On Sat, May 18, 2019 at 07:44:26PM -0700, Yonatan Zunger wrote:
> Essentially, a "noop with" improves code health,
I don't think I can accept that as a mere assertion. I'd like to see
some concrete evidence that it does.
> > > For example, this is a good pattern:
> > >
> > > with functionReturningFile(...) as input:
> > > doSomething(input)
Right.
> > > There are many cases where an Optional[file] makes sense as a parameter,
> > > as well; for example, an optional debug output stream, or an input
> > > source which may either be a file (if provided) or some non-file source
> > > (by default). Likewise, there are many cases where a function may
> > > naturally return an Optional[file], e.g. "open the file if the user has
> > > provided the filename."
If you need to use the word "if" to describe what a piece of code does,
then it makes sense for the code to include the "if" keyword.
> > > However, the following is /not/ valid Python:
> > >
> > > with functionReturningOptionalFile(...) as input:
> > > doSomething(input)
Nor would any of these be safe:
functionReturningFileOrNone().write("hello world")
(expression_returning_list_or_None).append(1)
functionReturningNumberOrNone() + 1
especially since it is so easy to accidentally return None when you
didn't intend to:
def functionReturningNumber(arg):
if arg > 0:
return something
# oops forgot the case arg <= 0
So I think that it is a mistake to treat None where it would normally
not be accepted as a no-op. It is better practice to explicitly test for
None and branch, or to use a some sort of None-aware operator or wrapper.
> > > To handle this case, one has a few options. One may only use the 'with'
> > > in the known safe cases:
> > >
> > > inputFile = functionReturningOptionalFile(...)
> > > if inputFile:
> >
> > Or more expicitly, 'if inputFile is not None:'
> >
> > > with inputFile as input:
> > > doSomething(input)
> > > else:
> > > doSomething(None)
Can you give an example of real-world code where the body of the with
and the body of the else are identical?
I would expect the more common case will be:
if file := MaybeFileOrNone() is None:
handle(None)
else:
with file as f:
process(f)
or for those using pre-walrus operator Pythons:
file = MaybeFileOrNone()
if file is None:
handle(None)
else:
with file as f:
process(f)
--
Steven
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