How do I flatten a nested list in Python?
Asked by
CreativeDuck42
I'm working on a program to flatten a nested list structure in Python. For example, I have a list like [[1,2,3],[4,5],6,7,[8,9],10] that contains both sublists and integer elements. I'd love to know the best way to achieve this using list comprehension or any other method you recommend!
Answers
I played around with this out of curiosity, but I don't recommend this method:
```python
l = [1, 2, [3, 4], 5, 6, [[7], 8], 9]
print(eval('[' + str(l).replace('[', '').replace(']', '') + ']'))
```
It works, but it’s super unsafe. Just stick to the safer methods!
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Another way you could do it is by using recursion. Check this out:
```python
def flatten_recursion(lst):
if not lst:
return []
elif not isinstance(lst, list):
return [lst]
else:
return flatten_recursion(lst[0]) + flatten_recursion(lst[1:])
example_list = [[1,2,3],[4,5],6,7,[8,[9]],10]
print(flatten_recursion(example_list)) # Outputs: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
```
This handles deep nesting really well, just remember to use `isinstance(lst, list)` instead of checking the type directly!
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You can flatten a nested list using list comprehension like this: `[subitem for item in nested for subitem in (item if isinstance(item, list) else [item])]`. Just a heads up, I personally find list comprehensions messy when dealing with more than one nested list, but it's definitely one way to do it!
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You can definitely do it with a simple loop as well. Just use this approach:
```python
flat = [x for i in a for x in (i if isinstance(i, list) else [i])]
```
It’s clean and efficient!
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Answers are reviewed before publication.
CuriousCat90: Totally agree! Using isinstance is much safer because it works with subclassing too.