On Sat, Mar 8, 2014 at 5:58 AM, Jim J. Jewett
(Thu Mar 6 23:26:47 CET 2014) Chris Angelico responded:
On Fri, Mar 7, 2014 at 7:29 AM, Jim J. Jewett <jimjjewett at gmail.com> wrote:
[ note that "x if y" already occurs in multiple contexts, and always evaluates y before x. ]
Yes, but that's still out of order.
Yeah, but local consistency is more important than global guidelines. :D
I don't see except expressions as fundamentally more associated with if/else than with, say, an or chain, which works left to right. Aside from both being ternary operators, of course, which isn't really much of a justification.
The other thing to note is that it's somewhat ambiguous. Until you find that there isn't an else clause, it could be the equally valid "expr except (default if cond else other_default)", with the actual "if Exception" part still to come.
True -- and I'm not a parser expert. But my belief is that the current parser allows lookahead for exactly one token, and that the "else" would fit within that limit.
... humans reading the code have to assume style guides mightn't be followed.
True ... but I hope any non-trivial use of this (including use with a non-trivial ternary if) will look bad enough to serve as its own warning.
Not sure whether the parser would be able to handle it or not, but the human would have to if the machine can, and that's going to be a source of confusion. I'd rather avoid it if I can. Remember, everywhere else in Python, the word "if" is followed by something that's interpreted as a boolean. You wouldn't expect to see this somewhere: if None: do_stuff() do_stuff() if sys else do_other_stuff() So it'd cause a mental glitch to see some other constant and always-true expression there: ... if ZeroDivisionError ... You don't expect ZeroDivisionError ever to be false. Seeing it following an if would leave you wondering if maybe it is. IMO that syntax is abuse of the 'if' keyword, hammering it into a hole it's not designed to fit into.
I do think parentheses help, (but are less important when there is only a single "if")
Analysis of the Python standard library suggests that the single-if situation is *by far* the most common, to the extent that it'd hardly impact the stdlib at all to add multiple except clauses to the proposal. Do you have a strong use-case for the more full syntax? It tends to get long, which rather damages the readability. A number of people have said that if the except expression goes over a line break, it should probably be a statement instead. I'm not sure that extreme is fully justified, but it certainly does have merit. In a codebase of mine I wouldn't *forbid* breaking an except expression over multiple lines, but it'd certainly be a reason to consider whether it's really necessary or not.
and I strongly prefer that they [the parentheses] be internal (which you fear looks too much like calling a function named except). In that case, it is:
expr1 except (expr3 if expr2)
I'm still not really seeing how this is better. With the colon version, it looks very much like dict display, only with different brackets around it; in some fonts, that'll be very easily confused. With the if, it looks like an incomplete expression waiting for its else. And I'm still not enamored of constructing syntax that has the evaluation order (a) not simple left-to-right, like most expressions are, and (b) different from the except statement, which puts the exception_list ahead of the suite.
Agreed ... the "say it like you would in English" applies only to the "expr if expr" form (proposed here and) used by comprehensions:
[1/x for x in data if x]
Sure. That usage makes a _lot_ of sense, I really like list comprehensions. There's no room in them for an 'else' clause though, so it's very strictly one-way. Makes them a tricky comparison for this.
I can't speak to the lambda precedent, but I do know that I personally often stumble when trying to parse it, so I don't consider it a good model.
The only confusion I have with lambda is its precedence, which trips me up now and then:
def f(x): ... return lambda y: x,y ... f(5) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 2, in f NameError: name 'y' is not defined
That's not returning a function that returns a tuple. It's returning a tuple of a function and the global y, and the function will ignore its arg.
The other three inline uses (dict display, slide notation, and function parameter annotation) are effectively conjunction operators, saying that expr1 and expr2 are bound more tightly than you would assume if they were separated by commas. They only occur inside a fairly close bracket (of some sort), and if the bracket isn't *very* close, then there are usually multiple associates-colons inside the same bracket.
Not sure that really helps. This isn't going to be as tight as a slice, and it's most often not going to have multiple colons inside the brackets.
