Can we assign an object instead of a string? about jinja2-simple-tags

[pix666]

Yes, the render method in both StandaloneTag and ContainerTag can indeed return any object, not just strings. When using the assignment feature with the as keyword, the result is treated the same as any regular variable in the context.

Example:

from jinja2_simple_tags import StandaloneTag


class Person:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

    def __str__(self):
        return f"{self.name}, {self.age} years old"

    def greet(self, other_person):
        return f"Hello, {other_person}! My name is {self.name}."


class PersonTag(StandaloneTag):
    tags = {"person"}

    def render(self, name, age):
        return Person(name, age)

{% person 'Kevin', 12 as user %}

Name: {{ user.name }}
Age: {{ user.age }}
Greeting: {{ user.greet("Mike") }}

Result:

Name: Kevin
Age: 12
Greeting: Hello, Mike! My name is Kevin.

[Meng6]

Hi @pix666, thanks so much for your reply! The above example works perfectly for the StandaloneTag. However, I kept getting the following error while using the ContainerTag. Could you please give me some suggestions? Thanks!

TypeError: sequence item 0: expected str instance, Person found

Here is my PersonTag class:

class PersonTag(ContainerTag):
    tags = {"person"}

    def render(self, name, age, caller=None):
        return Person(name, age)

Jinja template:

{% person 'Kevin', 12 as user %}{% endperson %}
Name: {{ user.name }}
Age: {{ user.age }}
Greeting: {{ user.greet("Mike") }}

[pix666]

Oh, yes, you are right. I discovered that there is a significant difference between the nodes.Assign and nodes.AssignBlock (which is used in ContainerTag) in Jinja2. nodes.AssignBlock uses the concat function, which is a synonym for "".join(). Therefore, block tags MUST return a string value, and I don't see a simple way to bypass this limitation.

https://github.com/pallets/jinja/blob/3fd91e4d11bdd131d8c12805177dbe74d85e7b82/src/jinja2/compiler.py#L1582

However, if you really need to achieve the desired behavior, you can create a custom subclass of CodeGenerator. This is a hacky way to change the behavior of the Jinja2 compiler. Here's an example:

from jinja2 import Environment, nodes
from jinja2.compiler import CodeGenerator, Frame

from jinja2_simple_tags import ContainerTag


class CustomCodeGenerator(CodeGenerator):
    def visit_AssignBlock(self, node: nodes.AssignBlock, frame: Frame) -> None:
        self.push_assign_tracking()
        block_frame = frame.inner()
        # This is a special case.  Since a set block always captures we
        # will disable output checks.  This way one can use set blocks
        # toplevel even in extended templates.
        block_frame.require_output_check = False
        block_frame.symbols.analyze_node(node)
        self.enter_frame(block_frame)
        self.buffer(block_frame)
        self.blockvisit(node.body, block_frame)
        self.newline(node)

        self.writeline(f"if len({block_frame.buffer}) == 1 and not isinstance({block_frame.buffer}[0], str):")
        self.indent()
        self.newline()
        self.visit(node.target, frame)
        self.write(f" = {block_frame.buffer}[0]")
        self.outdent()
        self.writeline("else:")
        self.indent()
        self.newline()
        self.visit(node.target, frame)
        self.write(" = (Markup if context.eval_ctx.autoescape else identity)(")
        if node.filter is not None:
            self.visit_Filter(node.filter, block_frame)
        else:
            self.write(f"concat({block_frame.buffer})")
        self.write(")")
        self.outdent()

        self.pop_assign_tracking(frame)
        self.leave_frame(block_frame)


class Person:
    def __init__(self, name: str, age: int, bio: str = ""):
        self.name = name
        self.age = age
        self.bio = bio

    def greet(self, other_person):
        return f"Hello, {other_person}! My name is {self.name}."


class PersonTag(ContainerTag):
    tags = {"person"}

    def render(self, name, age, caller=None):
        bio = caller().strip()
        return Person(name, age, bio)


class TestContainerTagAssignment:
    def setup_method(self):
        self.env = Environment(
            extensions=[PersonTag],
        )
        self.env.code_generator_class = CustomCodeGenerator

    def test_output(self):
        template = self.env.from_string(
            "{% person 'Kevin', 12 as person %}"
            "  I love playing soccer with my friends."
            "{% endperson %}\n"
            ""
            "Name: {{ person.name }}\n"
            "Age: {{ person.age }}\n"
            "Greeting: {{ person.greet(\"Mike\") }}\n"
            "Bio: {{ person.bio }}\n"
        )
        assert template.render({}).strip() == (
            "Name: Kevin\n"
            "Age: 12\n"
            "Greeting: Hello, Mike! My name is Kevin.\n"
            "Bio: I love playing soccer with my friends."
        )

[Meng6]

It works! Thanks so much for your help!