New submission from Dutcho <dut...@ziggo.nl>:
>From Python 3.7, `functools.singledispatch` makes the `register()` attribute
>of the generic function infer the type of the first argument automatically for
>functions annotated with types. That's great for DRY.
However, in 3.7 and 3.8, no check is made that the *first* parameter of the
registered function is actually annotated; *any* annotation suffices, even the
*return* one.
Example:
```
>>> @functools.singledispatch
... def func(arg):...
>>> @func.register
... def _int(arg) -> int:...
>>> @func.register
... def _str(arg) -> str:...
```
No errors happen, although the return type, *not* `arg`, is annotated.
This results in:
```
>>> func.registry
mappingproxy({<class 'object'>: <function func>, <class 'int'>: <function
_int>, <class 'str'>: <function _str>})
```
Obviously, that doesn't dispatch correctly.
Note that un-annotated functions *are* caught:
```
>>> @func.register
... def _no_annotation(arg): ...
Traceback (most recent call last):
...
TypeError: Invalid first argument to `register()`: <function _no_annotation
at 0x000001D769A43D30>. Use either `@register(some_class)` or plain `@register`
on an annotated function.
```
----------
components: Library (Lib)
messages: 367824
nosy: Dutcho
priority: normal
severity: normal
status: open
title: functools.singledispatch doesn't verify annotation is on FIRST parameter
type: behavior
versions: Python 3.7
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue40464>
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