Add decorator to turn regular functions in Result returning ones

Add a as_result() helper to make a decorator to turn a function into one
that returns a Result: Regular return values are turned into
Ok(return_value). Raised exceptions of the specified exception type(s)
are turned into Err(exc).

The decorator is signature-preserving, except for wrapping the return
type into a Result, of course.

For type annotations, this depends on typing.ParamSpec which requires
Python 3.10+ (or use typing_extensions); see
PEP612 (https://www.python.org/dev/peps/pep-0612/).

This is currently not fully supported by Mypy; see
python/mypy#8645

Calling decorated functions works without errors from Mypy, but will
not be type-safe, i.e. it will behave as if it is calling a function
like f(*args: Any, **kwargs: Any)

Fixes rustedpy#33.

CHANGELOG.md

@@ -13,6 +13,8 @@ Possible log types:
 
 ## [Unreleased]
 
+- `[added]` `as_result` decorator to turn regular functions into
+  `Result` returning ones (#33, 71)
 - `[removed]` Drop support for Python 3.6 (#49)
 - `[added]` Implement `unwrap_or_else` (#74)
 

README.rst

@@ -1,3 +1,4 @@
+======
 Result
 ======
 
@@ -90,7 +91,7 @@ be OK or not, without resorting to custom exceptions.
 
 
 API
----
+===
 
 Creating an instance:
 
@@ -199,7 +200,6 @@ returns the error value if ``Err``, otherwise it raises an ``UnwrapError``:
     >>>res2.unwrap_err()
     'nay'
 
-
 A custom error message can be displayed instead by using ``expect`` and ``expect_err``:
 
 .. sourcecode:: python
@@ -261,10 +261,45 @@ To save memory, both the ``Ok`` and ``Err`` classes are ‘slotted’,
 i.e. they define ``__slots__``. This means assigning arbitrary
 attributes to instances will raise ``AttributeError``.
 
+The ``as_result()`` decorator can be used to quickly turn ‘normal’
+functions into ``Result`` returning ones by specifying one or more
+exception types:
 
-FAQ
--------
+.. sourcecode:: python
+
+    @as_result(ValueError, IndexError)
+    def f(value: int) -> int:
+        if value < 0:
+            raise ValueError
+        else:
+            return value
 
+    res = f(12)  # Ok[12]
+    res = f(-1)  # Err[ValueError(-1)]
+
+``Exception`` (or even ``BaseException``) can be specified to create a
+‘catch all’ ``Result`` return type. This is effectively the same as
+``try`` followed by ``except Exception``, which is not considered good
+practice in most scenarios, and hence this requires explicit opt-in.
+
+Since ``as_result`` is a regular decorator, it can be used to wrap
+existing functions (also from other libraries), albeit with a slightly
+unconventional syntax (without the usual ``@``):
+
+.. sourcecode:: python
+
+    import third_party
+
+    x = third_party.do_something(...)  # could raise; who knows?
+
+    safe_do_something = as_result(Exception)(third_party.do_something)
+
+    res = safe_do_something(...)  # Ok(...) or Err(...)
+    if isinstance(res, Ok):
+        print(res.value)
+
+FAQ
+===
 
 - **Why do I get the "Cannot infer type argument" error with MyPy?**
 
@@ -274,9 +309,7 @@ Using ``if isinstance(res, Ok)`` instead of ``if res.is_ok()`` will help in some
 Otherwise using `one of these workarounds
 <https://github.com/python/mypy/issues/3889#issuecomment-325997911>`_ can help.
 
-
-
 License
--------
+=======
 
 MIT License

setup.cfg

@@ -22,6 +22,8 @@ classifiers =
 
 [options]
 include_package_data = True
+install_requires =
+    typing_extensions;python_version<'3.10'
 package_dir =
     =src
 packages = find:

src/result/__init__.py

@@ -1,10 +1,11 @@
-from .result import Err, Ok, OkErr, Result, UnwrapError
+from .result import Err, Ok, OkErr, Result, UnwrapError, as_result
 
 __all__ = [
     "Err",
     "Ok",
     "OkErr",
     "Result",
     "UnwrapError",
+    "as_result",
 ]
 __version__ = "0.7.0"

src/result/result.py

@@ -1,11 +1,33 @@
 from __future__ import annotations
 
-from typing import Any, Callable, Generic, NoReturn, TypeVar, Union, cast, overload
+import functools
+import inspect
+import sys
+from typing import (
+    Any,
+    Callable,
+    Generic,
+    NoReturn,
+    Type,
+    TypeVar,
+    Union,
+    cast,
+    overload,
+)
+
+if sys.version_info[:2] >= (3, 10):
+    from typing import ParamSpec
+else:
+    from typing_extensions import ParamSpec
+
 
