Python 3.10: ranges in pattern matching
Hi everyone! I've just read about pattern matching in Python 3.10, and it sounds really nice. I've also found out that Rust supports ranges in pattern matching. https://doc.rust-lang.org/book/ch18-03-pattern-syntax.html#matching-ranges-o... fn main() { let x = 5; match x { 1..=5 => println!("one through five"), _ => println!("something else"), } } Can we have something similar please? I think this would be a nice addition to Python's pattern matching.
-1. This would require definition of what is a range and which types it support. Defining ranges of user-defined classes doesn't make sense and it would also be hard to implement.
On Thu, May 13, 2021 at 6:38 PM Shreyan Avigyan <pythonshreyan09@gmail.com> wrote:
-1. This would require definition of what is a range and which types it support. Defining ranges of user-defined classes doesn't make sense and it would also be hard to implement.
There are two obvious definitions of a range: a range object, and a pair of inequalities. I'm not sure whether a range object is supported, but it's possible to use a built-in type with a guard as a range check: match thing: case int(n) if 1 <= n < 5: ... Or you can omit the type check and just use the guard: match thing: case n if 1 <= n < 5: ... Whether this is too cumbersome to be useful is up to the OP. (I'm not familiar with Rust so I don't know what "1..=5" means and whether or not each endpoint is included, so if I got the openness of the range wrong, just change whether it's "<=" or "<" as needed.) ChrisA
On Mon, May 17, 2021 at 5:46 PM Shreyan Avigyan <pythonshreyan09@gmail.com> wrote:
What do you mean by "an object in range"? Please be very specific here. I gave two interpretations, both of which make plausible sense within Python, and it's not clear which one you're addressing, or if you're talking about something completely different. What exactly is "range checking" other than a pair of inequalities? ChrisA
Chris:
Range here means that a range or a list. Of What? A list between two numbers, characters, etc. Python interprets numbers, characters as a kind of PyObject. Yes it makes sense to have ranges of int and str but not for others. It doesn't make sense to have ranges for some and not for others. And as you demonstrated there's a current approach to use <= x <= to have ranges only for the required types. The current approach is very good.
On Mon, May 17, 2021 at 08:31:17AM -0000, Shreyan Avigyan wrote:
I'm sorry, I cannot decipher what you mean here. I can't tell whether you are talking about element containment (the `in` operator) or range checking in the sense of `low <= value <= high`. I thought you were talking about classical range checking but then you mentioned lists and you confused me. Especially the comment about "a kind of PyObject" -- all values in Python are objects, so I don't understand why you mention it. Can you explain what happens if you try to match the value 15 against the following "ranges" (match or no match) please? 1. range(2, 20) 3. [2, 20] 3. [2, 20, 15] 4. [None, 'hello'] Now how about the value 12.5 -- will that match? -- Steve
Range here means low <= value <= high. I also don't understand what Valentine suggested and meant by range. I thought Valentine means low..=high as low <= value <= high and meant to say that functionality already exists. I'm really sorry for confusing everyone.
Scala also has "match" cases that are Class redefinable, used for regex for example About integers ranges, we already have the master class : builtins.range pip install funcoperators deals with the issue like this : for i in 1 /irange/ 5: print(i) # will print 1 2 3 4 5 Where irange = infix(lambda a,b: range(a, 1+b)) About matching, switch case does not exist in python because if/elif/else cascade exists if (x := 5) in irange(1, 5): println(stuff1) elif x == 8 println(stuff2) else: println(stuff3) and with factorization : print(stuff1 if (x := 5) in irange(1, 5) else stuff2 if x == 8 else stuff3) Le mer. 12 mai 2021 à 21:41, Valentin Dymchishin <valentindymchishin@ya.ru> a écrit :
On Tue, May 18, 2021 at 8:26 PM Ir. Robert Vanden Eynde <robertve92@gmail.com> wrote:
Hmm, switch statements exist in a lot of other languages that also have if/elif/else. A closer comparison, for a lot of switch blocks, is a dispatch table based on a dictionary. For more complex switching, there's the concept of pattern matching, which has been debated on-and-off for a good while, and is now being added to the language; but the main reason for not needing a classic switch block is probably the dict lookup option. ChrisA
-1. This would require definition of what is a range and which types it support. Defining ranges of user-defined classes doesn't make sense and it would also be hard to implement.
