[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, 18 Jan 2021 12:41:45 -0800 Chris Barker via Python-Dev wrote: > > And in "real world" code, I've done just this -- building a system for > saving / restoring dataclasses to/from JSON. In that case, each of the > dataclasses knows how to save itself and build itself from JSON-compatible > python objects (numbers, dicts, strings, lists) -- so again, no need for > pattern matching there either. And what I really like about the approach of > putting all the logic in the "nodes" is that I can make new types of nodes > without having to touch the code at the "top" that visits those nodes. Note that another approach is the little-used `functools.singledispatch`. Regards Antoine. ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/PD5PIBOLSHL3I7TC7XDCEDKRPRHAJPEY/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, Nov 23, 2020 at 8:20 AM Brian Coleman
wrote:
> Take as an example a function designed to process a tree of nodes similar
> to that which might be output by a JSON parser. There are 4 types of node:
>
> - A node representing JSON strings
> - A node representing JSON numbers
> - A node representing JSON arrays
> - A node representing JSON dictionaries
>
> The function transforms a tree of nodes, beginning at the root node, and
> proceeding recursively through each child node in turn. The result is a
> Python object, with the following transformation applied to each node type:
>
> - A JSON string `->` Python `str`
> - A JSON number `->` Python `float`
> - A JSON array `->` Python `list`
> - A JSON dictionary `->` Python `dict`
>
> I have implemented this function using 3 different approaches:
>
> - The visitor pattern
> - `isinstance` checks against the node type
> - Pattern matching
>
I've always thought that the alternative to a "switch case" construct in
Python (and I suppose most OO languages) is subclassing and method
overriding. I guess that's what the "visitor pattern" is here, but it seems
to be adding a bunch of unnecessary bolierplate.
So: given that you have a special "Node" object anyway, the thing to do is
to have those Node object know how to unpack themselves. Then the top
"traverse the tree" function becomes a single method or attribute access:
tree = node_tree.value
here's what the Nodes look like in this example:
class Node:
def __init__(self, val):
self._value = val
@property
def value(self):
return self._value
class StringNode(Node):
pass
class NumberNode(Node):
pass
class ListNode(Node):
@property
def value(self):
return [item.value for item in self._value]
class DictNode(Node):
@property
def value(self):
return {k: item.value for k, item in self._value.items()}
Of course, this requires that you have control of the Node objects, rather
than getting them from some other library -- but that seems to be what all
the examples here are anyway.
If you do need to parse out a tree of object that are not "special"
already, then you need to do some type of pattern matching / isinstance
checking. In this case, I wrote a function that builds up a tree of Nodes
from arbitrary Python objects:
def make_nodes_from_obj(obj):
if isinstance(obj, str):
return StringNode(obj)
if isinstance(obj, Real):
return NumberNode(obj)
if isinstance(obj, Sequence):
return ListNode([make_nodes_from_obj(item) for item in obj])
if isinstance(obj, Mapping):
return DictNode({k: make_nodes_from_obj(item)
for k, item in obj.items()})
And that could benefit from pattern matching, I suppose, though it's not
very compelling to me.
And in "real world" code, I've done just this -- building a system for
saving / restoring dataclasses to/from JSON. In that case, each of the
dataclasses knows how to save itself and build itself from JSON-compatible
python objects (numbers, dicts, strings, lists) -- so again, no need for
pattern matching there either. And what I really like about the approach of
putting all the logic in the "nodes" is that I can make new types of nodes
without having to touch the code at the "top" that visits those nodes.
In short -- I'm still looking for a more compelling example :-)
-CHB
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
NOAA/NOS/OR(206) 526-6959 voice
7600 Sand Point Way NE (206) 526-6329 fax
Seattle, WA 98115 (206) 526-6317 main reception
chris.bar...@noaa.gov
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
David Mertz wrote:
> On Mon, Nov 23, 2020 at 9:02 PM Brian Coleman brianfcole...@gmail.com
> wrote:
> > Basically, I
> > agree matching/destructuring is a powerful idea. But I also
> > wonder how much genuinely better it is than a library that does not
> > require
> > a language change. For example, I could create a library to allow this:
> > m = Matcher(arbitrary_expression)
> > if m.case("StringNode(s)"):
> > process_string(m.val.s)
> > elif m.case("[a, 5, 6, b]"):
> > process_two_free_vars(m.val.a, m.val.b)
> > What you are proposing here is very similar in effect to executing pattern
> > matching statements using eval. What is the advantage of implementing the
> > pattern matching functionality in a library if the user interface for that
> > functionality has the same pitfalls as eval?
