[jira] [Created] (ARROW-18411) [Python] MapType comparison ignores nullable flag of item_field

["&res (Jira)" <ji...@apache.org>]

&res created ARROW-18411:
----------------------------

             Summary: [Python] MapType comparison ignores nullable flag of item_field
                 Key: ARROW-18411
                 URL: https://issues.apache.org/jira/browse/ARROW-18411
             Project: Apache Arrow
          Issue Type: Bug
          Components: Python
         Environment: pyarrow==10.0.1

            Reporter: &res


By default MapType value fields are nullable:
{code:java}
 pa.map_(pa.string(), pa.int32()).item_field.nullable == True {code}
It is possible to mark the value field of a MapType as not-nullable:
{code:java}
 pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=False)).item_field.nullable == False{code}
But comparing these two types, that are semantically different, returns True:
{code:java}
pa.map_(pa.string(), pa.int32()) == pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=False)) # Returns True {code}
So it looks like the comparison omits the nullable flag. 
{code:java}
import pyarrow as pa
import pytest

print(pa.__version__)

map_type = pa.map_(pa.string(), pa.int32())
pa.array(
[[("one", 1), ("two", 2), ("null", None)]],
map_type
)

with pytest.raises(pa.ArrowInvalid, match=r"Invalid Map: key field can not contain null values"):
pa.array(
[[("one", 1), ("two", 2), (None, None)]],
map_type
)

map_type = pa.map_(pa.string(), pa.int32())
pa.array(
[[("one", 1), ("two", 2), ("null", None)]],
map_type
)

non_null_map_type = pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=False))
nullable_map_type = pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=True))

pa.array(
[[("one", 1), ("two", 2), ("null", None)]],
map_type
)


assert nullable_map_type == map_type # Should be different
assert str(nullable_map_type) == str(map_type)
assert non_null_map_type == map_type
assert non_null_map_type.item_type == map_type.item_type
assert non_null_map_type.item_field != map_type.item_field
assert non_null_map_type.item_field.nullable != map_type.item_field.nullable
assert non_null_map_type.item_field.name == map_type.item_field.name
assert str(non_null_map_type) != str(map_type.item_field.name){code}



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