Support inference of `input_variables` from `jinja2` template (#3013)

`langchain.prompts.PromptTemplate` is unable to infer `input_variables`
from jinja2 template.

```python
# Using langchain v0.0.141
template_string = """\
Hello world
Your variable: {{ var }}
{# This will not get rendered #}

{% if verbose %}
Congrats! You just turned on verbose mode and got extra messages!
{% endif %}
"""

template = PromptTemplate.from_template(template_string, template_format="jinja2")
print(template.input_variables) # Output ['# This will not get rendered #', '% endif %', '% if verbose %']
```

---------

Co-authored-by: engkheng <ongengkheng929@example.com>

langchain/prompts/prompt.py

@@ -3,8 +3,9 @@ from __future__ import annotations
 
 from pathlib import Path
 from string import Formatter
-from typing import Any, Dict, List, Union
+from typing import Any, Dict, List, Set, Union
 
+from jinja2 import Environment, meta
 from pydantic import Extra, root_validator
 
 from langchain.prompts.base import (
@@ -14,6 +15,13 @@ from langchain.prompts.base import (
 )
 
 
+def _get_jinja2_variables_from_template(template: str) -> Set[str]:
+    env = Environment()
+    ast = env.parse(template)
+    variables = meta.find_undeclared_variables(ast)
+    return variables
+
+
 class PromptTemplate(StringPromptTemplate):
     """Schema to represent a prompt for an LLM.
 
@@ -125,9 +133,15 @@ class PromptTemplate(StringPromptTemplate):
     @classmethod
     def from_template(cls, template: str, **kwargs: Any) -> PromptTemplate:
         """Load a prompt template from a template."""
-        input_variables = {
-            v for _, v, _, _ in Formatter().parse(template) if v is not None
-        }
+        if "template_format" in kwargs and kwargs["template_format"] == "jinja2":
+            # Get the variables for the template
+            input_variables = _get_jinja2_variables_from_template(template)
+
+        else:
+            input_variables = {
+                v for _, v, _, _ in Formatter().parse(template) if v is not None
+            }
+
         return cls(
             input_variables=list(sorted(input_variables)), template=template, **kwargs
         )

tests/unit_tests/prompts/test_prompt.py

@@ -145,3 +145,70 @@ def test_partial() -> None:
     assert new_result == "This is a 3 test."
     result = prompt.format(foo="foo")
     assert result == "This is a foo test."
+
+
+def test_prompt_from_jinja2_template() -> None:
+    """Test prompts can be constructed from a jinja2 template."""
+    # Empty input variable.
+    template = """Hello there
+There is no variable here {
+Will it get confused{ }? 
+    """
+    prompt = PromptTemplate.from_template(template, template_format="jinja2")
+    expected_prompt = PromptTemplate(
+        template=template, input_variables=[], template_format="jinja2"
+    )
+    assert prompt == expected_prompt
+
+    # Multiple input variables.
+    template = """\
+Hello world
+
+Your variable: {{ foo }}
+
+{# This will not get rendered #}
+
+{% if bar %}
+You just set bar boolean variable to true
+{% endif %}
+
+{% for i in foo_list %}
+{{ i }}
+{% endfor %}
+"""
+    prompt = PromptTemplate.from_template(template, template_format="jinja2")
+    expected_prompt = PromptTemplate(
+        template=template,
+        input_variables=["bar", "foo", "foo_list"],
+        template_format="jinja2",
+    )
+
+    assert prompt == expected_prompt
+
+    # Multiple input variables with repeats.
+    template = """\
+Hello world
+
+Your variable: {{ foo }}
+
+{# This will not get rendered #}
+
+{% if bar %}
+You just set bar boolean variable to true
+{% endif %}
+
+{% for i in foo_list %}
+{{ i }}
+{% endfor %}
+
+{% if bar %}
+Your variable again: {{ foo }}
+{% endif %}
+"""
+    prompt = PromptTemplate.from_template(template, template_format="jinja2")
+    expected_prompt = PromptTemplate(
+        template=template,
+        input_variables=["bar", "foo", "foo_list"],
+        template_format="jinja2",
+    )
+    assert prompt == expected_prompt