ARROW-17131: [Python] add public-facing field() to UnionType

python/pyarrow/types.pxi

@@ -620,6 +620,23 @@ cdef class UnionType(DataType):
         for i in range(len(self)):
             yield self[i]
 
+    def field(self, i):
+        """
+        Return a child field by its numeric index.
+
+        Parameters
+        ----------
+        i : int
+
+        Returns
+        -------
+        pyarrow.Field
+        """
+        if isinstance(i, int):
+            return DataType.field(self, i)
+        else:
+            raise TypeError('Expected integer')
+
     def __getitem__(self, i):
         """
         Return a child field by its index.