Python - Group contiguous strings in List

When working with mixed lists containing both strings and numbers, you might need to group contiguous string elements together while keeping non-string elements separate. Python's itertools.groupby() function combined with a custom key function provides an elegant solution.

Understanding the Problem

Given a list with mixed data types, we want to group consecutive string elements into sublists while leaving non-string elements as individual items ?

Solution Using groupby()

We'll create a helper function to identify strings and use groupby() to group contiguous elements ?

from itertools import groupby

def string_check(elem):
    return isinstance(elem, str)

def group_string(my_list):
    for key, grp in groupby(my_list, key=string_check):
        if key:
            yield list(grp)
        else:
            yield from grp

my_list = [52, 11, 'py', 'th', 'on', 11, 52, 'i', 's', 18, 'f', 'un', 99]

print("The list is :")
print(my_list)

my_result = [*group_string(my_list)]

print("The result is:")
print(my_result)

The list is :
[52, 11, 'py', 'th', 'on', 11, 52, 'i', 's', 18, 'f', 'un', 99]
The result is:
[52, 11, ['py', 'th', 'on'], 11, 52, ['i', 's'], 18, ['f', 'un'], 99]

How It Works

The string_check() function returns True for string elements and False for others. The groupby() function groups consecutive elements that return the same key value ?

• string_check() − Identifies if an element is a string using isinstance()
• groupby() − Groups consecutive elements with the same key (string or non-string)
• yield list(grp) − Returns string groups as lists
• yield from grp − Returns non-string elements individually

Alternative Approach Using Manual Iteration

Here's a more explicit approach without using groupby() ?

def group_strings_manual(data):
    result = []
    current_group = []
    
    for item in data:
        if isinstance(item, str):
            current_group.append(item)
        else:
            if current_group:
                result.append(current_group)
                current_group = []
            result.append(item)
    
    # Add remaining group if exists
    if current_group:
        result.append(current_group)
    
    return result

my_list = [52, 11, 'py', 'th', 'on', 11, 52, 'i', 's', 18, 'f', 'un', 99]
result = group_strings_manual(my_list)
print(result)

[52, 11, ['py', 'th', 'on'], 11, 52, ['i', 's'], 18, ['f', 'un'], 99]

Comparison

Conclusion

Use itertools.groupby() with a key function to efficiently group contiguous string elements in mixed-type lists. The yield approach creates a memory-efficient generator that can handle large datasets.