> > I don't understand the similarity with eval that you are
> suggesting.
> My hypothetical library would store the value passed as initializer to
> Matcher, and provide a method .case(). That method would need
> to do
> some moderately complicated parsing of the pattern passed into it, but
> using parsing techniques, not any eval() call. Btw. I modified my above
> strawman just slightly to what might be a bit better API.
> If there are any free variables in the pattern, they would be provided by
> the Matcher object. For example, they could be attributes of the property
> m.val. Or I suppose we could make them attributes of the Matcher object
> itself, e.g. m.a and m.b, but I think name conflicts with e.g.
> m.case. Anyway, it's a strawman not an API design.
> If the pattern looked kinda like a class constructor (i.e. exactly the same
> as PEP 634 rules), the .case() method would do an isinstance()
> check
> before doing its other stuff. If the pattern looks like a list, it would
> scan the freevars inside it and match the constant values. And so on.
> The actual return value from .m.case(...) would be True or False (or at
> least something truthy or falsy). However, it MIGHT create some new
> attributes (or keys, or something else) on the Matcher object if the
> pattern actually matched.
> I agree the above is slightly less readable than PEP 635 syntax, but it
> seems to have the entire power of the proposal without changing Python
> syntax.
Because syntax errors in the patterns passed to the `case` method are detected
at runtime, rather than at compile time, it is necessary to ensure that you
have code coverage of every call to a `case` method to be confident that there
are no syntax errors in the patterns.
By comparison, if the pattern matching syntax is built into the language, the
compiler will detect syntax errors in any and all patterns in `case` clauses. I
think that this is a major advantage as compared to implementing pattern
matching via a library.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
Hi Eric,
On 23/11/2020 9:32 pm, Eric V. Smith wrote:
On 11/23/2020 3:44 PM, David Mertz wrote:
I have a little bit of skepticism about the pattern matching syntax,
for similar reasons to those Larry expresses, and that Steve Dower
mentioned on Discourse.
Basically, I agree matching/destructuring is a powerful idea. But I
also wonder how much genuinely better it is than a library that does
not require a language change. For example, I could create a library
to allow this:
m = Matcher(arbitrary_expression)
if m.case("StringNode(s)"):
process_string(m.val)
elif m.case("[a, 5, 6, b]"):
process_two_free_vars(*m.values)
elif m.case("PairNone(a, b)"):
a, b = m.values
process_pair(a, b)
elif m.case("DictNode"):
foo = {key, process_node(child_node) for key, child_node in
m.values.items()}
I don't disagree that the pattern mini-language looks nice as syntax.
But there's nothing about that mini-language that couldn't be put in a
library (with the caveat that patterns would need to be quoted in some
way).
I just commented on Steve's post over on Discourse. The problem with
this is that the called function (m.case, here) needs to have access to
the caller's namespace in order to resolve the expressions, such as
StringNode and PairNone. This is one of the reasons f-strings weren't
implemented as a function, and is also the source of many headaches with
string type annotations.
My conclusion is that if you want something that operates on DSLs
(especially ones that can't be evaluated as expressions), the compiler
is going to need to know about it somehow so it can help you with it. I
wish there were a general-purpose mechanism for this. Maybe it's PEP
638, although I haven't really investigated it much, and pattern
matching might be a bad fit for it.
Hygienic macros (PEP 638) solve two problems with a string based library
(in my obviously biased opinion).
1. The pattern is parsed by the normal parser, so must have correct
syntax, and the contents are visible to IDEs and editors.
if m.case("StringNode(s)"): the pattern is just a string.
case!(StringNode(s)): the pattern is validated Python syntax.
2. The transformation is done at compile time, so the generated code
will execute in the correct context. Basically, the macro generates the
correct series of if/elifs for you.
Cheers,
Mark.
Eric
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/2020 4:49 PM, David Mertz wrote:
On Mon, Nov 23, 2020, 4:32 PM Eric V. Smith
I just commented on Steve's post over on Discourse. The problem
with this is that the called function (m.case, here) needs to have
access to the caller's namespace in order to resolve the
expressions, such as StringNode and PairNone. This is one of the
reasons f-strings weren't implemented as a function, and is also
the source of many headaches with string type annotations.
