[python] Adding Unknown and Void transforms

python/iceberg/api/transforms/transforms.py

@@ -22,6 +22,8 @@
 from .identity import Identity
 from .timestamps import Timestamps
 from .truncate import Truncate
+from .unknown_transform import UnknownTransform
+from .void_transform import VoidTransform
 from ..types import (TypeID)
 
 
@@ -60,7 +62,10 @@ def from_string(type_var, transform):
         elif type_var.type_id == TypeID.DATE:
             return Dates(transform.lower(), transform.lower())
 
-        raise RuntimeError("Unknown transform: %s" % transform)
+        if transform.lower() == "void":
+            return VoidTransform.get()
+
+        return UnknownTransform(type_var, transform)
 
     @staticmethod
     def identity(type_var):
@@ -109,3 +114,7 @@ def bucket(type_var, num_buckets):
     @staticmethod
     def truncate(type_var, width):
         return Truncate.get(type_var, width)
+
+    @staticmethod
+    def always_null():
+        return VoidTransform.get()

python/iceberg/api/transforms/unknown_transform.py

@@ -0,0 +1,61 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+from typing import Union
+
+from iceberg.api.types import StringType, Type
+
+from .transform import Transform
+
+
+class UnknownTransform(Transform):
+
+    def __init__(self, source_type: Type, transform: str):
+        self.source_type = source_type
+        self.transform = transform
+
+    def apply(self, value):
+        raise AttributeError(f"Cannot apply unsupported transform: {self.transform}")
+
+    def can_transform(self, type_var) -> bool:
+        # assume the transform function can be applied for this type because unknown transform is only used when parsing
+        # a transform in an existing table. a different Iceberg version must have already validated it.
+        return self.source_type == type_var
+
+    def get_result_type(self, source_type):
+        # the actual result type is not known
+        return StringType.get()
+
+    def project(self, name, predicate):
+        return None
+
+    def project_strict(self, name, predicate):
+        return None
+
+    def __str__(self):
+        return self.transform
+
+    def __eq__(self, other: Union['UnknownTransform', Transform, object]):
+        if id(self) == id(other):
+            return True
+        elif not isinstance(other, UnknownTransform):
+            return False
+
+        return self.source_type == other.source_type and self.transform == other.transform
+
+    def __hash__(self):
+        return hash((self.source_type, self.transform))

python/iceberg/api/transforms/void_transform.py

@@ -0,0 +1,52 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+from .transform import Transform
+
+
+class VoidTransform(Transform):
+    _INSTANCE = None
+
+    @staticmethod
+    def get():
+        if VoidTransform._INSTANCE is None:
+            VoidTransform._INSTANCE = VoidTransform()
+        return VoidTransform._INSTANCE
+
+    def __init__(self):
+        pass
+
+    def apply(self, value):
+        return None
+
+    def can_transform(self, type_var):
+        return True
+
+    def get_result_type(self, source_type):
+        return source_type
+
+    def project(self, name, predicate):
+        return None
+
+    def project_strict(self, name, predicate):
+        return None
+
+    def to_human_string(self, value):
+        return "null"
+
+    def __str__(self):
+        return "void"