I've taken a look through PEP 622 and I've been thinking about how it could be used with sympy.
In principle case/match and destructuring should be useful for sympy because sympy has a class Basic which defines a common structure for ~1000 subclasses. There are a lot of places where it is necessary to dispatch on the type of some object including in places that are performance sensitive so those would seem like good candidates for case/match. However the PEP doesn't quite seem as I hoped because it only handles positional arguments indirectly and it does not seem to directly handle types with variadic positional args.
The objects I refer to in sympy represent mathematical expressions e.g.:
from sympy import * x, y = symbols('x, y') expr = x**2 + 2*x*y expr
x**2 + 2*x*y
You can see the structure of the object explicitly using sympy's srepr function:
Add(Pow(Symbol('x'), Integer(2)), Mul(Integer(2), Symbol('x'), Symbol('y')))
There are a bunch of classes there (Add, Pow, Symbol, Mul, Integer) but these are a tiny subset of the possibilities. The key feature of Basic instances is that they have an .args attribute which can be used to rebuild the object like:
x**2 + 2*x*y
type(expr)(*expr.args) == expr
This is known as the func-args invariant in sympy and is used to destructure and rebuild the expression tree in different ways e.g. for performing a substitution:
10*y + 25
All Basic classes are strictly constructed using positional only arguments and not keyword arguments. In the PEP it seems that we can handle positional arguments when their number is fixed by the type. For example a simplified version of Pow could be:
def __init__(self, base, exp): self.args = (base, exp)
__match_args__ == ("base", "exp")
@property def base(self): return self.args
@property def exp(self): return self.args
Then I could match Pow in case/match with
obj = Pow(Symbol('x'), Integer(4))
match obj: case Pow(base, exp): # do stuff with base, exp
It seems awkward and inefficient though to go through __match_args__ and the base and exp property-methods to match the positional arguments when they are already available as a tuple in obj.args. Note that performance is a concern: just dispatching on isinstance() has a measurable overhead in sympy code which is almost always CPU-bound.
The main problem though is with variadic positional arguments. For example sympy has a symbolic Tuple class which is much like a regular python tuple except that it takes multiple positional args rather than a single iterable arg:
class Tuple: def __init__(self, *args): self.args = args
So now how do I match a 2-Tuple of two integers? I can't use __match_args__ because that's a class attribute and different instances have different numbers of args. It seems I can do this:
obj = Tuple(2, 4)
match obj: case Tuple(args=(2, 4)):
That's awkward though because it doesn't match the constructor syntax which strictly uses positional-only args. It also doesn't scale well with nesting:
obj = Tuple(Tuple(1, 2), Tuple(3, 4))
match obj: case Tuple(args=(Tuple(args=(1, 2)), Tuple(args=(3, 4))): # handle ((1, 2), (3, 4)) case
Another option would be to fake a single positional argument for matching purposes:
class Tuple: __match_args__ == ("args",) def __init__(self, *args): self.args = args
match obj: case Tuple((Tuple((1, 2)), Tuple((3, 4)))):
This requires an extra level of brackets for each node and also doesn't match the actual constructor syntax: evaluating that pattern in sympy turns each Tuple into a 1-Tuple containing another Tuple of the args:
t = Tuple((Tuple((1, 2)), Tuple((3, 4)))) print(srepr(t))
Tuple(Tuple(Tuple(Tuple(Integer(1), Integer(2))), Tuple(Tuple(Integer(3), Integer(4)))))
I've used Tuple in the examples above but the same applies to all variadic Basic classes: Add, Mul, And, Or, FiniteSet, Union, Intersection, ProductSet, ...
From a first glimpse of the proposal I thought I could do matches like this:
match obj: case Add(Mul(x, y), Mul(z, t)) if y == t: case Add(*terms): case Mul(coeff, *factors): case And(Or(A, B), Or(C, D)) if B == D: case Union(Interval(x1, y1), Interval(x2, y2)) if y1 == x2: case Union(Interval(x, y), FiniteSet(*p)) | Union(FiniteSet(*p), Interval(x, y)): case Union(*sets):
Knowing the sympy codebase each of those patterns would look quite natural because they resemble the constructors for the corresponding objects (as intended in the PEP). It seems instead that many of these constructors would need to have args= so it becomes:
match obj: case Add(args=(Mul(args=(x, y)), Mul(args=(z, t)))) if y == t: case Add(args=terms): case Mul(args=(coeff, *factors)): case And(args=(Or(args=(A, B)), Or(args=(C, D)))) if C == D: case Union(args=(Interval(x1, y1), Interval(x2, y2))) if y1 == x2: case Union(args=(Interval(x, y), FiniteSet(args=p))) | Union(args=(FiniteSet(args=p), Interval(x, y))): case Union(args=sets):
Each of these looks less natural as they don't match the constructors and the syntax gets messier with nesting.