On 25/06/2020 16:48, Tim Peters wrote:
[Tim]
See reply to Glenn. Can you give an example of a dotted name that is
not a constant value pattern? An example of a non-dotted name that is?
If you can't do either (and I cannot)), then that's simply what "if
[Rhodri James <rho...@kynesim.co.uk>]
case long.chain.of.attributes:
[Tim]
That's a dotted name and so is a constant value pattern - read the PEP.
Every dotted name in a pattern is looked up using normal Python
name resolution rules, and the value is used for comparison by
equality with the matching expression (same as for literals).
[Rhodri]
Then I am surprised, which is worse. "long.chain.of.attributes" looks
like an assignment target, and I would have expected the case to have
been a name pattern.
As always, I don't care whether something is obvious at first glance.
I care whether something can be learned with reasonable effort, and
"sticks" _after_ it's learned. There's essentially nothing truly
obvious about programming.
This, from the PEP, is the entire grammar for a "name pattern'"
name_pattern: NAME !('.' | '(' | '=')
That's it. A plain name not followed by a dot, left paren, or equality sign.
While it may or may not surprise any given person at first glance,
it's very simple. Put a fraction of the effort into learning it as
you're willing to expend on articulating surprise, and it would
already be far behind you ;-)
Well, now is the time for expressing surprise :-p
As I've said before, one of my main problems with the PEP is as you go
through it, more and more special cases and surprises appear, and the
consequences of earlier surprises generate more special cases and
surprises. You claim not unreasonably that it's easy to remember that
"_" is special in matches. Maybe you're right, but that decision has
consequences spelled out later in the PEP that are less easy to
remember. Another example: I had not previously thought the definition
of name patterns to be surprising, but apparently they are (it just
surprised me, at any rate). That consequently makes the definition of
constant value patterns, which I was already iffy about, really quite
surprising.
Each individual learning curve might be small, but cumulative total by
the time you reach the end of the PEP is large. Simple match statements
will, with adequate squinting, look recognisably like other areas of
Python. Complex match statements won't. And that's a problem for
anyone who wants to be able to read someone else's code.
Bear in mind I am predominantly a C programmer who uses Python from time
to time for tools and glue. If I have to put in effort to learn new
special-case rules in Python, that's an active discouragement; I'm
frankly unlikely to bother, and more likely to write those tools and
glue in C instead. I'm certainly much less likely to use someone else's
tools and glue if I have to re-read the spec to remind myself what all
the gotchas are.
--
Rhodri James *-* Kynesim Ltd
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