langchain/libs/experimental/langchain_experimental/pydantic_v1/__init__.py

import typing
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.

# It's currently impossible to support mypy for both pydantic v1 and v2 at once:
# https://github.com/pydantic/pydantic/issues/6022
#
# In the lint environment, pydantic is currently v1.
# When we upgrade it to pydantic v2, we'll need
# to replace this with `from pydantic.v1 import *`.
if typing.TYPE_CHECKING:
    from pydantic import *  # noqa: F403
else:
    try:
        from pydantic.v1 import *  # noqa: F403
    except ImportError:
        from pydantic import *  # noqa: F403

try:
    _PYDANTIC_MAJOR_VERSION: int = int(metadata.version("pydantic").split(".")[0])
except metadata.PackageNotFoundError:
    _PYDANTIC_MAJOR_VERSION = 0