ARROW-16651 : [Python] Casting Table to new schema ignores nullability of fields

python/pyarrow/table.pxi

@@ -3395,11 +3395,16 @@ cdef class Table(_PandasConvertible):
             Field field
             list newcols = []
 
-        if self.schema.names != target_schema.names:
+        field_names = self.schema.names
+        if field_names != target_schema.names:
             raise ValueError("Target schema's field names are not matching "
                              "the table's field names: {!r}, {!r}"
                              .format(self.schema.names, target_schema.names))
 
+        for name in field_names:
+            if self.schema.field(name).nullable and not target_schema.field(name).nullable:
+                raise RuntimeError(
+                    "Casting a nullable field {!r} to non-nullable".format(name))
         for column, field in zip(self.itercolumns(), target_schema):
             casted = column.cast(field.type, safe=safe, options=options)
             newcols.append(casted)

python/pyarrow/tests/test_table.py

@@ -2192,3 +2192,12 @@ def test_table_join_many_columns():
         "col6": ["A", "B", None, "Z"],
         "col7": ["A", "B", None, "Z"],
     })
+
+
+def test_table_cast_invalid():
+    # Casting a nullable field to non-nullable should be invalid!
+    table = pa.table({'a': [None, 1], 'b': [None, True]})
+    new_schema = pa.schema([pa.field("a", "int64", nullable=True),
+                            pa.field("b", "bool", nullable=False)])
+    with pytest.raises(RuntimeError):
+        table.cast(new_schema)