How to flatten a nested list in python?

Question

How can I convert:

THIS = \
['logging',
 ['logging', 'loggers',
  ['logging', 'loggers', 'MYAPP',
   ['logging', 'loggers', 'MYAPP', '-handlers'],
   ['logging', 'loggers', 'MYAPP', 'propagate']
  ]
 ],
 ['logging', 'version']
]

into:

THAT = [
    ['logging'],
    ['logging', 'version'],
    ['logging', 'loggers'],
    ['logging', 'loggers', 'MYAPP'],
    ['logging', 'loggers', 'MYAPP', '-handlers'],
    ['logging', 'loggers', 'MYAPP', 'propagate']
]

in python (it doesn't need to be sorted, just flattened)?

I've tried lots of things but can't find how to solve this.

Does this answer your question? How do I make a flat list out of a list of lists? — questionto42
– questionto42, Commented Jul 8, 2022 at 18:38

Imran · Accepted Answer · 2014-02-07 15:31:11Z

3

Solved with recursive generator

def flatten(items):
    non_list_items = []

    for item in items:
        if isinstance(item, list):
            for inner_item in flatten(item):
                yield inner_item
        else:
            non_list_items.append(item)

    yield non_list_items

Testing against your input:

from pprint import pprint

>>> pprint(sorted(flatten(THIS)))
[['logging'],
 ['logging', 'loggers'],
 ['logging', 'loggers', 'MYAPP'],
 ['logging', 'loggers', 'MYAPP', '-handlers'],
 ['logging', 'loggers', 'MYAPP', 'propagate'],
 ['logging', 'version']]

answered Feb 7, 2014 at 15:31

Imran

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1 Comment

jbrown Over a year ago

Thanks I've been trying for ages to get this

wheaties · Accepted Answer · 2014-02-07 15:32:09Z

2

This is where a recursive function really shines:

def flatten(myList):
  def inner(current, acc):
    items = []
    for x in myList:
      if isinstance(x, list):
        acc.extend(inner(x, []))
      else:
        items.append(x)
    acc.extend(items)
    return acc

  return inner(myList, [])

which I believe should do the trick.

edited Feb 7, 2014 at 15:32

answered Feb 7, 2014 at 15:26

wheaties

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