Without (preferably internal) parentheses, it will instead look like a long line with a colon near the end, and a short continuation suite that got moved up a line because it was only one statement long.
def nullfunc(self, a): pass
expr1 except expr3: expr2
That should have expr2 and exp3 the other way round - the bit before the colon is evaluated before the bit after the colon. What about putting it all in parens, which is the current proposal? There's no way you can have a statement with a one-line suite on the same line, all enclosed in parens. (expr1 except expr2: expr3)
All forms involving the 'as' capturing clause have been deferred ...
Nick is right that you should specify whether it is deferred or rejected, because the simplest implementation may lock you into too broad a scope if it is added later.
Put it this way: It's deferred until there's a non-closure means of creating a sub-scope.
The problem is that once it is deployed as leaking into the parent scope, backwards compatibility may force it to always leak into the parent scope. (You could document the leakage as a bug or as implementation-defined, but ... those choices are also sub-optimal.)
It'll never be deployed as leaking, for the same reason that the current 'except' statement doesn't leak: it creates a refloop. The entire notion of an 'as' clause is deferred. If there's no way to create a sub-scope that isn't a closure, then there will be no 'as' clause in an except expression. It's that simple. Then later, if such a feature is added, it can be retrofitted without breaking anything. But there's little need of it; the cases where the exception needs to be captured are usually the ones that need the full statement anyway. Of all 'except' statements in the stdlib, almost exactly one in five uses 'as'; of the ones simple enough to potentially be changed to this form, just 7 out of 236 do. (And those seven are among the more dubious ones.) So it's no great loss to the proposal, assuming the stdlib is indicative. (If you have a good-sized Py3 codebase to try it on, feel free to enhance those stats with more data. The script's part of my repo.)
The fact that short-circuiting will prevent false_expr from even being evaluated means that "(true_expr if cond)" is a useful mental approximation, even though "true_expr if cond" is already to the left.
Hrm, I don't really like that way of describing it. To me, the entire expression is one unit - you can't parenthesize it. It's like parenthesizing a chained comparison: if 5 < x < 10: if (5 < x) < 10: if 5 < (x < 10): You can't add parens around one part of the expression; it completely changes the meaning of it.
Commutivity and Associativity are pretty strong biases; testing order of execution for apparently parallel expressions (such as different arguments to the same function in the same call) is a source of trick questions.
In C, the answer is usually "Implementation-defined". In Python, the answer is usually "Left to right".
The replacement value and the exception that triggers it are clearly associated more closely with each other than either is to the primary expression. Therefore, they *should* be grouped more tightly.
Maybe. On the other hand, the replacement value and the original expression are two things of the same broad type, and the other expression is an exception list, so the primary and the replacement should be grouped more tightly. You're going to get this or that. I don't think this is a strong reason for parentheses.
This is a proposal with no perfect syntax, but I don't really see any upside to *not* grouping the exception with its exceptional result. At the moment, I don't know how to spell that closer grouping without at least one of:
Parentheses or brackets of some sort
A line break/new suite: - kills the proposal
An inline associates-colon: - needs brackets anyhow, or it will be mistaken for a suite-introduction-colon.
A new grouping operator, such as "->" - risk that it will be misinterpreted
Relying on convention and taste: (main_expr except exc_expr fallback_expr)
Definitely not the last one. It needs some kind of syntactic separation, not just whitespace (what if the fallback_expr is a negative number?). The entire expression should be representing a single thought, or else it probably should be written as a statement. Like the thought "Get me the current working directory, None if there isn't one". pwd = (os.getcwd() except OSError: None) Or: "Append the IP, or failing that the network address". ips.append(ip.ip except AttributeError: ip.network_address) With those, there's really no separation between "Get the current working directory", "if there isn't one", and "None". Parenthesizing the part after 'except' makes good sense if you want to have multiple except clauses, but there's no reason to do that, so we're back with this very simple case. I'd rather not mandate any parens at all, personally, but as was said in another thread, you don't need to see a Frenchman on the parapet to know where the wind's blowing. ChrisA