Some background: the reason we decided not to do this is because cases are
allowed to omit positional sub-patterns when matching. For example, it would be
possible to write this:
```
>>> match n:
... case complex(42j):
... ...
...
```
Read that carefully! It would match any complex number with a *real* part equal
to 42j. Which is to say, it would actually match *nothing*.
I personally prefer the (admittedly wordier) status quo:
```
>>> match n:
... case complex(real=r, imag=i):
... ...
...
```
The problem with slice is much bigger, though: the expected attributes actually
change depending on how many "args" there are, so a static mapping just isn't
possible. Again, the status quo is much more explicit, for better or worse:
```
>>> match i:
... case slice(start=lo, stop=hi, step=by):
... pass
...
```
At one point, the original pattern matching proposal (and implementation)
included support for a `__match_args_required__` attribute that could
optionally be provided alongside `__match_args__`. With that, you could then
set `complex.__match_args__ = ("real", "imag")` and
`complex.__match_args_required__ = 2`, and get an error like:
```
>>> match n:
... case complex(42j):
... ...
...
TypeError: complex() requires 2 positional sub-patterns (1 given)
```
Similarly, you could set `slice.__match_args__ = ("start", "stop", "step")` and
`slice.__match_args_required__ = 2`, and get an error like:
```
>>> match i:
... case slice(stop):
... ...
...
TypeError: slice() requires at least 2 positional sub-patterns (1 given)
```
We eventually dropped `__match_args_required__` to simplify the proposal,
though. For anyone curious, all of our discussions during this period are
archived in the issue tracker at: https://github.com/gvanrossum/patma/issues.
Brandt
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