chore(core): add ruff rule PLC0415 (#32351)

See https://docs.astral.sh/ruff/rules/import-outside-top-level/

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/langchain_core/prompts/message.py

@@ -88,7 +88,8 @@ class BaseMessagePromptTemplate(Serializable, ABC):
         Returns:
             Combined prompt template.
         """
-        from langchain_core.prompts.chat import ChatPromptTemplate
+        # Import locally to avoid circular import.
+        from langchain_core.prompts.chat import ChatPromptTemplate  # noqa: PLC0415
 
         prompt = ChatPromptTemplate(messages=[self])
         return prompt + other

libs/core/langchain_core/prompts/string.py

@@ -15,6 +15,14 @@ from langchain_core.utils import get_colored_text, mustache
 from langchain_core.utils.formatting import formatter
 from langchain_core.utils.interactive_env import is_interactive_env
 
+try:
+    from jinja2 import Environment, meta
+    from jinja2.sandbox import SandboxedEnvironment
+
+    _HAS_JINJA2 = True
+except ImportError:
+    _HAS_JINJA2 = False
+
 PromptTemplateFormat = Literal["f-string", "mustache", "jinja2"]
 
 
@@ -40,9 +48,7 @@ def jinja2_formatter(template: str, /, **kwargs: Any) -> str:
     Raises:
         ImportError: If jinja2 is not installed.
     """
-    try:
-        from jinja2.sandbox import SandboxedEnvironment
-    except ImportError as e:
+    if not _HAS_JINJA2:
         msg = (
             "jinja2 not installed, which is needed to use the jinja2_formatter. "
             "Please install it with `pip install jinja2`."
@@ -50,7 +56,7 @@ def jinja2_formatter(template: str, /, **kwargs: Any) -> str:
             "Do not expand jinja2 templates using unverified or user-controlled "
             "inputs as that can result in arbitrary Python code execution."
         )
-        raise ImportError(msg) from e
+        raise ImportError(msg)
 
     # This uses a sandboxed environment to prevent arbitrary code execution.
     # Jinja2 uses an opt-out rather than opt-in approach for sand-boxing.
@@ -88,14 +94,12 @@ def validate_jinja2(template: str, input_variables: list[str]) -> None:
 
 
 def _get_jinja2_variables_from_template(template: str) -> set[str]:
-    try:
-        from jinja2 import Environment, meta
-    except ImportError as e:
+    if not _HAS_JINJA2:
         msg = (
             "jinja2 not installed, which is needed to use the jinja2_formatter. "
             "Please install it with `pip install jinja2`."
         )
-        raise ImportError(msg) from e
+        raise ImportError(msg)
     env = Environment()  # noqa: S701
     ast = env.parse(template)
     return meta.find_undeclared_variables(ast)