core: fix "template" not allowed as prompt param (#26060)

- **Description:**  fix "template" not allowed as prompt param
- **Issue:** #26058
- **Dependencies:** none


- [ ] **Add tests and docs**: If you're adding a new integration, please
include
1. a test for the integration, preferably unit tests that do not rely on
network access,
2. an example notebook showing its use. It lives in
`docs/docs/integrations` directory.


- [ ] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

---------

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/core/langchain_core/prompts/string.py

@@ -17,7 +17,7 @@ from langchain_core.utils.formatting import formatter
 from langchain_core.utils.interactive_env import is_interactive_env
 
 
-def jinja2_formatter(template: str, **kwargs: Any) -> str:
+def jinja2_formatter(template: str, /, **kwargs: Any) -> str:
     """Format a template using jinja2.
 
     *Security warning*:
@@ -99,7 +99,7 @@ def _get_jinja2_variables_from_template(template: str) -> set[str]:
     return variables
 
 
-def mustache_formatter(template: str, **kwargs: Any) -> str:
+def mustache_formatter(template: str, /, **kwargs: Any) -> str:
     """Format a template using mustache.
 
     Args:

libs/core/tests/unit_tests/prompts/test_prompt.py

@@ -640,3 +640,22 @@ def test_prompt_missing_vars_error() -> None:
 
     # Check helper text has right number of braces
     assert "'{{goingtobemissing}}'" in str(e.value.args[0])
+
+
+def test_prompt_with_template_variable_name_fstring() -> None:
+    template = "This is a {template} test."
+    prompt = PromptTemplate.from_template(template, template_format="f-string")
+    assert prompt.invoke({"template": "bar"}).to_string() == "This is a bar test."
+
+
+def test_prompt_with_template_variable_name_mustache() -> None:
+    template = "This is a {{template}} test."
+    prompt = PromptTemplate.from_template(template, template_format="mustache")
+    assert prompt.invoke({"template": "bar"}).to_string() == "This is a bar test."
+
+
+@pytest.mark.requires("jinja2")
+def test_prompt_with_template_variable_name_jinja2() -> None:
+    template = "This is a {{template}} test."
+    prompt = PromptTemplate.from_template(template, template_format="jinja2")
+    assert prompt.invoke({"template": "bar"}).to_string() == "This is a bar test."