Re: [Python-ideas] Proposal: Allowing any variable to be used in a 'with... as...' expression

19 May 2019

      On Sat, May 18, 2019 at 07:44:26PM -0700, Yonatan Zunger wrote:
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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.
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For example, this is a good pattern:
with functionReturningFile(...) as input:
    doSomething(input)
Right.
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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.
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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.
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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

Re: [Python-ideas] Proposal: Allowing any variable to be used in a 'with... as...' expression

Steven D'Aprano