# Python Flatten a List of Lists

> Flatten regular lists with itertools.chain.from_iterable() or list comprehension, or use nested loops for irregular lists

**URL:** https://sentry.io/answers/making-a-list-out-of-a-list-of-lists-in-python/

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## The Problem

You want to flatten both regular and irregular lists of lists. Nested for loops could be used to solve this problem.

But are there any simpler solutions in Python?

## The Solution

### Using Python Libraries

The Python `itertools` module is a simple and readable method for flattening regular lists of lists:

```python
import itertools

regular_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat_list = list(itertools.chain.from_iterable(regular_list))
print(flat_list)
```

**Output:**

```
[1, 2, 3, 4, 5, 6, 7, 8, 9]
```

### Using List Comprehension

Use list comprehension to flatten a regular list:

```python
regular_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat_list = [item for inner_list in regular_list for item in inner_list]
print(flat_list)
```

**Output:**

```
[1, 2, 3, 4, 5, 6, 7, 8, 9]
```

### Using Nested Loops

Using nested loops might not be a shortcut, but this method is useful for flattening both regular and irregular lists of lists:

```python
def flatten_list(list_of_lists):
    flattened_list = []

    for element in list_of_lists:
        if type(element) is list:
            for item in element:
                flattened_list.append(item)
        else:
            flattened_list.append(element)
    return flattened_list

# Regular list of lists
regular_list_of_lists = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

# Irregular list of lists
irregular_list_of_lists = [1, [2, 3, 4], 5, [6, 7], 8, 9]

print(flatten_list(regular_list_of_lists))
print(flatten_list(irregular_list_of_lists))
```

**Output:**

```
[1, 2, 3, 4, 5, 6, 7, 8, 9]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
```

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*Source: [sentry.io/answers/making-a-list-out-of-a-list-of-lists-in-python/](https://sentry.io/answers/making-a-list-out-of-a-list-of-lists-in-python/)*