Is this really true though? Yes, it would require magic of the sort
zero-argument `super()` does. But can't the Matcher constructor
capture the locals one step up the stack on instance creation? That's
largely why my strawman version is slightly different from Steve's
strawman.
Beyond the issue of stack access being frowned on, there are some
practical problems. One that's given in PEP 498 is closures accessing
variables that aren't otherwise referenced:
def outer(x):
... def inner():
... return 'x={x}'.format_map(locals())
... return inner
...
outer(42)()
Traceback (most recent call last):
File "", line 1, in
File "", line 3, in inner
KeyError: 'x'
Whereas an f-string in that scenario does work:
>>> def outer(x):
... def inner():
... return f'x={x}'
... return inner
...
>>> outer(42)()
'x=42'
I'd put the question this way: assuming Matcher can be written (with a
bit of stack magic), and assuming that the strings inside m.case()
calls are exactly the same mini-languague PEP 634 proposes, would you
want a syntax change?
No, I wouldn't!
Something that gets brought up every now and again is: what if there
were a way to pass an AST to a function, such that it received the AST
instead of the evaluated value for a parameter. Let's say that backticks
(`) meant "compute the AST of the enclosed expression", and that was
passed it to the function. (I always choose backticks for this example
because we all know it isn't going to happen.)
Then you write your original example using backticks instead of quotes,
and m.case would get an AST it could inspect. It would probably still
need some help in evaluating the AST nodes in the caller's context, but
at least it would be theoretically possible to do so with the compiler's
assistance.
Another option would be the function itself saying "give me an AST
instead of evaluating a particular parameter". But that's all but
impossible, since the compiler couldn't look at the called function to
know it wants to do that.
I think we're in python-ideas land now.
Eric
That's not rhetorical, I'm of mixed feeling myself.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/2020 3:44 PM, David Mertz wrote:
I have a little bit of skepticism about the pattern matching syntax,
for similar reasons to those Larry expresses, and that Steve Dower
mentioned on Discourse.
Basically, I agree matching/destructuring is a powerful idea. But I
also wonder how much genuinely better it is than a library that does
not require a language change. For example, I could create a library
to allow this:
m = Matcher(arbitrary_expression)
if m.case("StringNode(s)"):
process_string(m.val)
elif m.case("[a, 5, 6, b]"):
process_two_free_vars(*m.values)
elif m.case("PairNone(a, b)"):
a, b = m.values
process_pair(a, b)
elif m.case("DictNode"):
foo = {key, process_node(child_node) for key, child_node in
m.values.items()}
I don't disagree that the pattern mini-language looks nice as syntax.
But there's nothing about that mini-language that couldn't be put in a
library (with the caveat that patterns would need to be quoted in some
way).
I just commented on Steve's post over on Discourse. The problem with
this is that the called function (m.case, here) needs to have access to
the caller's namespace in order to resolve the expressions, such as
StringNode and PairNone. This is one of the reasons f-strings weren't
implemented as a function, and is also the source of many headaches with
string type annotations.
My conclusion is that if you want something that operates on DSLs
(especially ones that can't be evaluated as expressions), the compiler
is going to need to know about it somehow so it can help you with it. I
wish there were a general-purpose mechanism for this. Maybe it's PEP
638, although I haven't really investigated it much, and pattern
matching might be a bad fit for it.
Eric
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/2020 5:44 PM, David Mertz wrote: I'd put the question this way: assuming Matcher can be written (with a bit of stack magic), and assuming that the strings inside m.case() calls are exactly the same mini-languague PEP 634 proposes, would you want a syntax change? No, I wouldn't! Is one of us mixing up negations? Are you saying you do NOT support PEP 634 (i.e. syntax change)? A reasonable enough opinion that several core developers hold. Just trying to understand. Sorry I wasn't clear. I wouldn't want a syntax change specific to matching if it could be done with a library. I just don't think it can be done with a library without other language changes. But I think those other language changes could be used in ways outside of just matching. Eric ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/B3ARZOBDOK7TGEGSJ4VI745ISXOGYDI2/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
> > I'd put the question this way: assuming Matcher can be written (with a bit > of stack magic), and assuming that the strings inside m.case() calls are > exactly the same mini-languague PEP 634 proposes, would you want a syntax > change? > > No, I wouldn't! > Is one of us mixing up negations? Are you saying you do NOT support PEP 634 (i.e. syntax change)? A reasonable enough opinion that several core developers hold. Just trying to understand. ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/RFWLULNGRNEYSAGQ2KKAT3ZUFBEU2BFV/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 24/11/20 9:44 am, David Mertz wrote:
m = Matcher(arbitrary_expression)
if m.case("StringNode(s)"):
process_string(m.val)
elif m.case("[a, 5, 6, b]"):
process_two_free_vars(*m.values)
elif m.case("PairNone(a, b)"):
a, b = m.values
process_pair(a, b)
elif m.case("DictNode"):
foo = {key, process_node(child_node) for key, child_node in
m.values.items()}
How does the Matcher object know the meanings of "StringNode",
"PairNode", etc.? Presumably they're bound to classes in the
namespace of the caller, but the Matcher doesn't know anything
about that namespace.
