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GH-47380: [Python] Apply maps_as_pydicts to Nested MapScalar Values #47454

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Merged
raulcd merged 5 commits into from
Oct 7, 2025
Merged

GH-47380: [Python] Apply maps_as_pydicts to Nested MapScalar Values #47454

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ffbc346
GH-47380: [Python] Apply maps_as_pydicts to Nested Map Values
jo-migo Aug 28, 2025
60b963c
Simplify key iteration
jo-migo Sep 10, 2025
e542e37
Retain existing unit test and introduce new one
jo-migo Sep 22, 2025
985c315
Handle empty values array in map struct, add corresponding test
jo-migo Oct 6, 2025
45b5431
Merge branch 'apache:main' into fix-nested-maps-as-pydicts
jo-migo Oct 6, 2025
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7 changes: 5 additions & 2 deletions python/pyarrow/scalar.pxi
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Original file line number Diff line number Diff line change
Expand Up @@ -1173,7 +1173,10 @@ cdef class MapScalar(ListScalar, Mapping):
if not maps_as_pydicts:
return list(self)
result_dict = {}
for key, value in self:
if self.values is None:
return result_dict

for key, value in zip(self.keys(), self.values.field(self.type.item_field.name)):
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@raulcd raulcd Oct 6, 2025

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What was the issue with the previous iterator?
I am unsure I understand what was wrong with the previous iteration and what are we fixing here.

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@jonasdedden jonasdedden Oct 6, 2025

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Not the author of the MR, but the previous for key, value in self would result in a call of self.__iter__().

That thing is defined above this function, and does yield (k.as_py(), v.as_py()) directly. So it's hardcoded to the default maps_as_pydicts behaviour, which is incompatible to what we want here. The values, if they happen to be map types, basically would be yielded in the non-dict way (as the list of (key, value) tuples).

It's also not possible to adjust the __iter__() function because by definition it has to have no parameters, so it has to be opinionated in some sense about how to handle maps.

So in this case, we have to loop over the keys and values manually and then do the as_py() call on the value type with the correct maps_as_pydicts parameter ourselves.

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@raulcd raulcd Oct 6, 2025

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I see, that makes sense, thanks for the clarification, should we validate self.values is not None as we currently do on __iter__?

        arr = self.values
        if arr is None:
            return
        for k, v in zip(arr.field(self.type.key_field.name), arr.field(self.type.item_field.name)):
            yield (k.as_py(), v.as_py())

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@jonasdedden jonasdedden Oct 6, 2025

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I guess that would make sense, also adding a test for whether an empty map works could make sense? @jo-migo ?

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@jo-migo jo-migo Oct 6, 2025

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Sure, I have added an explicit check for self.values is None and corresponding test.

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@jo-migo jo-migo Oct 6, 2025

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As for the context, check out the PR description for a (hopefully) straightforward minimal example, but it's just like @jonded94 says. The problem with the current implementation is that round trips from python -> arrow -> python are broken at the moment for nested map columns ---

basically we want

{'x': {'a': {'1': 1}}} -> {'x': {'a': {'1': 1}}}

but the current behaviour with maps_as_pydicts is:

{'x': {'a': {'1': 1}}} -> {'x': {'a': [('1', 1)]}}

if key in result_dict:
if maps_as_pydicts == "strict":
raise KeyError(
Expand All @@ -1183,7 +1186,7 @@ cdef class MapScalar(ListScalar, Mapping):
else:
warnings.warn(
f"Encountered key '{key}' which was already encountered.")
result_dict[key] = value
result_dict[key] = value.as_py(maps_as_pydicts=maps_as_pydicts)
return result_dict

def keys(self):
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31 changes: 30 additions & 1 deletion python/pyarrow/tests/test_scalars.py
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Original file line number Diff line number Diff line change
Expand Up @@ -956,7 +956,7 @@ def test_map_scalar_as_py_with_custom_field_name():
).as_py() == [("foo", "bar")]


def test_nested_map_types_with_maps_as_pydicts():
def test_map_types_with_maps_as_pydicts():
ty = pa.struct([
pa.field('x', pa.map_(pa.string(), pa.int8())),
pa.field('y', pa.list_(pa.map_(pa.string(), pa.int8()))),
Expand All @@ -966,3 +966,32 @@ def test_nested_map_types_with_maps_as_pydicts():
s = pa.scalar(v, type=ty)

assert s.as_py(maps_as_pydicts="strict") == v


def test_nested_map_types_with_maps_as_pydicts():
ty = pa.struct(
[
pa.field('x', pa.map_(pa.string(), pa.map_(pa.string(), pa.int8()))),
pa.field(
'y', pa.list_(pa.map_(pa.string(), pa.map_(pa.string(), pa.int8())))
),
]
)

v = {'x': {'a': {'1': 1}}, 'y': [{'b': {'2': 2}}, {'c': {'3': 3}}]}
s = pa.scalar(v, type=ty)

assert s.as_py(maps_as_pydicts="strict") == v


def test_map_scalar_with_empty_values():
map_type = pa.struct(
[
pa.field('x', pa.map_(pa.string(), pa.string())),
]
)

v = {'x': {}}
s = pa.scalar(v, type=map_type)

assert s.as_py(maps_as_pydicts="strict") == v
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