Create a module for pydantic v1 (langchain-ai#187)

Create a module for pydantic v1 code, so we don't have to worry about
conditional imports or try/except blocks in the codebase.

langserve/playground.py

@@ -7,10 +7,7 @@
 from fastapi.responses import Response
 from langchain.schema.runnable import Runnable
 
-try:
-    from pydantic.v1 import BaseModel
-except ImportError:
-    from pydantic import BaseModel
+from langserve.pydantic_v1 import BaseModel
 
 
 class PlaygroundTemplate(Template):

langserve/pydantic_v1.py

@@ -0,0 +1,25 @@
+from importlib import metadata
+
+## Create namespaces for pydantic v1 and v2.
+# This code must stay at the top of the file before other modules may
+# attempt to import pydantic since it adds pydantic_v1 and pydantic_v2 to sys.modules.
+#
+# This hack is done for the following reasons:
+# * Langchain will attempt to remain compatible with both pydantic v1 and v2 since
+#   both dependencies and dependents may be stuck on either version of v1 or v2.
+# * Creating namespaces for pydantic v1 and v2 should allow us to write code that
+#   unambiguously uses either v1 or v2 API.
+# * This change is easier to roll out and roll back.
+
+try:
+    # F401: imported but unused
+    from pydantic.v1 import BaseModel, Field, ValidationError  # noqa: F401
+except ImportError:
+    from pydantic import BaseModel, Field, ValidationError  # noqa: F401
+
+
+# This is not a pydantic v1 thing, but it feels too small to create a new module for.
+try:
+    _PYDANTIC_MAJOR_VERSION: int = int(metadata.version("pydantic").split(".")[0])
+except metadata.PackageNotFoundError:
+    _PYDANTIC_MAJOR_VERSION = -1

langserve/schema.py

@@ -2,13 +2,9 @@
 from typing import Dict, List, Optional, Union
 from uuid import UUID
 
-from langserve.pydantic import PYDANTIC_MAJOR_VERSION
+from pydantic import BaseModel  # Floats between v1 and v2
 
-if PYDANTIC_MAJOR_VERSION == 2:
-    from pydantic.v1 import BaseModel as BaseModelV1
-else:
-    from pydantic import BaseModel as BaseModelV1
-from pydantic import BaseModel
+from langserve.pydantic_v1 import BaseModel as BaseModelV1
 
 
 class CustomUserType(BaseModelV1):

langserve/serialization.py

@@ -39,14 +39,9 @@
     LLMResult,
 )
 
+from langserve.pydantic_v1 import BaseModel, ValidationError
 from langserve.validation import CallbackEvent
 
-try:
-    from pydantic.v1 import BaseModel, ValidationError
-except ImportError:
-    from pydantic import BaseModel, ValidationError
-
-
 logger = logging.getLogger(__name__)
 
 

langserve/server.py

@@ -45,7 +45,7 @@
 from langserve.callbacks import AsyncEventAggregatorCallback, CallbackEventDict
 from langserve.lzstring import LZString
 from langserve.playground import serve_playground
-from langserve.pydantic import PYDANTIC_MAJOR_VERSION
+from langserve.pydantic_v1 import _PYDANTIC_MAJOR_VERSION
 from langserve.schema import (
     BatchResponseMetadata,
     CustomUserType,
@@ -1045,8 +1045,8 @@ async def feedback(feedback_create_req: FeedbackCreateRequest) -> Feedback:
     #######################################
     # Documentation variants of end points.
     #######################################
-    # At the moment, we only support pydantic 1.x
-    if PYDANTIC_MAJOR_VERSION == 1:
+    # At the moment, we only support pydantic 1.x for documentation
+    if _PYDANTIC_MAJOR_VERSION == 1:
 
         @app.post(
             namespace + "/c/{config_hash}/invoke",