge to happen in descending order, resulting in [8, 7, 6, 5, 4, 3, 2, 1].

When to Use heapq.merge()?

You should use heapq.merge() when:

Merging Sorted Data: You have multiple already sorted sequences and want to merge them into a single sorted sequence efficiently.
Efficient Merging: When dealing with large datasets, heapq.merge() is much more efficient than sorting the entire dataset.
Handling Infinite Sequences: Since heapq.merge() returns an iterator, it is useful when dealing with infinite sequences, as it doesn’t require all the data to be in memory at once.

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