Am 09.11.21 um 10:50 schrieb Chris Angelico:
On Tue, Nov 9, 2021 at 8:38 PM Sebastian Rittau <srit...@rittau.biz> wrote:
Currently, Python doesn't allow non-default arguments after default
arguments:
>>> def foo(x=None, y): pass
File "<stdin>", line 1
def foo(x=None, y): pass
^
SyntaxError: non-default argument follows default argument
I believe that at the time this was introduced, no use cases for this
were known and this is is supposed to prevent a source of bugs. I have
two use cases for this, one fringe, but valid, the other more important:
The fringe use case: Suppose you have a function that takes a 2D
coordinate value as separate "x" and "y" arguments. The "x" argument is
optional, the "y" argument isn't. Currently there are two ways to do
this, none of them particularly great:
def foo(y, x): # reverse x and y arguments, confusing
...
def foo(x, y=None): # Treat the x argument as y if only one argument is
provided
if y is None:
x, y = y, x
...
To me, the "natural" solution looks like this:
def foo(x=None, y): ...
# Called like this:
foo(1, 2)
foo(y=2)
This could also be useful when evolving APIs. For example there is a
function "bar" that takes two required arguments. In a later version,
the first argument gains a useful default, the second doesn't. There is
no sensible way to evolve the API at the moment.
What would this mean, though:
foo(2)
Is that legal?
No. This would be equal to foo(x=2) (same as now), meaning the required
argument "y" is missing.
I would instead recommend making the parameters keyword-only, which
would allow any of them to have defaults or not have defaults. In
terms of useful API design, this is usually more helpful than having
an early parameter omitted.
This might be better API design (although I don't think Python should be
opinionated about this outside the standard library), but this still
leaves the API change example and the very real problem of @overloads
unsolved.
- Sebastian
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