--
Greg
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 24/11/20 9:31 am, Brian Coleman wrote: In my opinion, the object oriented solution to this problem is to use the visitor pattern. Which is a good thing only if you believe that OO solutions are always better than non-OO ones. IMO, the visitor pattern is a workaround for when your language doesn't permit introspection on types. It's unnecessary in Python and better avoided. -- Greg ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/XZ2EKVMQXZ5DEV5TWLBKGNU5JWFMIX4Q/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/20 1:49 PM, David Mertz wrote: On Mon, Nov 23, 2020, 4:32 PM Eric V. Smith I just commented on Steve's post over on Discourse. The problem with this is that the called function (m.case, here) needs to have access to the caller's namespace in order to resolve the expressions, such as StringNode and PairNone. This is one of the reasons f-strings weren't implemented as a function, and is also the source of many headaches with string type annotations. Is this really true though? Yes, it would require magic of the sort zero-argument `super()` does. But can't the Matcher constructor capture the locals one step up the stack on instance creation? That's largely why my strawman version is slightly different from Steve's strawman. I'd put the question this way: assuming Matcher can be written (with a bit of stack magic), and assuming that the strings inside m.case() calls are exactly the same mini-languague PEP 634 proposes, would you want a syntax change? That's not rhetorical, I'm of mixed feeling myself. I would, yes. Writing Python code in strings to be processed by another function/library is a pain: - no syntax highlighting because the code is in a string - no syntax checking because the code is in a string - lots of quotes because the code is in a string All in all, it feels very inelegant to me. -- ~Ethan~ ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/VPG7WYZ35QFR7X2OA73ET3IM5C4HGHMV/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, Nov 23, 2020 at 08:44:21PM +, David Mertz wrote:
> Basically, I agree matching/destructuring is a powerful idea. But I also
> wonder how much genuinely better it is than a library that does not require
> a language change. For example, I could create a library to allow this:
>
> m = Matcher(arbitrary_expression)
> if m.case("StringNode(s)"):
> process_string(m.val)
> elif m.case("[a, 5, 6, b]"):
> process_two_free_vars(*m.values)
> elif m.case("PairNone(a, b)"):
> a, b = m.values
> process_pair(a, b)
> elif m.case("DictNode"):
> foo = {key, process_node(child_node) for key, child_node in
> m.values.items()}
Look at all those strings. So much for syntax highlighting and
compile-time syntax checking. Who needs them anyway?
# Perfectly legal syntax.
if m.case("StringNode[s)"):
...
elif m.case("[a, 5 6, b"):
...
It's a good thing we already have comprehensions, because if we didn't
have them, people would argue that they aren't necessary, we can just
write a `comprehension` function that takes the loop expression as a
string:
# [(a + int(''.join(b)))*c) for a, b, c in items]
comprehension("(a + int(''.join(b)))*c)", items]
Imagine how silly we would be to dedicate actual syntax to
comprehensions, when we can write a library to do it. It's all this
syntactic sugar (comprehensions, decorators, f-strings, async, with
statements, etc) that is killing Python.
*wink*
> I don't disagree that the pattern mini-language looks nice as syntax. But
> there's nothing about that mini-language that couldn't be put in a library
> (with the caveat that patterns would need to be quoted in some way).
One thing that we get with pattern matching syntax is the absense of
certain *misfeatures*.
# Oops, I forgot to use `elif`, now I have fall-through semantics
# which is widely agreed to be a bad idea.
m = Matcher([4, 5, 6, 7])
if m.case("[a, 5, 6, b]"):
print("first case")
if m.case("[4, a, b, 7]"):
print("second case")
As the library author, I hope you are prepared for many, many bug
reports about why people's code behaves differently if they use `if`
compared to `elif`.
