5 Best Ways to Convert a Python List of Lists to List of Tuples

π‘ Problem Formulation: Converting a list of lists to a list of tuples is a common requirement in Python programming when you need to ensure the immutability of the inner lists or to work with hashable collections like sets. Given an input like `[[1, 2], [3, 4], [5, 6]]`, the desired output is `[(1, 2), (3, 4), (5, 6)]`. This article outlines five efficient methods to achieve this transformation.

Method 1: Using a List Comprehension

The list comprehension method is a concise way to convert a list of lists into a list of tuples. It is pythonic and the go-to solution for such a transformation in a single line of code. This method iterates over each inner list and converts it to a tuple, collecting them into a new list.

Here’s an example:

```list_of_lists = [[1, 2], [3, 4], [5, 6]]
list_of_tuples = [tuple(inner_list) for inner_list in list_of_lists]
print(list_of_tuples)```

Output:

`[(1, 2), (3, 4), (5, 6)]`

This code example defines a list of lists and uses a list comprehension where `tuple(inner_list)` converts each inner list `inner_list` within `list_of_lists` to a tuple, creating a new list of tuples `list_of_tuples`.

Method 2: Using the map() Function

The `map()` function applies a given function to each item of an iterable and returns a list of the results in Python 2.x or an iterator in Python 3.x. When combined with `tuple`, it can be used to convert each inner list of a list of lists into a tuple.

Here’s an example:

```list_of_lists = [[1, 'a'], [3, 'b'], [5, 'c']]
list_of_tuples = list(map(tuple, list_of_lists))
print(list_of_tuples)```

Output:

`[(1, 'a'), (3, 'b'), (5, 'c')]`

Here, `map(tuple, list_of_lists)` applies the `tuple` function to each element in `list_of_lists`, converting them to tuples. The `list` constructor is then used to convert the resulting iterator to a list.

Method 3: Using List Unpacking

The list unpacking method utilizes the splat operator (*) to unpack inner lists directly into tuples. It is useful when working with a known and fixed number of elements in inner lists.

Here’s an example:

```list_of_lists = [[1, 'x'], [2, 'y'], [3, 'z']]
list_of_tuples = [(a, b) for [a, b] in list_of_lists]
print(list_of_tuples)```

Output:

`[(1, 'x'), (2, 'y'), (3, 'z')]`

This snippet showcases the use of list unpacking within a list comprehension. Each inner list is unpacked into variables `a` and `b`, which are used to form a tuple explicitly.

Method 4: Using a For Loop

For those who prefer classic programming patterns, the for-loop method iterates over the list of lists and converts each sublist into a tuple explicitly, appending it to the result list.

Here’s an example:

```list_of_lists = [[10, 20], [30, 40], [50, 60]]
list_of_tuples = []
for sublist in list_of_lists:
list_of_tuples.append(tuple(sublist))
print(list_of_tuples)```

Output:

`[(10, 20), (30, 40), (50, 60)]`

This method starts by defining an empty list `list_of_tuples`. Then, a for loop iterates over each sublist in `list_of_lists`, converting them to tuples, and appends these to `list_of_tuples`.

Bonus One-Liner Method 5: Using the zip() Function

If the list of lists contains parallel elements that should be combined into tuples (such as columns of a matrix), the `zip()` function is an elegant solution. It pairs up the i-th elements of all the sublists into tuples.

Here’s an example:

```list_of_lists = [[1, 3, 5], ['a', 'b', 'c']]
list_of_tuples = list(zip(*list_of_lists))
print(list_of_tuples)```

Output:

`[(1, 'a'), (3, 'b'), (5, 'c')]`

By using the splat operator (*) with `zip()`, each sublist in `list_of_lists` is unpacked and their corresponding items are combined into tuples.

Summary/Discussion

• Method 1: List Comprehension. This method is concise and effective for most cases without any add-on conditions. However, it doesn’t cater to situations where the sublists are of unequal length or non-existent.
• Method 2: map() Function. It is very readable and works well with functional programming styles. Yet, it requires wrapping with a list constructor in Python 3.x to see the transformed results as a list.
• Method 3: List Unpacking. Perfect for predictable and sequence-structured data. It loses flexibility when dealing with varying sized inner lists and can raise errors if the data does not match the unpacking pattern.
• Method 4: For Loop. Ideal for beginners or cases requiring more complex transformations within the loop. It is, however, more verbose and can be slower compared to list comprehensions.
• Method 5: zip() Function. Best used when dealing with transposed lists of lists (matrix columns into rows). Its use is limited to cases where sublists are the same length, and understanding its functionality might be tricky for some.