 T = TypeVar("T", covariant=True)  # Success type
 E = TypeVar("E", covariant=True)  # Error type
 U = TypeVar("U")
 F = TypeVar("F")
+P = ParamSpec("P")
+R = TypeVar("R")
+TBE = TypeVar("TBE", bound=BaseException)
 
 
 class Ok(Generic[T]):
@@ -287,3 +309,43 @@ def result(self) -> Result[Any, Any]:
         Returns the original result.
         """
         return self._result
+
+
+def as_result(
+    *exceptions: Type[TBE],
+) -> Callable[[Callable[P, R]], Callable[P, Result[R, TBE]]]:
+    """
+    Make a decorator to turn a function into one that returns a ``Result``.
+
+    Regular return values are turned into ``Ok(return_value)``. Raised
+    exceptions of the specified exception type(s) are turned into ``Err(exc)``.
+    """
+    # Note: type annotations for signature-preserving decorators via ParamSpec
+    # are currently not fully supported by Mypy 0.930; see
+    # https://github.com/python/mypy/issues/8645
+    #
+    # The ‘type: ignore’ comments below are for our own linting purposes.
+    # Calling code works without errors from Mypy, but will also not be
+    # type-safe, i.e. it will behave as if it is calling a function like
+    # f(*args: Any, **kwargs: Any)
+    if not exceptions or not all(
+        inspect.isclass(exception) and issubclass(exception, BaseException)
+        for exception in exceptions
+    ):
+        raise TypeError("as_result() requires one or more exception types")
+
+    def decorator(f: Callable[P, R]) -> Callable[P, Result[R, TBE]]:
+        """
+        Decorator to turn a function into one that returns a ``Result``.
+        """
+
+        @functools.wraps(f)
+        def wrapper(*args: P.args, **kwargs: P.kwargs) -> Result[R, TBE]:
+            try:
+                return Ok(f(*args, **kwargs))
+            except exceptions as exc:
+                return Err(exc)
+
+        return wrapper
+
+    return decorator

tests/test_result.py

@@ -2,7 +2,7 @@
 
 import pytest
 
-from result import Err, Ok, OkErr, Result, UnwrapError
+from result import Err, Ok, OkErr, Result, UnwrapError, as_result
 
 
 def test_ok_factories() -> None:
@@ -197,3 +197,71 @@ def test_slots() -> None:
         o.some_arbitrary_attribute = 1  # type: ignore[attr-defined]
     with pytest.raises(AttributeError):
         n.some_arbitrary_attribute = 1  # type: ignore[attr-defined]
+
+
+def test_as_result() -> None:
+    """
+    ``as_result()`` turns functions into ones that return a ``Result``.
+    """
+
+    @as_result(ValueError)
+    def good(value: int) -> int:
+        return value
+
+    @as_result(IndexError, ValueError)
+    def bad(value: int) -> int:
+        raise ValueError
+
+    good_result = good(123)
+    bad_result = bad(123)
+
+    assert isinstance(good_result, Ok)
+    assert good_result.unwrap() == 123
+    assert isinstance(bad_result, Err)
+    assert isinstance(bad_result.unwrap_err(), ValueError)
+
+
+def test_as_result_other_exception() -> None:
+    """
+    ``as_result()`` only catches the specified exceptions.
+    """
+
+    @as_result(ValueError)
+    def f() -> int:
+        raise IndexError
+
+    with pytest.raises(IndexError):
+        f()
+
+
+def test_as_result_invalid_usage() -> None:
+    """
+    Invalid use of ``as_result()`` raises reasonable errors.
+    """
+    message = "requires one or more exception types"
+
+    with pytest.raises(TypeError, match=message):
+
+        @as_result()  # No exception types specified
+        def f() -> int:
+            return 1
+
+    with pytest.raises(TypeError, match=message):
+
+        @as_result("not an exception type")  # type: ignore[arg-type]
+        def g() -> int:
+            return 1
+
+
+def test_as_result_type_checking() -> None:
+    """
+    The ``as_result()`` is a signature-preserving decorator.
+    """
+
+    @as_result(ValueError)
+    def f(a: int) -> int:
+        return a
+
+    res: Result[int, ValueError]
+    res = f(123)  # No mypy error here.
+    assert res.ok() == 123