On Thu, May 13, 2021 at 6:38 PM Shreyan Avigyan <pythonshreyan09@gmail.com> wrote:
-1. This would require definition of what is a range and which types it support. Defining ranges of user-defined classes doesn't make sense and it would also be hard to implement.
There are two obvious definitions of a range: a range object, and a pair of inequalities. I'm not sure whether a range object is supported, but it's possible to use a built-in type with a guard as a range check: match thing: case int(n) if 1 <= n < 5: ... Or you can omit the type check and just use the guard: match thing: case n if 1 <= n < 5: ... Whether this is too cumbersome to be useful is up to the OP. (I'm not familiar with Rust so I don't know what "1..=5" means and whether or not each endpoint is included, so if I got the openness of the range wrong, just change whether it's "<=" or "<" as needed.) ChrisA
On Mon, May 17, 2021 at 5:46 PM Shreyan Avigyan <pythonshreyan09@gmail.com> wrote:
What do you mean by "an object in range"? Please be very specific here. I gave two interpretations, both of which make plausible sense within Python, and it's not clear which one you're addressing, or if you're talking about something completely different. What exactly is "range checking" other than a pair of inequalities? ChrisA
Chris:
Range here means that a range or a list. Of What? A list between two numbers, characters, etc. Python interprets numbers, characters as a kind of PyObject. Yes it makes sense to have ranges of int and str but not for others. It doesn't make sense to have ranges for some and not for others. And as you demonstrated there's a current approach to use <= x <= to have ranges only for the required types. The current approach is very good.
On Mon, May 17, 2021 at 08:31:17AM -0000, Shreyan Avigyan wrote:
I'm sorry, I cannot decipher what you mean here. I can't tell whether you are talking about element containment (the `in` operator) or range checking in the sense of `low <= value <= high`. I thought you were talking about classical range checking but then you mentioned lists and you confused me. Especially the comment about "a kind of PyObject" -- all values in Python are objects, so I don't understand why you mention it. Can you explain what happens if you try to match the value 15 against the following "ranges" (match or no match) please? 1. range(2, 20) 3. [2, 20] 3. [2, 20, 15] 4. [None, 'hello'] Now how about the value 12.5 -- will that match? -- Steve
Range here means low <= value <= high. I also don't understand what Valentine suggested and meant by range. I thought Valentine means low..=high as low <= value <= high and meant to say that functionality already exists. I'm really sorry for confusing everyone.
Scala also has "match" cases that are Class redefinable, used for regex for example About integers ranges, we already have the master class : builtins.range pip install funcoperators deals with the issue like this : for i in 1 /irange/ 5: print(i) # will print 1 2 3 4 5 Where irange = infix(lambda a,b: range(a, 1+b)) About matching, switch case does not exist in python because if/elif/else cascade exists if (x := 5) in irange(1, 5): println(stuff1) elif x == 8 println(stuff2) else: println(stuff3) and with factorization : print(stuff1 if (x := 5) in irange(1, 5) else stuff2 if x == 8 else stuff3) Le mer. 12 mai 2021 à 21:41, Valentin Dymchishin <valentindymchishin@ya.ru> a écrit :
On Tue, May 18, 2021 at 8:26 PM Ir. Robert Vanden Eynde <robertve92@gmail.com> wrote:
Hmm, switch statements exist in a lot of other languages that also have if/elif/else. A closer comparison, for a lot of switch blocks, is a dispatch table based on a dictionary. For more complex switching, there's the concept of pattern matching, which has been debated on-and-off for a good while, and is now being added to the language; but the main reason for not needing a classic switch block is probably the dict lookup option. ChrisA
participants (5)
-
Chris Angelico -
Ir. Robert Vanden Eynde -
Shreyan Avigyan -
Steven D'Aprano -
Valentin Dymchishin