Another misfeature: the ability to scatter your pattern matching cases
all over the place.
m = Matcher(expression)
do_this()
do_that()
if m.case(something):
process("this case")
do_another_thing()
class Spam:
# snip five pages of code
if m.case(something_else):
print("Did you forget we were inside a match pseudo-block?")
Objection: "But coders won't do that!"
No, *sensible* coders won't do that. With pattern matching syntax, even
the other sort of coder can't do that.
Objection: "But they could put five pages of code inside a case block
too."
True, but only if it is *conditional* to that specific case. You can't
mix unconditional code and cases. And the extra indentation will hint
that something odd is going on.
Another misfeature: the ability to modify the value being matched in the
middle of the pattern matching pseudo-block.
m = Matcher(something)
if m.case(spam):
process("case 1")
m = Matcher(another_thing)
if m.case(eggs):
process("case 2")
People can write obfuscated, confusing, poor-quality code with anything,
but syntax can limit their opportunities to do so.
--
Steve
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, Nov 23, 2020, 4:32 PM Eric V. Smith > I just commented on Steve's post over on Discourse. The problem with this > is that the called function (m.case, here) needs to have access to the > caller's namespace in order to resolve the expressions, such as StringNode > and PairNone. This is one of the reasons f-strings weren't implemented as a > function, and is also the source of many headaches with string type > annotations. > Is this really true though? Yes, it would require magic of the sort zero-argument `super()` does. But can't the Matcher constructor capture the locals one step up the stack on instance creation? That's largely why my strawman version is slightly different from Steve's strawman. I'd put the question this way: assuming Matcher can be written (with a bit of stack magic), and assuming that the strings inside m.case() calls are exactly the same mini-languague PEP 634 proposes, would you want a syntax change? That's not rhetorical, I'm of mixed feeling myself. ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/ZQBAYYR5KJFJVXVYQQ3TIS5JLJZDM3K3/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
David Mertz wrote:
> On Mon, Nov 23, 2020 at 9:02 PM Brian Coleman brianfcole...@gmail.com
> wrote:
> > Basically, I
> > agree matching/destructuring is a powerful idea. But I also
> > wonder how much genuinely better it is than a library that does not
> > require
> > a language change. For example, I could create a library to allow this:
> > m = Matcher(arbitrary_expression)
> > if m.case("StringNode(s)"):
> > process_string(m.val.s)
> > elif m.case("[a, 5, 6, b]"):
> > process_two_free_vars(m.val.a, m.val.b)
> > What you are proposing here is very similar in effect to executing pattern
> > matching statements using eval. What is the advantage of implementing the
> > pattern matching functionality in a library if the user interface for that
> > functionality has the same pitfalls as eval?
> > I don't understand the similarity with eval that you are
> suggesting.
> My hypothetical library would store the value passed as initializer to
> Matcher, and provide a method .case(). That method would need
> to do
> some moderately complicated parsing of the pattern passed into it, but
> using parsing techniques, not any eval() call. Btw. I modified my above
> strawman just slightly to what might be a bit better API.
> If there are any free variables in the pattern, they would be provided by
> the Matcher object. For example, they could be attributes of the property
> m.val. Or I suppose we could make them attributes of the Matcher object
> itself, e.g. m.a and m.b, but I think name conflicts with e.g.
> m.case. Anyway, it's a strawman not an API design.
> If the pattern looked kinda like a class constructor (i.e. exactly the same
> as PEP 634 rules), the .case() method would do an isinstance()
> check
> before doing its other stuff. If the pattern looks like a list, it would
> scan the freevars inside it and match the constant values. And so on.
> The actual return value from .m.case(...) would be True or False (or at
> least something truthy or falsy). However, it MIGHT create some new
> attributes (or keys, or something else) on the Matcher object if the
> pattern actually matched.
> I agree the above is slightly less readable than PEP 635 syntax, but it
> seems to have the entire power of the proposal without changing Python
> syntax.
To be more precise, the similarity that I see to `eval` is that syntax errors
in the patterns that are passed to the `case` method cannot be detected at
compile time and instead will be detected at runtime. Building the syntax into
the language gives the advantage of producing a syntax error at compile time.
