Jinja2 validation changed to issue warnings rather than issuing exceptions. (#7161)

- Description: If their are missing or extra variables when validating
Jinja 2 template then a warning is issued rather than raising an
exception. This allows for better flexibility for the developer as
described in #7044. Also changed the relevant test so pytest is checking
for raised warnings rather than exceptions.
  - Issue: #7044 
  - Tag maintainer: @hwchase17, @baskaryan

---------

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

langchain/prompts/base.py

@@ -1,6 +1,7 @@
 """BasePrompt schema definition."""
 from __future__ import annotations
 
+import warnings
 from abc import ABC
 from typing import Any, Callable, Dict, List, Set
 
@@ -26,7 +27,7 @@ def jinja2_formatter(template: str, **kwargs: Any) -> str:
 def validate_jinja2(template: str, input_variables: List[str]) -> None:
     """
     Validate that the input variables are valid for the template.
-    Raise an exception if missing or extra variables are found.
+    Issues an warning if missing or extra variables are found.
 
     Args:
         template: The template string.
@@ -37,15 +38,15 @@ def validate_jinja2(template: str, input_variables: List[str]) -> None:
     missing_variables = valid_variables - input_variables_set
     extra_variables = input_variables_set - valid_variables
 
-    error_message = ""
+    warning_message = ""
     if missing_variables:
-        error_message += f"Missing variables: {missing_variables} "
+        warning_message += f"Missing variables: {missing_variables} "
 
     if extra_variables:
-        error_message += f"Extra variables: {extra_variables}"
+        warning_message += f"Extra variables: {extra_variables}"
 
-    if error_message:
-        raise KeyError(error_message.strip())
+    if warning_message:
+        warnings.warn(warning_message.strip())
 
 
 def _get_jinja2_variables_from_template(template: str) -> Set[str]:

tests/unit_tests/prompts/test_few_shot.py

@@ -227,7 +227,7 @@ def test_prompt_jinja2_missing_input_variables(
     suffix = "Ending with {{ bar }}"
 
     # Test when missing in suffix
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         FewShotPromptTemplate(
             input_variables=[],
             suffix=suffix,
@@ -237,7 +237,7 @@ def test_prompt_jinja2_missing_input_variables(
         )
 
     # Test when missing in prefix
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         FewShotPromptTemplate(
             input_variables=["bar"],
             suffix=suffix,
@@ -254,7 +254,7 @@ def test_prompt_jinja2_extra_input_variables(
     """Test error is raised when there are too many input variables."""
     prefix = "Starting with {{ foo }}"
     suffix = "Ending with {{ bar }}"
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         FewShotPromptTemplate(
             input_variables=["bar", "foo", "extra", "thing"],
             suffix=suffix,

tests/unit_tests/prompts/test_prompt.py

@@ -218,7 +218,7 @@ def test_prompt_jinja2_missing_input_variables() -> None:
     """Test error is raised when input variables are not provided."""
     template = "This is a {{ foo }} test."
     input_variables: list = []
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         PromptTemplate(
             input_variables=input_variables, template=template, template_format="jinja2"
         )
@@ -228,7 +228,7 @@ def test_prompt_jinja2_extra_input_variables() -> None:
     """Test error is raised when there are too many input variables."""
     template = "This is a {{ foo }} test."
     input_variables = ["foo", "bar"]
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         PromptTemplate(
             input_variables=input_variables, template=template, template_format="jinja2"
         )
@@ -238,7 +238,7 @@ def test_prompt_jinja2_wrong_input_variables() -> None:
     """Test error is raised when name of input variable is wrong."""
     template = "This is a {{ foo }} test."
     input_variables = ["bar"]
-    with pytest.raises(ValueError):
+    with pytest.warns(UserWarning):
         PromptTemplate(
             input_variables=input_variables, template=template, template_format="jinja2"
         )