Validate input_variables when using jinja2 templates (#3140)

`langchain.prompts.PromptTemplate` and
`langchain.prompts.FewShotPromptTemplate` do not validate
`input_variables` when initialized as `jinja2` template.

```python
# Using langchain v0.0.144
template = """"\
Your variable: {{ foo }}
{% if bar %}
You just set bar boolean variable to true
{% endif %}
"""

# Missing variable, should raise ValueError
prompt_template = PromptTemplate(template=template, 
                                 input_variables=["bar"], 
                                 template_format="jinja2", 
                                 validate_template=True)

# Extra variable, should raise ValueError
prompt_template = PromptTemplate(template=template, 
                                 input_variables=["bar", "foo", "extra", "thing"], 
                                 template_format="jinja2", 
                                 validate_template=True)
```

tests/unit_tests/prompts/test_few_shot.py

@@ -1,4 +1,6 @@
 """Test few shot prompt template."""
+from typing import Dict, List, Tuple
+
 import pytest
 
 from langchain.prompts.few_shot import FewShotPromptTemplate
@@ -9,6 +11,25 @@ EXAMPLE_PROMPT = PromptTemplate(
 )
 
 
+@pytest.fixture()
+def example_jinja2_prompt() -> Tuple[PromptTemplate, List[Dict[str, str]]]:
+    example_template = "{{ word }}: {{ antonym }}"
+
+    examples = [
+        {"word": "happy", "antonym": "sad"},
+        {"word": "tall", "antonym": "short"},
+    ]
+
+    return (
+        PromptTemplate(
+            input_variables=["word", "antonym"],
+            template=example_template,
+            template_format="jinja2",
+        ),
+        examples,
+    )
+
+
 def test_suffix_only() -> None:
     """Test prompt works with just a suffix."""
     suffix = "This is a {foo} test."
@@ -174,3 +195,71 @@ def test_partial() -> None:
         "Now you try to talk about party."
     )
     assert output == expected_output
+
+
+def test_prompt_jinja2_functionality(
+    example_jinja2_prompt: Tuple[PromptTemplate, List[Dict[str, str]]]
+) -> None:
+    prefix = "Starting with {{ foo }}"
+    suffix = "Ending with {{ bar }}"
+
+    prompt = FewShotPromptTemplate(
+        input_variables=["foo", "bar"],
+        suffix=suffix,
+        prefix=prefix,
+        examples=example_jinja2_prompt[1],
+        example_prompt=example_jinja2_prompt[0],
+        template_format="jinja2",
+    )
+    output = prompt.format(foo="hello", bar="bye")
+    expected_output = (
+        "Starting with hello\n\n" "happy: sad\n\n" "tall: short\n\n" "Ending with bye"
+    )
+
+    assert output == expected_output
+
+
+def test_prompt_jinja2_missing_input_variables(
+    example_jinja2_prompt: Tuple[PromptTemplate, List[Dict[str, str]]]
+) -> None:
+    """Test error is raised when input variables are not provided."""
+    prefix = "Starting with {{ foo }}"
+    suffix = "Ending with {{ bar }}"
+
+    # Test when missing in suffix
+    with pytest.raises(ValueError):
+        FewShotPromptTemplate(
+            input_variables=[],
+            suffix=suffix,
+            examples=example_jinja2_prompt[1],
+            example_prompt=example_jinja2_prompt[0],
+            template_format="jinja2",
+        )
+
+    # Test when missing in prefix
+    with pytest.raises(ValueError):
+        FewShotPromptTemplate(
+            input_variables=["bar"],
+            suffix=suffix,
+            prefix=prefix,
+            examples=example_jinja2_prompt[1],
+            example_prompt=example_jinja2_prompt[0],
+            template_format="jinja2",
+        )
+
+
+def test_prompt_jinja2_extra_input_variables(
+    example_jinja2_prompt: Tuple[PromptTemplate, List[Dict[str, str]]]
+) -> None:
+    """Test error is raised when there are too many input variables."""
+    prefix = "Starting with {{ foo }}"
+    suffix = "Ending with {{ bar }}"
+    with pytest.raises(ValueError):
+        FewShotPromptTemplate(
+            input_variables=["bar", "foo", "extra", "thing"],
+            suffix=suffix,
+            prefix=prefix,
+            examples=example_jinja2_prompt[1],
+            example_prompt=example_jinja2_prompt[0],
+            template_format="jinja2",
+        )

tests/unit_tests/prompts/test_prompt.py

@@ -212,3 +212,33 @@ Your variable again: {{ foo }}
         template_format="jinja2",
     )
     assert prompt == expected_prompt
+
+
+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):
+        PromptTemplate(
+            input_variables=input_variables, template=template, template_format="jinja2"
+        )
+
+
+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):
+        PromptTemplate(
+            input_variables=input_variables, template=template, template_format="jinja2"
+        )
+
+
+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):
+        PromptTemplate(
+            input_variables=input_variables, template=template, template_format="jinja2"
+        )