Just a pointer related to this, typeguard is abandoned. kttps://github.com/agronholm/typeguard/issues/198

On 16 Oct 2021, at 06:13, Steven D'Aprano <steve@pearwood.info> wrote: Be careful about believing what you are told.

Indeed, MyPy will correctly raise errors if you assign {None: []} to a variable annotated with dict[str, Number]. However, you'll find that MyPy also raises an error if you assign {'foo': 4} to a variable annotated with dict[str, Number]: https://mypy-play.net/?mypy=latest&python=3.10&gist=de7faea3b4d6a0ffefc64acf3bfa7b77 The issue isn't a matter of false negatives from Mypy; it's a matter of false positives. For more, see the (rather long — sorry!) answer I gave Sebastian to a previous question of his on Stack Overflow: https://stackoverflow.com/questions/69334475/how-to-hint-at-number-types-i-e... Having said that, I absolutely agree that type-hinting doesn't seem to have much to do with this specific problem, and that a UserDict subclass is probably the way to go here. Best, Alex

On 16 Oct 2021, at 06:13, Steven D'Aprano <steve@pearwood.info> wrote:

On Fri, Oct 15, 2021 at 06:17:12PM +0200, Sebastian M. Ernst wrote:

...

Data = Dict[str, Number]

@typechecked def foo(bar: Data): print(bar) ```

Yes, this is using run-time checks (typeguard), which works just fine. Only strings as keys and Number objects as values are going through. (I was told that MyPy does not get this at the moment.)

Be careful about believing what you are told.

[steve ~]$ cat test.py from numbers import Number from typing import Dict Data = Dict[str, Number] a: Data = {None: []}

[steve ~]$ mypy test.py test.py:4: error: Dict entry 0 has incompatible type "None": "List[<nothing>]"; expected "str": "Number" Found 1 error in 1 file (checked 1 source file)

...

The issue is that `bar` itself still allows "everything" to go in (and out):

```python @typechecked def foo2(bar: Data): bar[1.0] = b'should not be allowed' ```

The Python interpreter intentionally doesn't know or care about typing annotations. The interpreter should absolutely allow that.

However mypy correctly flags that assignment as a type error.

How about typeguard? That's for the typeguard developers to answer, but my guess is that they will say that the typechecked decorator can enforce type checking of the input parameters and perhaps even the return result, but there is nothing they can do about what happens inside the body of the function.

...

PEP 589 introduces typed dictionaries, but for a fixed set of predefined keys (similar to struct-like constructs in other languages). In contrast, I am looking for an arbitrary number of typed keys/value pairs.

Something like this?

from numbers import Number from collections import UserDict

class RestrictedDict(UserDict): def __setitem__(self, key, value): if not isinstance(key, str): raise TypeError('key must be a string') if not isinstance(value, Number): raise TypeError('value must be a number') super().__setitem__(key, value)

(I think that is the only method that needs to be overloaded.)

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