249 – Recursive structural pattern matching

249 – Recursive structural pattern matching#

You can traverse nested data structures by using structural pattern matching recursively, like in the example below:

def flatten(obj):
    match obj:
        case list(values):
            return [*flatten(v) for v in values]
        case _:
            return [obj]

When called with a list that is deeply nested and lacking uniform structure, the nesting is successively unravelled by the recursive calls and pattern matching is used to detect the structural base case(s):

flat = flatten(
  [
    [
      [1, 2],
      [3, 4],
      [
        [5],
        [6]
      ]
    ],
    [7, 8]
  ]
)
print(flat)  # [1, 2, 3, 4, 5, 6, 7, 8]

Note that the current implementation of flatten uses unpacking inside comprehensions, which is a feature that is new in Python 3.15.