Description

When using mutmut 3.4.0 with async functions (async def), the generated trampoline wrapper is synchronous (def), which causes the coroutine to not be awaited.

Environment

• mutmut version: 3.4.0
• Python version: 3.12.3
• OS: Linux (WSL2)

Steps to Reproduce

1. Have a codebase with async functions like:

async def get_current_user(
    credentials: Annotated[HTTPAuthorizationCredentials, Depends(security)],
    db: Annotated[AsyncSession, Depends(get_db)],
) -> User:
    """Get current authenticated user from JWT token."""
    # ... async code
    user = await service.get_user_by_id(uuid.UUID(user_id))
    return user

2. Run mutmut run

3. Observe the generated trampoline in mutants/ directory

Expected Behavior

The trampoline for async functions should be:

async def get_current_user(*args, **kwargs):
    result = await _mutmut_trampoline(x_get_current_user__mutmut_orig, x_get_current_user__mutmut_mutants, args, kwargs)
    return result

Actual Behavior

The generated trampoline is synchronous:

def get_current_user(*args, **kwargs):
    result = _mutmut_trampoline(x_get_current_user__mutmut_orig, x_get_current_user__mutmut_mutants, args, kwargs)
    return result 

This causes the error:

RuntimeWarning: coroutine 'x_get_current_user__mutmut_orig' was never awaited

And tests fail because the coroutine is returned instead of being awaited.

Root Cause

In trampoline_templates.py, the build_trampoline function always generates a synchronous def:

def {orig_name}({'self, ' if class_name is not None else ''}*args, **kwargs):
    result = {trampoline_name}(...)
    return result 

It should detect if the original function is async and generate:

async def {orig_name}({'self, ' if class_name is not None else ''}*args, **kwargs):
    result = await {trampoline_name}(...)
    return result 

Related

• problems with mutations to async generators #406 - Fixed async generators, but not regular async functions
• Support async generators #407 - PR that added async generator support

Workaround

Currently excluding files with async functions using do_not_mutate in pyproject.toml:

[tool.mutmut]
do_not_mutate = [
    "**/dependencies.py",
    "**/routes.py", 
    "**/service.py",
    "**/database.py",
]

This significantly limits mutation testing coverage for async Python codebases (FastAPI, asyncio, etc.).