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Python groupby method to remove all consecutive duplicates

Last Updated : 10 Jan, 2025

In Python, the groupby method from the itertools module provides a powerful way to group consecutive identical elements in an iterable. This functionality can be effectively utilized to remove all consecutive duplicates from a sequence. By retaining only the first occurrence of each group of identical elements, we can clean up a list or string while preserving its overall structure.

Let’s start with a simple example to see how the groupby method can help us remove consecutive duplicates:

from itertools import groupby

# Example list with consecutive duplicates
s = [1, 1, 2, 3, 3, 4]

# Removing consecutive duplicates using groupby
res = [key for key, _ in groupby(s)]
print(res) 

• groupby function groups consecutive identical elements in the sequence.
• For each group, the key (unique element) is extracted and added to the result list.
• The final output is a list with all consecutive duplicates removed.

itertools.groupby(iterable, key=None)

Parameters

• iterable: The input iterable to be grouped.
• key (optional): A function to specify a criterion for grouping. By default, consecutive identical elements are grouped.

Return Type

• Returns an iterator that generates groups of consecutive identical elements from the input iterable.

Before diving into the implementation, consider a list with repetitive elements:

from itertools import groupby

# List with consecutive duplicates
s = ["a", "a", "b", "b", "c"]

# Remove consecutive duplicates
res = [key for key, _ in groupby(s)]
print(res)  

• groupby method groups consecutive identical elements.
• Key variable represents the unique elements in each group, forming the final cleaned list.

Let’s apply the groupby method to a string:

from itertools import groupby

# String with consecutive duplicates
s = "aabbcc"

# Remove consecutive duplicates
res = "".join(key for key, _ in groupby(s))
print(res)  

• By using "".join(), the unique keys are concatenated into a single string without consecutive duplicates.
• The process works similarly to lists, treating characters as elements.

Strings can be processed in a case-sensitive manner by default. Here’s how it works:

from itertools import groupby

# String with mixed-case duplicates
s = "aaAAaabb"

# Remove consecutive duplicates
res = "".join(key for key, _ in groupby(s))
print(res)  

• Consecutive duplicates are removed without altering the case.
• The method groups identical elements as they appear, maintaining case sensitivity.

Key parameter can be used for advanced grouping scenarios. For instance, treating uppercase and lowercase letters as identical:

from itertools import groupby

# String with mixed-case duplicates
s = "aaAAaabb"

# Remove consecutive duplicates ignoring case
res = "".join(key for key, _ in groupby(s, key=str.lower))
print(res) 

• str.lower function is passed as the key, ensuring that grouping ignores case differences.
• This creates a more flexible approach to grouping elements.

Yes, the groupby method works with any iterable, including tuples and generators:

from itertools import groupby
# Tuple with consecutive duplicates
a = (1, 1, 2, 2, 3)
res = [key for key, _ in groupby(a)]
print(res)  # Output: [1, 2, 3]

The groupby method only groups consecutive identical elements. Non-consecutive duplicates remain in the result:

from itertools import groupby
# List with non-consecutive duplicates
s = [1, 2, 1, 2]
res = [key for key, _ in groupby(s)]
print(res)  # Output: [1, 2, 1, 2]

Absolutely. The method works seamlessly with numeric data:

from itertools import groupby
# Numeric data
n = [1, 1, 2, 2, 3, 3]
res = [key for key, _ in groupby(n)]
print(res)  # Output: [1, 2, 3]

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