Defined in awkward.operations.ak_transform on line 25.

ak.transform(transformation, array, *more_arrays, depth_context=None, lateral_context=None, allow_records=True, broadcast_parameters_rule='intersect', left_broadcast=True, right_broadcast=True, numpy_to_regular=False, regular_to_jagged=False, return_value='simplified', expect_return_value=False, highlevel=True, behavior=None, attrs=None)#
  • transformation (callable) – Function to apply to each node of the array. See below for details.

  • array – Array-like data (anything ak.to_layout recognizes), but not an ak.Record or ak.record.Record.

  • more_arrays – Additional arrays to be broadcasted together (with first array) and used together in the transformation. See below for details.

  • depth_context (None or dict) – User data to propagate through the transformation. New data added to depth_context is available to the entire subtree at which it is added, but no other subtrees. For example, data added during the transformation will not be in the original depth_context after the transformation.

  • lateral_context (None or dict) – User data to propagate through the transformation. New data added to lateral_context is available at any later step of the depth-first walk over the tree, including other subtrees. For example, data added during the transformation will be in the original lateral_context after the transformation.

  • allow_records (bool) – If False and the recursive walk encounters any ak.contents.RecordArray nodes, an error is raised.

  • broadcast_parameters_rule (str) – Rule for broadcasting parameters, one of: - "intersect" - "all_or_nothing" - "one_to_one" - "none"

  • left_broadcast (bool) – If more_arrays are provided, the parameter determines whether the arrays are left-broadcasted, which is Awkward-like broadcasting.

  • right_broadcast (bool) – If more_arrays are provided, the parameter determines whether the arrays are right-broadcasted, which is NumPy-like broadcasting.

  • numpy_to_regular (bool) – If True, multidimensional ak.contents.NumpyArray nodes are converted into ak.contents.RegularArray nodes before calling transformation.

  • regular_to_jagged (bool) – If True, regular-type lists are converted into variable-length lists before calling transformation.

  • return_value ("none", "original", ``"simplified") – this function is None; if "original", untouched nodes surrounding the ones replaced by the transformation are returned in their original state; if "simplified", the ak.Content.simplified constructor is used on the surrounding nodes to ensure that option-type and union-type nodes are not nested inappropriately. Note that if return_value is "none", the only way to get information out of this function is through the lateral_context.

  • expect_return_value (bool) – If True, raise a RuntimeError if the transformer does not terminate the recursion.

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

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

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

Applies a transformation function to every node of an Awkward array or arrays to either obtain a transformed copy or extract data from a walk over the arrays’ low-level layout nodes.

This is a public interface to the infrastructure that is used to implement most Awkward Array operations. As such, it’s very powerful, but low-level.

Here is a “hello world” example:

>>> def say_hello(layout, depth, **kwargs):
...     print("Hello", type(layout).__name__, "at", depth)
>>> array = ak.Array([[1.1, 2.2, "three"], [], None, [4.4, 5.5]])
>>> ak.transform(say_hello, array, return_value="none")
Hello IndexedOptionArray at 1
Hello ListOffsetArray at 1
Hello UnionArray at 2
Hello NumpyArray at 2
Hello ListOffsetArray at 2
Hello NumpyArray at 3

In the above, say_hello is called on every node of the array, which has a lot of nodes because it has nested lists, missing data, and a union of different types. The data types are low-level “layouts,” subclasses of ak.contents.Content, rather than high-level ak.Array.

The primary purpose of this function is to allow you to edit one level of structure without having to worry about what it’s embedded in. Suppose, for instance, you want to apply NumPy’s np.round function to numerical data, regardless of what lists or other structures they’re embedded in.

The return value must be a subclass of ak.contents.Content (to replace the array node) or None (to leave the array node unchanged).

>>> def rounder(layout, **kwargs):
...     if layout.is_numpy:
...         return ak.contents.NumpyArray(
...             np.round(layout.data).astype(np.int32)
...         )
>>> array = ak.Array(
... [[[[[1.1, 2.2, 3.3], []], None], []],
...  [[[[4.4, 5.5]]]]]
... )
>>> ak.transform(rounder, array).show(type=True)
type: 2 * var * var * option[var * var * int32]
[[[[[1, 2, 3], []], None], []],
 [[[[4, 6]]]]]

If you pass multiple arrays to this function (more_arrays), those arrays will be broadcasted and all inputs, at the same level of depth and structure, will be passed to the transformation function as a group.

Here is an example with broadcasting:

>>> def combine(layouts, **kwargs):
...     assert len(layouts) == 2
...     if layouts[0].is_numpy and layouts[1].is_numpy:
...         return ak.contents.NumpyArray(
...             layouts[0].data + 10 * layouts[1].data
...         )
>>> array1 = ak.Array([[1, 2, 3], [], None, [4, 5]])
>>> array2 = ak.Array([1, 2, 3, 4])
>>> ak.transform(combine, array1, array2)
<Array [[11, 12, 13], [], None, [44, 45]] type='4 * option[var * int64]'>

The 1 and 4 from array2 are broadcasted to the [1, 2, 3] and the [4, 5] of array1, and the other elements disappear because they are broadcasted with an empty list and a missing value. Note that the first argument of this transformation function is a list of layouts, not a single layout. There are always 2 layouts because 2 arrays were passed to ak.transform.

