Guido van Rossum wrote:
> I'm happy to present a new PEP for the python-dev community to review. This
> is joint work with Brandt Bucher, Tobias Kohn, Ivan Levkivskyi and Talin.
> Many people have thought about extending Python with a form of pattern
> matching similar to that found in Scala, Rust, F#, Haskell and other
> languages with a functional flavor. The topic has come up regularly on
> python-ideas (most recently yesterday :-).
> I'll mostly let the PEP speak for itself:
>
> Published: https://www.python.org/dev/peps/pep-0622/
> (*)
> Source: https://github.com/python/peps/blob/master/pep-0622.rst
>
> (*) The published version will hopefully be available soon.
> I want to clarify that the design space for such a match statement is
> enormous. For many key decisions the authors have clashed, in some cases we
> have gone back and forth several times, and a few uncomfortable compromises
> were struck. It is quite possible that some major design decisions will
> have to be revisited before this PEP can be accepted. Nevertheless, we're
> happy with the current proposal, and we have provided ample discussion in
> the PEP under the headings of Rejected Ideas and Deferred Ideas. Please
> read those before proposing changes!
> I'd like to end with the contents of the README of the repo where we've
> worked on the draft, which is shorter and gives a gentler introduction than
> the PEP itself:
> # Pattern Matching
> This repo contains a draft PEP proposing a match statement.
> Origins
> The work has several origins:
>
> Many statically compiled languages (especially functional ones) have
> a match expression, for example
> Scala,
> Rust,
> F#;
> Several extensive discussions on python-ideas, culminating in a
> summarizing
> blog
> post
> by Tobias Kohn;
> An independently developed draft
> PEP
> by Ivan Levkivskyi.
>
> Implementation
> A full reference implementation written by Brandt Bucher is available
> as a fork) of
> the CPython repo. This is readily converted to a pull
> request).
> Examples
> Some example
> code is available
> from this repo.
> Tutorial
> A match statement takes an expression and compares it to successive
> patterns given as one or more case blocks. This is superficially
> similar to a switch statement in C, Java or JavaScript (an many
> other languages), but much more powerful.
> The simplest form compares a target value against one or more literals:
> def http_error(status):
> match status:
> case 400:
> return "Bad request"
> case 401:
> return "Unauthorized"
> case 403:
> return "Forbidden"
> case 404:
> return "Not found"
> case 418:
> return "I'm a teapot"
> case _:
> return "Something else"
>
> Note the last block: the "variable name" _ acts as a wildcard
> and
> never fails to match.
> You can combine several literals in a single pattern using | ("or"):
> case 401|403|404:
> return "Not allowed"
>
> Patterns can look like unpacking assignments, and can be used to bind
> variables:
> # The target is an (x, y) tuple
> match point:
> case (0, 0):
> print("Origin")
> case (0, y):
> print(f"Y={y}")
> case (x, 0):
> print(f"X={x}")
> case (x, y):
> print(f"X={x}, Y={y}")
> case _:
> raise ValueError("Not a point")
>
> Study that one carefully! The first pattern has two literals, and can
> be thought of as an extension of the literal pattern shown above. But
> the next two patterns combine a literal and a variable, and the
> variable is extracted from the target value (point). The fourth
> pattern is a double extraction, which makes it conceptually similar to
> the unpacking assignment (x, y) = point.
> If you are using classes to structure your data (e.g. data classes)
> you can use the class name followed by an argument list resembling a
> constructor, but with the ability to extract variables:
> from dataclasses import dataclass
>
> @dataclass
> class Point:
> x: int
> y: int
>
> def whereis(point):
> match point:
> case Point(0, 0):
> print("Origin")
> case Point(0, y):
> print(f"Y={y}")
> case Point(x, 0):
> print(f"X={x}")
> case Point():
> print("Somewhere else")
> case _:
> print("Not a point")
>
> We can use keyword parameters too. The following patterns are all
> equivalent (and all bind the y attribute to the var
> variable):
> Point(1, var)
> Point(1, y=var)
> Point(x=1, y=var)
> Point(y=var, x=1)
>
> Patterns can be arbitrarily nested. For example, if we have a short
> list of points, we could match it like this:
> match points:
> case []:
> print("No points")
> case [Point(0, 0)]:
> print("The origin")
> case [Point(x, y)]:
> print(f"Single point {x}, {y}")
> case [Point(0, y1), Point(0, y2)]:
> print(f"Two on the Y axis at {y1}, {y2}")
> case _:
> print("Something else")
>
> We can add an if clause to a pattern, known as a "guard". If the
> guard is false, match goes on to try the next case block. Note
> that variable extraction happens before the guard is evaluated:
> match point:
> case Point(x, y) if x == y:
> print(f"Y=X at {x}")
> case Point(x, y):
> print(f"Not on the diagonal")
>
> Several other key features:
>
> Like unpacking assignments, tuple and list patterns have exactly the
> same meaning and actually match arbitrary sequences. An important
> exception is that they don't match iterators or strings.
> (Technically, the target must be an instance of
> collections.abc.Sequence.)
>
> Sequence patterns support wildcards: [x, y, *rest] and (x, y,
> *rest) work similar to wildcards in unpacking assignments. The
> name after * may also be _, so (x, y, *_) matches a
> sequence
> of at least two items without binding the remaining items.
>
> Mapping patterns: {"bandwidth": b, "latency": l} extracts the
> "bandwidth" and "latency" values from a dict. Unlike sequence
> patterns, extra keys are ignored. A wildcard **rest is also
> supported. (But **_ would be redundant, so it not allowed.)
>
> Subpatterns may be extracted using the walrus (:=) operator:
> case (Point(x1, y1), p2 := Point(x2, y2)): ...
>
>
> Patterns may use named constants. These must be dotted names; a
> single name can be made into a constant value by prefixing it with a
> dot to prevent it from being interpreted as a variable extraction:
> RED, GREEN, BLUE = 0, 1, 2
>
> match color:
> case .RED:
> print("I see red!")
> case .GREEN:
> print("Grass is green")
> case .BLUE:
> print("I'm feeling the blues :(")
>
>
> Classes can customize how they are matched by defining a
> __match__() method.
> Read the PEP
> for details.
Wow, I totally didn't see this coming, not after seeing what seems like a lot
of rejected ideas on this topic (there was at least one PEP already that
proposed this, right?). I have to admire the authors' determination to write
such a lengthy and (from skimming it) complex and comprehensive proposal *and*
providing a reference implementation on top of that, the amount of work
(including internal bikeshedding) must've been substantial.
Needless to say it's +1 from my humble person, big time, and I wouldn't want
the comment below to detract from that.
So, now for the one thing that makes me unhappy: the rejected idea to make it
an expression. In my short experience with pattern matching, mainly in Rust,
roughly half (very vague estimate) of its usefulness came from it being an
expression. It's even small things like
let i = match i {
9 => 10,
10 => 9,
_ => i,
};
and
let mut file: Box<Write> = match filename.as_ref() {
"-" => Box::new(io::stdout()),
_ => Box::new(File::create(filename).expect("Cannot open file for
writing")),
};
and it adds up. I'm not sure how to approach this with Python syntax and I'll
think about this, but I feel that it'd be a huge missed opportunity to not have
this.
Jakub
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