It also makes it easier for linters and type checkers to validate the pattern
matching clauses if the syntax is built into the language.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, Nov 23, 2020 at 9:02 PM Brian Coleman
wrote:
> > Basically, I agree matching/destructuring is a powerful idea. But I also
> > wonder how much genuinely better it is than a library that does not
> require
> > a language change. For example, I could create a library to allow this:
>
> m = Matcher(arbitrary_expression)
> if m.case("StringNode(s)"):
> process_string(m.val.s)
> elif m.case("[a, 5, 6, b]"):
> process_two_free_vars(m.val.a, m.val.b)
> What you are proposing here is very similar in effect to executing pattern
> matching statements using `eval`. What is the advantage of implementing the
> pattern matching functionality in a library if the user interface for that
> functionality has the same pitfalls as `eval`?
>
I don't understand the similarity with `eval` that you are suggesting.
My hypothetical library would store the value passed as initializer to
`Matcher`, and provide a method `.case()`. That method would need to do
some moderately complicated parsing of the pattern passed into it, but
using parsing techniques, not any eval() call. Btw. I modified my above
strawman just slightly to what might be a bit better API.
If there are any free variables in the pattern, they would be provided by
the Matcher object. For example, they could be attributes of the property
`m.val`. Or I suppose we could make them attributes of the Matcher object
itself, e.g. `m.a` and `m.b`, but I think name conflicts with e.g.
`m.case`. Anyway, it's a strawman not an API design.
If the pattern looked kinda like a class constructor (i.e. exactly the same
as PEP 634 rules), the `.case()` method would do an `isinstance()` check
before doing its other stuff. If the pattern looks like a list, it would
scan the freevars inside it and match the constant values. And so on.
The actual return value from `.m.case(...)` would be True or False (or at
least something truthy or falsy). However, it MIGHT create some new
attributes (or keys, or something else) on the Matcher object if the
pattern actually matched.
I agree the above is slightly less readable than PEP 635 syntax, but it
seems to have the entire power of the proposal without changing Python
syntax.
--
The dead increasingly dominate and strangle both the living and the
not-yet born. Vampiric capital and undead corporate persons abuse
the lives and control the thoughts of homo faber. Ideas, once born,
become abortifacients against new conceptions.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
David Mertz wrote:
> I have a little bit of skepticism about the pattern matching syntax, for
> similar reasons to those Larry expresses, and that Steve Dower mentioned on
> Discourse.
> Basically, I agree matching/destructuring is a powerful idea. But I also
> wonder how much genuinely better it is than a library that does not require
> a language change. For example, I could create a library to allow this:
> m = Matcher(arbitrary_expression)
> if m.case("StringNode(s)"):
> process_string(m.val)
> elif m.case("[a, 5, 6, b]"):
> process_two_free_vars(*m.values)
> elif m.case("PairNone(a, b)"):
> a, b = m.values
> process_pair(a, b)
> elif m.case("DictNode"):
> foo = {key, process_node(child_node) for key, child_node in
> m.values.items()}
> I don't disagree that the pattern mini-language looks nice as syntax. But
> there's nothing about that mini-language that couldn't be put in a library
> (with the caveat that patterns would need to be quoted in some way).
What you are proposing here is very similar in effect to executing pattern
matching statements using `eval`. What is the advantage of implementing the
pattern matching functionality in a library if the user interface for that
functionality has the same pitfalls as `eval`?
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
I have a little bit of skepticism about the pattern matching syntax, for
similar reasons to those Larry expresses, and that Steve Dower mentioned on
Discourse.
Basically, I agree matching/destructuring is a powerful idea. But I also
wonder how much genuinely better it is than a library that does not require
a language change. For example, I could create a library to allow this:
m = Matcher(arbitrary_expression)
if m.case("StringNode(s)"):
process_string(m.val)
elif m.case("[a, 5, 6, b]"):
process_two_free_vars(*m.values)
elif m.case("PairNone(a, b)"):
a, b = m.values
process_pair(a, b)
elif m.case("DictNode"):
foo = {key, process_node(child_node) for key, child_node in
m.values.items()}
I don't disagree that the pattern mini-language looks nice as syntax. But
there's nothing about that mini-language that couldn't be put in a library
(with the caveat that patterns would need to be quoted in some way).