Signature of the transformation function#

If there is only one array, the first argument of transformation is a ak.contents.Content instance. If there are multiple arrays (more_arrays), the first argument is a list of ak.contents.Content instances.

All other arguments can be absorbed into a **kwargs because they will always be passed to your function by keyword. They are

  • depth (int): The current list depth, where 1 is the outermost array and

    higher numbers are deeper levels of list nesting. This does not count nesting of other data structures, such as option-types and records.

  • depth_context (None or dict): Any user-specified data. You can add to

    this dict during transformation; changes would only be seen in the subtree’s nodes.

  • lateral_context (None or dict): Any user-specified data. You can add to

    this dict during transformation; changes would be seen in any node visited later in the depth-first search.

  • continuation (callable): Zero-argument function that continues the

    recursion from this point in the walk, so that you can perform post-processing instead of pre-processing.

For completeness, the following arguments are also passed to transformation, but you usually won’t need them:

  • behavior (None or dict): Behavior that would be attached to the output

    array(s) if highlevel.

  • backend (array library / kernel library shim): Handle to the NumPy

    library, CuPy, etc., depending on the type of arrays.

  • options (dict): Options provided to ak.transform.

If there is only one array, the transformation function must either return None or return an ak.contents.Content.

If there are multiple arrays (more_arrays), then the transformation function may return one array or a tuple of arrays. (The preferred type is a tuple, even if it has length 1.)

The final return value of ak.transform is a new array or tuple of arrays constructed by replacing nodes when transformation returns a ak.contents.Content or tuple of ak.contents.Content, and leaving nodes unchanged when transformation returns None. If transformation returns length-1 tuples, the final output is an array, not a length-1 tuple.

If return_value is "none", ak.transform returns None. This is useful for functions that return non-array data through lateral_context. The other two choices, "original" and "simplified", determine how untouched array nodes, the ones that are _not_ modified by the transformation function, are returned. With "original", they are returned without modification, which might result in illegal combinations of option-type and union-type, which would raise an error. With "simplified", the surrounding array nodes are simplified upon reconstruction. For example, if the transformation puts a new ak.contents.ByteMaskedArray inside an existing ak.contents.ByteMaskedArray, the two will be consolidated into a single option-type array node.


The depth_context and lateral_context allow you to pass your own data into the transformation as well as communicate between calls of transformation on different nodes. The depth_context limits this communication to descendants of the subtree in which the data were added; lateral_context does not have this limit. (depth_context is shallow-copied at each node during descent; lateral_context is never copied.)

For example, consider this array:

>>> array = ak.Array([
...     [{"x": [1], "y": 1.1}, {"x": [1, 2], "y": 2.2}, {"x": [1, 2, 3], "y": 3.3}],
...     [],
...     [{"x": [1, 2, 3, 4], "y": 4.4}, {"x": [1, 2, 3, 4, 5], "y": 5.5}],
... ])

If we accumulate node type names using depth_context,

>>> def crawl(layout, depth_context, **kwargs):
...     depth_context["types"] = depth_context["types"] + (type(layout).__name__,)
...     print(depth_context["types"])
>>> context = {"types": ()}
>>> ak.transform(crawl, array, depth_context=context, return_value="none")
('ListOffsetArray', 'RecordArray')
('ListOffsetArray', 'RecordArray', 'ListOffsetArray')
('ListOffsetArray', 'RecordArray', 'ListOffsetArray', 'NumpyArray')
('ListOffsetArray', 'RecordArray', 'NumpyArray')
>>> context
{'types': ()}

The data in depth_context["types"] represents a path from the root of the tree to the current node. There is never, for instance, more than one leaf-type (ak.contents.NumpyArray) in the tuple. Also, the context is unchanged outside of the function.

On the other hand, if we do the same with a lateral_context,

>>> def crawl(layout, lateral_context, **kwargs):
...     lateral_context["types"] = lateral_context["types"] + (type(layout).__name__,)
...     print(lateral_context["types"])
>>> context = {"types": ()}
>>> ak.transform(crawl, array, lateral_context=context, return_value="none")
('ListOffsetArray', 'RecordArray')
('ListOffsetArray', 'RecordArray', 'ListOffsetArray')
('ListOffsetArray', 'RecordArray', 'ListOffsetArray', 'NumpyArray')
('ListOffsetArray', 'RecordArray', 'ListOffsetArray', 'NumpyArray', 'NumpyArray')
>>> context
{'types': ('ListOffsetArray', 'RecordArray', 'ListOffsetArray', 'NumpyArray', 'NumpyArray')}

The data accumulate through the walk over the tree. There are two leaf-types (ak.contents.NumpyArray) in the tuple because this tree has two leaves. The data are even available outside of the function, so lateral_context can be paired with return_value="none" to extract non-array data, rather than transforming the array.

