ak.flatten

Defined in awkward.operations.structure on line 1763.

ak.flatten(array, axis=1, highlevel=True, behavior=None)
Parameters
  • array – Data containing nested lists to flatten.

  • axis (None or int) – If None, the operation flattens all levels of nesting, returning a 1-dimensional array. Otherwise, it flattens at a specified depth. The outermost dimension is 0, followed by 1, etc., and negative values count backward from the innermost: -1 is the innermost dimension, -2 is the next level up, etc.

  • highlevel (bool) – If True, return an ak.Array; otherwise, return a low-level ak.layout.Content subclass.

  • behavior (None or dict) – Custom ak.behavior for the output array, if high-level.

Returns an array with one level of nesting removed by erasing the boundaries between consecutive lists. Since this operates on a level of nesting, axis=0 is a special case that only removes values at the top level that are equal to None.

Consider the following doubly nested array.

ak.Array([[
           [1.1, 2.2, 3.3],
           [],
           [4.4, 5.5],
           [6.6]],
          [],
          [
           [7.7],
           [8.8, 9.9]
          ]])

At axis=1, the outer lists (length 4, length 0, length 2) become a single list (of length 6).

>>> print(ak.flatten(array, axis=1))
[[1.1, 2.2, 3.3], [], [4.4, 5.5], [6.6], [7.7], [8.8, 9.9]]

At axis=2, the inner lists (lengths 3, 0, 2, 1, 1, and 2) become three lists (of lengths 6, 0, and 3).

>>> print(ak.flatten(array, axis=2))
[[1.1, 2.2, 3.3, 4.4, 5.5, 6.6], [], [7.7, 8.8, 9.9]]

There’s also an option to completely flatten the array with axis=None. This is useful for passing the data to a function that doesn’t care about nested structure, such as a plotting routine.

>>> print(ak.flatten(array, axis=None))
[1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8, 9.9]

Missing values are eliminated by flattening: there is no distinction between an empty list and a value of None at the level of flattening.

>>> array = ak.Array([[1.1, 2.2, 3.3], None, [4.4], [], [5.5]])
>>> ak.flatten(array, axis=1)
<Array [1.1, 2.2, 3.3, 4.4, 5.5] type='5 * float64'>

As a consequence, flattening at axis=0 does only one thing: it removes None values from the top level.

>>> ak.flatten(array, axis=0)
<Array [[1.1, 2.2, 3.3], [4.4], [], [5.5]] type='4 * var * float64'>

As a technical detail, the flattening operation can be trivial in a common case, ak.layout.ListOffsetArray in which the first offset is 0. In that case, the flattened data is simply the array node’s content.

>>> array.layout
<ListOffsetArray64>
    <offsets><Index64 i="[0 4 4 6]" offset="0" length="4"/></offsets>
    <content><ListOffsetArray64>
        <offsets><Index64 i="[0 3 3 5 6 7 9]" offset="0" length="7"/></offsets>
        <content>
            <NumpyArray format="d" shape="9" data="1.1 2.2 3.3 4.4 5.5 6.6 7.7 8.8 9.9"/>
        </content>
    </ListOffsetArray64></content>
</ListOffsetArray64>
>>> np.asarray(array.layout.content.content)
array([1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8, 9.9])

However, it is important to keep in mind that this is a special case: ak.flatten and content are not interchangeable!