--
The dead increasingly dominate and strangle both the living and the
not-yet born. Vampiric capital and undead corporate persons abuse
the lives and control the thoughts of homo faber. Ideas, once born,
become abortifacients against new conceptions.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
Antoine Pitrou wrote: > On Mon, 23 Nov 2020 16:15:12 - > "Brian Coleman" brianfcole...@gmail.com > wrote: > > Furthermore, Python has a regular expression module > > which implements it's own DSL for the purpose of matching string patterns. > > Regular > > expressions, in a similar way to pattern matching, > > allow string patterns to be expressed in a concise and declarative manner. > > Uh, without regular expressions, a lot of functions would be massively > more complicated and annoying to write. > However, your example shows that pattern matching barely makes > common code shorter (admittedly, on this _one_ example, but that's the > one you chose ;-)). > While I agree that regular expressions are far less Pythonic than the > proposed variant of pattern matching, they also have a tremendously > better cost/benefit ratio, IMHO. > Regards > Antoine. In my opinion, the object oriented solution to this problem is to use the visitor pattern. The solution using the visitor pattern is almost twice the length of the other solutions. Pattern matching is certainly no worse than the solution using a chain of `isinstance` checks in this case. However as the patterns to be checked against a candidate object become more complex, I believe that the pattern matching solution will retain the same level of elegance and obviousness that it possesses in this example, whereas the solution involving a chain of comparisons will quickly degrade in terms of obviousness. ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/UNESWNNQGXOI4W24H3HMD6UPDQQDUF2X/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
Larry Hastings wrote:
> On 11/23/20 8:15 AM, Brian Coleman wrote:
> > def process(root_node: Node):
> > def process_node(node: Node):
> > if isinstance(node, StringNode):
> > return node.value
> > elif isinstance(node, NumberNode):
> > return float(node.value)
> > elif isinstance(node, ListNode):
> > return [process_node(child_node) for child_node in
> > node.children]
> > elif isinstance(node, DictNode):
> > return {key: process_node(child_node) for key, child_node in
> > node.children}
> > else:
> > raise Exception('Unexpected node')
> > You don't need the "else". And you can change all your "elif"s into
> "if"s too. Now your "isinstance" version is 35 lines. Shorter than the
> pattern matching version, roughly the same speed, works in current
> Python, eminently readable to anybody conversant in current Python. A
> very reasonable solution to the problem.
> There should be one--and preferably only one--obvious way to do it,
> //arry/
It was not my intention to suggest that the solution implemented in the
shortest number of lines was superior.
I think that one of the advantages of the pattern matching implementation over
the sequence of `isinstance` checks is that there is no need to worry about
whether using `if`, `elif`or `else` is the best approach. There is a single
elegant and natural way to express the solution with pattern matching.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/20 12:05 PM, Chris Angelico wrote:
On Tue, Nov 24, 2020 at 7:00 AM Ethan Furman wrote:
On 11/23/20 11:06 AM, Larry Hastings wrote:
> On 11/23/20 8:15 AM, Brian Coleman wrote:
>> def process(root_node: Node):
>> def process_node(node: Node):
>> if isinstance(node, StringNode):
>> return node.value
>> elif isinstance(node, NumberNode):
>> return float(node.value)
>> elif isinstance(node, ListNode):
>> return [process_node(child_node) for child_node in
node.children]
>> elif isinstance(node, DictNode):
>> return {key: process_node(child_node) for key, child_node in
node.children}
>> else:
>> raise Exception('Unexpected node')
>
> You don't need the "else".
Without the "else" errors will pass silently.
> And you can change all your "elif"s into "if"s too.
On average, doubling your comparisons and therefore becoming slower.
I think the implication is that, since all the successful if
statements go into returns, you can leave off the "else" and just put
the "raise" immediately. It's a special case that applies ONLY to
situations where the pattern matching is the entire purpose of the
function.
Ah, of course. Thank you for correction.
--
~Ethan~
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, Nov 23, 2020, 2:58 PM Ethan Furman
> >> if isinstance(node, StringNode):
> >> return node.value
> >> elif isinstance(node, NumberNode):
> >> return float(node.value)
> >> elif isinstance(node, ListNode):
> >> return [process_node(child_node) for child_node in
> node.children]
> >> elif isinstance(node, DictNode):
> >> return {key: process_node(child_node) for key, child_node
> in node.children}
> >> else:
> >> raise Exception('Unexpected node')
> >
> > You don't need the "else".
>
> Without the "else" errors will pass silently.
>
Not in the particular example. It can just be a bunch of ifs, each of which
returns something. The last line of the function can be raise.