The visitation order is stable: a recursive walk always proceeds through the same tree in the same order.


The transformation function is given an input, untransformed layout or layouts. Some algorithms need to perform a correction on transformed outputs, so continuation() can be called at any point to continue descending but obtain the transformed result.

For example, this function inserts an option-type at every level of an array:

>>> def insert_optiontype(layout, continuation, **kwargs):
...     return ak.contents.UnmaskedArray(continuation())
>>> array = ak.Array([[[[[1.1, 2.2, 3.3], []]], []], [[[[4.4, 5.5]]]]])
>>> array.type.show()
2 * var * var * var * var * float64

>>> array2 = ak.transform(insert_optiontype, array)
>>> array2.type.show()
2 * option[var * option[var * option[var * option[var * ?float64]]]]

In the original array, every node is a ak.contents.ListOffsetArray except the leaf, which is a ak.contents.NumpyArray. The call to continuation() returns a ak.contents.ListOffsetArray with its contents transformed, which is the argument of a new ak.contents.UnmaskedArray.

To see this process as it happens, we can add print statements to the function.

>>> def insert_optiontype(input, continuation, **kwargs):
...     print("before", input.form.type)
...     output = ak.contents.UnmaskedArray(continuation())
...     print("after ", output.form.type)
...     return output
>>> ak.transform(insert_optiontype, array)
before var * var * var * var * float64
before var * var * var * float64
before var * var * float64
before var * float64
before float64
after  ?float64
after  option[var * ?float64]
after  option[var * option[var * ?float64]]
after  option[var * option[var * option[var * ?float64]]]
after  option[var * option[var * option[var * option[var * ?float64]]]]
<Array [[[[[1.1, ..., 3.3], ...]], ...], ...] type='2 * option[var * option...'>


When multiple arrays are provided (more_arrays), all of the arrays are broadcasted during the walk so that the transformation function is eventually provided with a list of layouts that have compatible types (for mathematical operations, etc.).

For instance, given these two arrays:

>>> array1 = ak.Array([[1, 2, 3], [], None, [4, 5]])
>>> array2 = ak.Array([10, 20, 30, 40])

The following single-array function shows the nodes encountered when walking down either one of them.

>>> def one_array(layout, **kwargs):
...     print(type(layout).__name__)
>>> ak.transform(one_array, array1, return_value="none")
>>> ak.transform(one_array, array2, return_value="none")

The first array has three nested nodes; the second has only one node.

However, when the following two-array function is applied,

>>> def two_arrays(layouts, **kwargs):
...     assert len(layouts) == 2
...     print(type(layouts[0]).__name__, ak.to_list(layouts[0]))
...     print(type(layouts[1]).__name__, ak.to_list(layouts[1]))
...     print()
>>> ak.transform(two_arrays, array1, array2)
RegularArray [[[1, 2, 3], [], None, [4, 5]]]
RegularArray [[10, 20, 30, 40]]

IndexedOptionArray [[1, 2, 3], [], None, [4, 5]]
NumpyArray [10, 20, 30, 40]

ListArray [[1, 2, 3], [], [4, 5]]
NumpyArray [10, 20, 40]

NumpyArray [1, 2, 3, 4, 5]
NumpyArray [10, 10, 10, 40, 40]

(<Array [[1, 2, 3], [], None, [4, 5]] type='4 * option[var * int64]'>,
 <Array [[10, 10, 10], [], None, [40, 40]] type='4 * option[var * int64]'>)

The incompatible types of the two arrays eventually becomes the same type by duplicating and removing values wherever necessary. If you cannot perform an operation on a ak.contents.ListArray and a ak.contents.NumpyArray, wait for a later iteration, in which both will be ak.contents.NumpyArray (if the original arrays are broadcastable).

The return value, without transformation, is the same as what ak.broadcast_arrays would return. See ak.broadcast_arrays for an explanation of left_broadcast and right_broadcast.

Broadcasting Parameters#

When broadcasting multiple arrays with parameters, there are different ways of assigning parameters to the outputs. The assignment of array parameters happens at every level above the transformation action.

The method of parameter assignment used by the broadcasting routine is controlled by the broadcast_parameters_rule option, which can take one of the following values:


The parameters of each output array will correspond to the intersection of the parameters from each of the input arrays.


If the parameters of the input arrays are all equal, then they will be used for each output array. Otherwise, the output arrays will not be given parameters.


If the number of output arrays matches the number of input arrays, then the output arrays are given the parameters of the input arrays. Otherwise, a ValueError is raised.


The output arrays will not be given parameters.

See also: ak.is_valid and ak.valid_when to check the validity of transformed outputs.