2023-03-10

Handling Undeclared Variables in Jinja

jinja2.meta.find_undeclared_variables

In Jinja2 templating, managing variables is a crucial aspect of creating dynamic content. Among these variables, undeclared variables can cause issues if not properly handled. Jinja2's jinja2.meta.find_undeclared_variables function is designed to help developers identify such variables within a template.

The find_undeclared_variables function inspects the abstract syntax tree (AST) of a Jinja2 template and returns a set of undeclared variables. These are the variables that are referenced in the template but not declared within the context.

To use find_undeclared_variables, you first need to import the required modules and parse your Jinja2 template:

python

from jinja2 import Environment, FileSystemLoader, meta

env = Environment(loader=FileSystemLoader('templates'))
template_source = env.loader.get_source(env, 'my_template.html')
parsed_content = env.parse(template_source)

Now you can call find_undeclared_variables to identify the undeclared variables:

python

undeclared_variables = meta.find_undeclared_variables(parsed_content)
print(undeclared_variables)

This will output a set of undeclared variables in the 'my_template.html' file, such as:

{'variable1', 'variable2', 'variable3'}

Example Template Setter Class

Here's a Jinja template setter class that renders a template while checking for undeclared variables. If there are undeclared variables, it raises a ValueError:

python

from dataclasses import dataclass, field
from jinja2 import Environment, meta, Template

@dataclass
class Jinja2TemplateSetter:
    query: str
    query_params: dict
    template: Template = field(init=False)
    parsed_content = field(init=False)

    def __post_init__(self):
        self.template = Template(self.query)
        self.parsed_content = self._parse_template()
        self._check_undeclared_variables()

    def _parse_template(self):
        return self.template.environment.parse(self.template.source)

    def _check_undeclared_variables(self):
        undeclared_variables = meta.find_undeclared_variables(self.parsed_content)
        missing_variables = undeclared_variables - set(self.query_params.keys())

        if missing_variables:
            raise ValueError(f"Undeclared variables in the query: {', '.join(missing_variables)}")

    def render_template(self):
        return self.template.render(self.query_params)

# Usage
if __name__ == "__main__":
    query = "{{ greeting }} {{ name }}, welcome to Jinja2!"
    query_params = {"greeting": "Hello", "name": "Alice"}

    try:
        template_setter = Jinja2TemplateSetter(query=query, query_params=query_params)
        print(template_setter.render_template())
    except ValueError as e:
        print(str(e))

Jinja2TemplateSetter class initializes the Jinja2 Template object and parses the template content in the __post_init__ method. It then checks for undeclared variables using the _check_undeclared_variables method. If there are any undeclared variables, a ValueError will be raised. The render_template method is used to render the template using the provided query_params.

Handling Undeclared Variables in Jinja

jinja2.meta.find_undeclared_variables

Example Template Setter Class

References

Jinja Templating Engine in Python

Streamlit

Ryusei Kakujo