Obviously, epic and else are very important in general. I think I wouldn't
use any for that particular code though.
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Tue, Nov 24, 2020 at 7:00 AM Ethan Furman wrote:
>
> On 11/23/20 11:06 AM, Larry Hastings wrote:
> > On 11/23/20 8:15 AM, Brian Coleman wrote:
> >> def process(root_node: Node):
> >> def process_node(node: Node):
> >> if isinstance(node, StringNode):
> >> return node.value
> >> elif isinstance(node, NumberNode):
> >> return float(node.value)
> >> elif isinstance(node, ListNode):
> >> return [process_node(child_node) for child_node in
> node.children]
> >> elif isinstance(node, DictNode):
> >> return {key: process_node(child_node) for key, child_node in
> node.children}
> >> else:
> >> raise Exception('Unexpected node')
> >
> > You don't need the "else".
>
> Without the "else" errors will pass silently.
>
> > And you can change all your "elif"s into "if"s too.
>
> On average, doubling your comparisons and therefore becoming slower.
I think the implication is that, since all the successful if
statements go into returns, you can leave off the "else" and just put
the "raise" immediately. It's a special case that applies ONLY to
situations where the pattern matching is the entire purpose of the
function.
ChrisA
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/20 11:06 AM, Larry Hastings wrote:
> On 11/23/20 8:15 AM, Brian Coleman wrote:
>> def process(root_node: Node):
>> def process_node(node: Node):
>> if isinstance(node, StringNode):
>> return node.value
>> elif isinstance(node, NumberNode):
>> return float(node.value)
>> elif isinstance(node, ListNode):
>> return [process_node(child_node) for child_node in
node.children]
>> elif isinstance(node, DictNode):
>> return {key: process_node(child_node) for key, child_node in
node.children}
>> else:
>> raise Exception('Unexpected node')
>
> You don't need the "else".
Without the "else" errors will pass silently.
> And you can change all your "elif"s into "if"s too.
On average, doubling your comparisons and therefore becoming slower.
> Now your "isinstance" version is 35 lines. Shorter than the pattern matching
version
But without error checking.
> roughly the same speed,
See above comment about becoming slower.
> works in current Python, eminently readable to anybody conversant in current Python. A very reasonable solution to
the problem.
Agreed. But (subjectively, of course) not as elegant as the match version, and deteriorates rapidly as complexity
increases.
> There should be one--and preferably only one--obvious way to do it,
But that is not a hard and fast rule or we wouldn't have decorators (zero lines saved), the "with" statement (two lines
saved), and four different ways spanning three different protocols to write an iterator [1].
--
~Ethan~
[1]
https://stackoverflow.com/questions/19151/build-a-basic-python-iterator/7542261#7542261
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[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On Mon, 23 Nov 2020 16:15:12 - "Brian Coleman" wrote: > > Furthermore, Python has a regular expression module which implements it's own > DSL for the purpose of matching string patterns. Regular expressions, in a > similar way to pattern matching, > allow string patterns to be expressed in a concise and declarative manner. Uh, without regular expressions, a lot of functions would be massively more complicated and annoying to write. However, your example shows that pattern matching barely makes common code shorter (admittedly, on this _one_ example, but that's the one you chose ;-)). While I agree that regular expressions are far less Pythonic than the proposed variant of pattern matching, they also have a tremendously better cost/benefit ratio, IMHO. Regards Antoine. ___ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@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/45MP5EZ4D4XIJAQR2WPXLXNMWOMAR3QT/ Code of Conduct: http://python.org/psf/codeofconduct/
[Python-Dev] Re: Advantages of pattern matching - a simple comparative analysis
On 11/23/20 8:15 AM, Brian Coleman wrote:
def process(root_node: Node):
def process_node(node: Node):
if isinstance(node, StringNode):
return node.value
elif isinstance(node, NumberNode):
return float(node.value)
elif isinstance(node, ListNode):
return [process_node(child_node) for child_node in node.children]
elif isinstance(node, DictNode):
return {key: process_node(child_node) for key, child_node in
node.children}
else:
raise Exception('Unexpected node')
You don't need the "else". And you can change all your "elif"s into
"if"s too. Now your "isinstance" version is 35 lines. Shorter than the
pattern matching version, roughly the same speed, works in current
Python, eminently readable to anybody conversant in current Python. A
very reasonable solution to the problem.
There should be one--and preferably only one--obvious way to do it,
//arry/
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