saferaise 0.0.1a2

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Checked exceptions for Python. Declare what your functions raise, and let saferaise enforce it - using your existing try/except blocks.

Python's exception system is powerful but unstructured: any function can raise anything, anywhere. saferaise brings discipline to error handling. Decorate functions with @raises, register your packages, and every try/except automatically tells saferaise which exceptions are being handled. No boilerplate, no wrappers - just your normal Python code, now validated.

saferaise is designed to complement static analysis tools, not replace them. Use it alongside type checkers (basedpyright, mypy) for static guarantees, and alongside linter rules like RET506 for documentation coverage. saferaise adds the runtime enforcement layer - catching what static tools can't, such as a @raises function being called outside any handling context.

*CAUTION*: Alpha: The API may change between releases, and using this right now might very well break your code in weird ways, so don't use it in production.

Quick Start

register must be called before a package is imported - and in a separate file from the code being instrumented. This is a hard requirement: the import hook only rewrites modules at load time, so any module imported before register is called will not be instrumented.

# entrypoint.py
import saferaise

saferaise.register("app")   # instrument app's try/except blocks before importing it

import app
with saferaise.enable():    # activate checking for this scope
    app.main()

# app.py
from saferaise import raises

@raises(ValueError)
def parse_input(raw: str) -> int:
    return int(raw)

def main():
    try:
        parse_input("abc")  # ValueError is caught here - @raises is satisfied
    except ValueError:
        print("bad input")

If parse_input is called outside a try/except that catches ValueError:

def main():
    parse_input("abc")  # UnwatchedRaiseError - nobody is catching ValueError

Preamble

This project was not particularly inspired by returns, but I'd like to note that it was after reading its README at 2am that I came up with the idea for saferaise. It was a lot of fun to build, and I learned a lot about Python's import system and AST manipulation.

Whether runtime-enforced checked exceptions are actually a good idea for Python is an open question - I have my own thoughts, and I'd love to hear yours. Feel free to open a discussion on GitHub.

How It Works

saferaise has two complementary mechanisms:

1. The Import Hook (register)

When you call saferaise.register("mypackage"), an import hook rewrites every try/except block in that package at load time. The body of each try is wrapped so the caught exception types are added to the watched set - the set of exceptions that @raises validates against.

This means your existing try/except blocks are all you need. No special syntax, no manual annotation of call sites.

saferaise.register("myapp")

# In myapp/service.py, this try/except:
try:
    result = do_something()  # if do_something() is @raises(KeyError), it just works
except KeyError:
    handle_missing_key()

register requires no cleanup and has no scope - it instruments the named package for the lifetime of the process. enable() is separate because it does have scope: it activates checking for a specific portion of code, and cleans up after itself. You can and should call enable() tightly around the code you want to validate.

2. The @raises Decorator

Decorates a function to declare its exceptions. When called inside an active watching context, it validates that every declared exception is in the watched set (i.e., someone upstream has a try/except for it).

@raises(ConnectionError, TimeoutError)
async def fetch(url: str) -> bytes:
    ...

Works with both sync and async functions. The watched set is tracked via contextvars, so concurrent async tasks are fully isolated. For threads, enable() must be called within each thread - contextvars give each thread its own context, so a parent thread's watched set is not inherited.

Subclass Handling

Exception subclasses are accepted when a parent is watched:

class AppError(Exception): ...
class NotFoundError(AppError): ...

try:
    find_user(42)  # @raises(NotFoundError) - OK, AppError covers it
except AppError:
    ...

Performance

AST rewriting happens at import time only, and only for packages explicitly passed to register. There is no per-call overhead from instrumentation.

The runtime check inside @raises is fail-fast: it iterates over the function's declared exceptions and checks each against the current watched set. This is O(N*M) in the worst case, where N is the number of exceptions declared by @raises and M is the size of the current watched set - but it exits on the first unhandled exception found. In practice, both N and M are small and the overhead is negligible.

If you are in a context where even this is a concern, disable() reduces the check to O(1) by bypassing it entirely. This means saferaise can be used at test time and dev time for full validation, and selectively disabled in hot paths at runtime if needed - the instrumentation stays in place, only the check is skipped.

Advanced Usage

For cases where you need to manage the watched set manually - entry points, test harnesses, or calling into third-party code - you can use the low-level context managers directly.

Symbol	Type	Description
enable()	Context manager	Activate exception watching with an empty set
disable()	Context manager	Bypass all @raises checks completely
unsafe(*exc)	Context manager	Add exceptions to the watched set manually

from saferaise import enable, unsafe, raises

@raises(ValueError)
def parse(raw: str) -> int:
    return int(raw)

with enable():
    with unsafe(ValueError):
        parse("abc")  # OK - ValueError is manually added to the watched set

unsafe() is intentionally named to signal that you are bypassing the normal flow. Prefer try/except with register() for all application code; reach for unsafe() only at bootstrapping boundaries where a try/except would be artificial.

disable() is distinct from unsafe(BaseException). unsafe(BaseException) satisfies the check - it marks all exceptions as handled. disable() skips the check entirely inside @raises, regardless of what is in the watched set. Use disable() when you want the instrumentation to remain in place but the enforcement to be inactive (e.g., in a production hot path, or temporarily during migration).

Full API Reference

Symbol	Type	Description
@raises(*exc)	Decorator	Declare exceptions a function may raise
register(*roots)	Function	Install the import hook for given package roots
enable()	Context manager	Activate exception watching (empty set)
disable()	Context manager	Bypass all @raises checks
unsafe(*exc)	Context manager	Add exceptions to the watched set

Errors

All errors inherit from SafeRaiseError, which itself inherits from BaseException rather than Exception. This is intentional: a bare except Exception block should never silently swallow a saferaise violation.

Error	When
UnwatchedRaiseError	A @raises function declares an exception not in the watched set
NotEnteredError	A context manager was exited without being entered
NameCollisionError	The import hook's injected name _saferaise_watch_exceptions conflicts with an existing module attribute

Development

# Install dev dependencies
uv sync --dev

# Run tests
uv run tox

# Type checking
uv run basedpyright

# Linting
uv run ruff check # Optiobally add --fix

# Formatting
uv run ruff format

Built With

• basedpyright
• ruff
• tox
• uv
• remark
• Love :blue_heart:

License

MIT License. See LICENSE for details.