jinja2 3.1.6: Jinja2 strips a single trailing newline from the end of a template by default, unless keep_trailing_newline is True.

# Jinja2 keep_trailing_newline Behavior

- **search_known_solution answered**: `NO_SAFE_MATCH`
- **Pinned Version**: 3.1.6
- **Observed Behavior**: By default, Jinja2 silently strips a single trailing newline from a template when rendering it, which contradicts the common expectation that template strings ending with a newline will render with that newline intact. Passing `keep_trailing_newline=True` to the `Environment` prevents this stripping. If multiple newlines exist at the end, exactly one is stripped.

When a competent model confidently writes code concatenating rendered strings expecting them to end in newlines, it silently produces improperly formatted text instead of throwing a visible error, making it a subtle trap for configurations generating line-separated outputs.

{"case":{"believed":"a template string ending with a newline will render with that newline intact by default","caseId":"case:sha256:589ee88e85e281b8a01dc5c1fc89557ee63793551e28198b136eefa3aad6a0b8","contract":["Environment().from_string('hello\\n').render() returns 'hello' rather than 'hello\\n', because the default is to strip exactly one trailing newline.","assert jinja2.Environment().from_string('hello\\n').render() == 'hello'","assert jinja2.Environment().from_string('hello\\n\\n').render() == 'hello\\n'","assert jinja2.Environment(keep_trailing_newline=True).from_string('hello\\n').render() == 'hello\\n'"],"goal":"Jinja2 strips a single trailing newline from the end of a template by default, unless keep_trailing_newline is True.","kind":"HOW","packages":["pkg:pypi/jinja2@3.1.6"],"schemaVersion":1,"symbols":["keep_trailing_newline"]},"contractCommand":["python","test/contract.py"],"environment":{"arch":"x64","ecosystem":"pypi","executionContext":"python","language":"python","os":"linux","packageManager":"pip","runtime":"python","schemaVersion":1},"license":"MIT-0","packages":["pkg:pypi/jinja2@3.1.6"],"schemaVersion":1,"symbols":["keep_trailing_newline"],"verifierAdapter":"python@1"}

Jinja2==3.1.6
MarkupSafe==3.0.3

import sys
import jinja2

env_default = jinja2.Environment()
env_keep = jinja2.Environment(keep_trailing_newline=True)

# A naive model expects a template ending with a newline to render with that newline.
# By default, Jinja2 strips a single trailing newline.
if env_default.from_string("hello\n").render() != "hello":
    print("Failed: default behavior did not strip the trailing newline.")
    sys.exit(1)

if env_default.from_string("hello\n\n").render() != "hello\n":
    print("Failed: default behavior did not strip exactly one trailing newline.")
    sys.exit(1)

# With keep_trailing_newline=True, the trailing newline is preserved.
if env_keep.from_string("hello\n").render() != "hello\n":
    print("Failed: keep_trailing_newline=True did not preserve the trailing newline.")
    sys.exit(1)

print("All assertions passed.")