[Python-ideas] revisit pep 377: good use case?
steve at pearwood.info
Thu Mar 1 01:00:03 CET 2012
Craig Yoshioka wrote:
> I've tried classes, decorators, and passing the conditional using 'as', as
> suggested by Michael, so I disagree that with is not suitable here since I have
> yet to find a better alternative. If you want I can give pretty concrete
> examples in the ways they aren't as good. Furthermore, I think it could be
> argued that it makes more sense to be able to safely skip the with body without
> the user of the with statement having to manually catch the exception
> themselves.... we don't make people catch the StopIteration exception manually
> when using iterators...
Sometimes we do. When you call next(it) manually, you are responsible for
catching the exception manually. It is only flow-control tools (e.g. for
loops, list comprehensions) that catch the exception for you.
> 1) I can't think of many instances in python where a block of code can not be
> conditionally executed safely:
> if - obvious
> functions - need to be called
> loops - can have 0 or more iterations
> try/except/finally - even here there is the same notion of the code blocks
> being conditionally executed, just a bit more scrambled
Classes are blocks of code. If you want to conditionally skip executing all,
or part, of a class block you wrap it in an if block. (Or try...except.) This
is as it should be: the class statement is not a flow control statement.
Nor is the def statement. In this case, the right concept is not in *calling*
the function body, but in *compiling* the function body. Again, if you want to
conditionally skip compiling all or part of the function body, you have to
wrap it in a flow control structure, if or try.
We don't have any concept of:
class K: body
def f(): body
where something inside the bodies invisibly determines whether or not the K
block executes or the f block compiles.
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