[Python] MapType comparison ignores nullable flag of item_field

[asfimport]

By default MapType value fields are nullable:

 pa.map_(pa.string(), pa.int32()).item_field.nullable == True 

It is possible to mark the value field of a MapType as not-nullable:

 pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=False)).item_field.nullable == False

But comparing these two types, that are semantically different, returns True:

pa.map_(pa.string(), pa.int32()) == pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=False)) # Returns True 

So it looks like the comparison omits the nullable flag. 

import pyarrow as pa

map_type = pa.map_(pa.string(), pa.int32())
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))
map_type_different_field_name = pa.map_(pa.string(), pa.field("value", pa.int32(), nullable=True))

assert nullable_map_type == map_type  # Wrong
assert str(nullable_map_type) == str(map_type)
assert str(non_null_map_type) == str(map_type) # Wrong
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 map_type == map_type_different_field_name # This makes sense

Environment: pyarrow==10.0.1

Reporter: &res / @0x26res
Assignee: Will Jones / @wjones127

Related issues:

• [C++] List types with different field names are not equal (is related to)

_{Note: This issue was originally created as ARROW-18411. Please see the migration documentation for further details.}

[asfimport]

Will Jones / @wjones127:
Thanks for reporting this. This will be fixed by #13851