Group Sublists by another List using Python

In Python, we can group sublists by another list using various methods like using a dictionary and using the itertools.groupby() function, using a nested list comprehension. Grouping sublists by another list can be useful when analyzing large datasets and categorization of data. It is also used in text analysis and Natural language processing. In this article, we will explore different methods to group sublists by another list in Python and understand their implementations.

Method 1:Using a Dictionary

A dictionary can be used in a very straightforward manner to group sublists by another list in Python. Let’s understand the usage of a dictionary for grouping sublists by another list with the help of an example.

Syntax

list_name.append(element)

Here, the element is the element that is to be added to the end of the list. Append methods place this element at the end of the list.

Example

In the below example, we define a function group_sublists that takes two parameters: sublists (the list of sublists) and grouping_list (the list that determines the grouping order). Inside the function, we create empty dictionary groups to store the sublists grouped by their keys. We iterate over each sublist in the sublists list. Assuming the first element of each sublist is the key, we extract it and check if it exists in the groups dictionary. If it does, we append the current sublist to the existing list of sublists for that key. Otherwise, we create a new key-value pair in the groups dictionary with the key and the current sublist as the value. Finally, we return a list comprehension that retrieves the grouped sublists in the order specified by the grouping_list.

def group_sublists(sublists, grouping_list):
    groups = {}

    for sublist in sublists:
        key = sublist[0]  # Assuming the first element of each sublist is the key
        if key in groups:
            groups[key].append(sublist)
        else:
            groups[key] = [sublist]

    return [groups[key] for key in grouping_list]

# Example usage
sublists = [[1, 'apple'], [2, 'banana'], [1, 'orange'], [2, 'grape']]
grouping_list = [1, 2]

result = group_sublists(sublists, grouping_list)
print(result)

Output

[[[1, 'apple'], [1, 'orange']], [[2, 'banana'], [2, 'grape']]]

Method 2:Using the itertools.groupby() Function

Python's itertools module provides a convenient function called groupby() that can be used to group elements based on a key function. Let’ understand this with the help of an example.

Syntax

list_name.append(element)

Here, the append() function is a list method used to add an element to the end of the list_name. It modifies the original list by adding the specified element as a new item.

itertools.groupby(iterable, key=None)

Here, iterable is the input iterable, which can be any sequence that you want to group and key=None is an optional parameter that can be a function that can be used as the key for grouping. If no key function is provided, the elements themselves are used as the keys for grouping.

Example

In the below example, we start by sorting the sublists based on the key (assuming it is the first element). Then, we create an empty list called result to store the grouped sublists. Next, we iterate over the groups generated by itertools.groupby(). The groupby() function takes two parameters: the iterable (in this case, sublists) and the key function (a lambda function that extracts the key from each sublist). It returns pairs of the key and an iterator containing the grouped sublists. Inside the loop, we check if the key exists in the grouping_list. If it does, we convert the iterator to a list using list(group) and append it to the result list. Finally, we return the result list containing the grouped sublists.

import itertools

def group_sublists(sublists, grouping_list):
    sublists.sort(key=lambda x: x[0])  # Sort the sublists based on the key

    result = []
    for key, group in itertools.groupby(sublists, lambda x: x[0]):
        if key in grouping_list:
            result.append(list(group))

    return result

# Example usage
sublists = [[1, 'apple'], [2, 'banana'], [1, 'orange'], [2, 'grape']]
grouping_list = [1, 2]

result = group_sublists(sublists, grouping_list)
print(result)

Output

[[[1, 'apple'], [1, 'orange']], [[2, 'banana'], [2, 'grape']]]

Method 3:Using a Nested List Comprehension

We can write nested list comprehension using Python, which can be used to group sublists by another list. Let’s see an example to see how we can implement this.

Syntax

[expression for item in list if condition]

Here, the syntax consists of square brackets enclosing an expression followed by a for loop that iterates over a list. Any if condition can also be added at the end of the expression to filter out certain elements.

Example

In the below example, we define the function group_sublists, which takes the sublists and grouping_list as parameters. We use a nested list comprehension to iterate over each key in the grouping_list. For each key, we iterate over the sublists and filter out only the sublists that have a matching key (assuming it is the first element). These filtered sublists are then collected into a new list, representing the grouped sublists for that key. The result is a list of lists, where each sublist contains the grouped sublists for a specific key.

def group_sublists(sublists, grouping_list):
    return [
        [sublist for sublist in sublists if sublist[0] == key]
        for key in grouping_list
    ]

# Example usage
sublists = [[1, 'apple'], [2, 'banana'], [1, 'orange'], [2, 'grape']]
grouping_list = [1, 2]

result = group_sublists(sublists, grouping_list)
print(result)

Output

[[[1, 'apple'], [1, 'orange']], [[2, 'banana'], [2, 'grape']]]

Conclusion

In this article, we discussed how we can group sublists by another list in Python. We discussed three approaches: using a dictionary and utilizing the itertools.groupby() function, and employing a nested list comprehension. Each method has its advantages and may be more suitable depending on the specific requirements of your program.