What's the Most Efficient Way to Flatten a Shallow List in Python?

Flattening Shallow Lists in Python

Flattening a shallow list of iterables is a common operation in Python. While there are several approaches, each offers varying performance and readability.

1. List Comprehension

A nested list comprehension can appear to flatten a list, but it results in a NameError since the variable used in the outer comprehension (e.g., menuitem) is not defined.

2. reduce

The reduce function allows for flattening using list.__add__ as the reduction operation. However, this method requires an additional list(x) call to convert Django QuerySet objects into lists.

3. itertools.chain

The itertools.chain function provides a more efficient option for list flattening. It uses a generator to yield elements from the iterables, avoiding the need for copying or list comprehension.

Code Example:

import itertools

list_of_menuitems = [['image00', 'image01'], ['image10'], []]

# Flatten using list comprehension (not recommended)
# [image for image in menuitem for menuitem in list_of_menuitems]

# Flatten using reduce
# reduce(list.__add__, (list(mi) for mi in list_of_menuitems))

# Flatten using itertools.chain
flattened_list = list(itertools.chain(*list_of_menuitems))

Chain is the preferred method for flattening shallow lists due to its performance and ease of use. It can handle any iterable, including Django QuerySets.

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