On Fri, 10 Jul 2020 at 10:33, Glenn Linderman <v+python@g.nevcal.com> wrote:
On 7/10/2020 1:21 AM, Stefano Borini wrote:
Just my 2 cents, I find it kind of annoying that the whole structure requires two levels of indentation to actually reach the operational code. This would be a first in python.
I would prefer an option akin to if elif elif else where each block is only one level deep. Me too.
That would also sidestep the dilemma of whether else: (if implemented) should be indented like case: or like match: because they would be the same.
match: t case ("rect", real, imag): return complex(real, imag) case ("polar", r, phi): return complex( r* cos(phi), r*sin(phi) else: return None
but it does make the match: block not a statement group, which was disturbing to some.
On the other hand, this has a correspondence to:
try: throw expression except (type of expression) as exc1: blah blah1 except (another type) as exc2: blah blah2 else: blah blah3
The problem of the try...except structure, with less indentation, is that, yes, it is OK for exceptions because normally you have 2 or 3 `except XXX` clauses, therefore it is usually easy to follow, if the number of vertical lines in the entire block of try-catch is low enough. But I have had cases with catching many exception types, each with its own block of 4 or 5 lines, adding up to a block of try-excepts that doesn't even fit in a single window of my editor. In that case, I always have wished for except clauses to be extra indented, to more easily distinguish where the try..except block ends. Therefore, I posit that the style of try...except indentation only works where the number of cases is small. But for the case of pattern matching, I expect the number of cases to be matched to be a lot higher than exception handling cases. Having cases to be matched be indented is, IMHO, a nice visual cue to help the reader understand where the pattern matching block ends.
In fact, one _could_ wrap this whole feature into the try: syntax... the match statement would be tried, and the cases would be special types of exception handlers:
try: match expression case ("rect", real, imag): return complex(real, imag) case ("polar", r, phi): return complex( r* cos(phi), r*sin(phi) else: return None
If the expression could fail to be calculated, one could have a mix of except clauses also to catch those, rather than needing to wrap the whole match expression in a separate try to handle that case [making the nesting even deeper :( ]
There might even be a use for using case clauses to extend "normal" exception handling, where the exception object could be tested for its content as well as its class to have different handling.
try: raise Exception("msg", 35, things) case Exception( x, "widgets"): blah blah 1 case Exception( x, "characters"): blah blah 2 else: blah blah 3
In this not-fully-thought-through scenario, maybe the keyword match isn't even needed: "raise expression" could do the job, or they could be aliases to signify intent.
In other words, a match expression would always "fail". The only mismatch here is that it points out the difference between try-else and match-else: try-else is executed if there is no failure, but if match always fails, else would never be appropriate, and case _: would be.
In any case, it does seem there is a strong correlation between match processing and try processing, that I didn't see during other discussions of the possible structural similarities. "match 3 / 0:" would clearly need to be wrapped in a try:
try: match x / y: case 43: print("wow, it is 43") case 22: print("22 seemed less likely than 43 for some reason") case _: print("You get what you get") except ZeroDivisionError as exc: print(f"But sometimes you get an exception {exc}")
or:
try: raise x / y case 43: print("wow, it is 43") case 22: print("22 seemed less likely than 43 for some reason") case exc := ZeroDivisionError: print(f"But sometimes you get an exception: {exc}") case _: print("You get what you get") _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/GDP2KKB3... Code of Conduct: http://python.org/psf/codeofconduct/
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