[Python-ideas] With expressions

[Robert Vanden Eynde]

>
> Expressionization may break the "one and only on obvious way" guideline,
> but it can offer concise, readable code in a lot of instances where a
> statement-based version would be clumsy and noisy, and there's already some
> precedent for it:
>
> function declaration => lambda
> for-loops => generator expressions and comprehensions
> if-else => ternary statements
>

Totally agree. That's the problem when being multi paradigm, but we already
have that "problem" and that's alright.


> With the exception of lambda, expressionized statements usually allow one
> to put the "meat before the vegetables" so to speak. That is; the highest
> value part of the expression comes first and all the book-keeping follows
>

As the code I showed, being just that:

filename = ...
lines = ...
parsed_data = ...

With implementation details? The highest value is there, the alg is clear.,
one fetches the filename, one preprocess the lines, then parse the data.



> One tactic that other expressionizations have taken is to limit the scope.
> For instance, the ternary operation only covers expressionization of
> "if-else" not "just if" or "if-elif-..." or "if-elif-...-else", and
> generator expressions don't allow the 'else' clause
> <http://book.pythontips.com/en/latest/for_-_else.html> of normal
> for-loops. So maybe you can obviate some of the edge cases by requiring an
> as clause or something. I don't know how that would help with the
> suppress(AttributeError) case thought...
>

About if elif elif else, ternary if does have that:

y = (x+1 if x < 0 else
       x-1 if x > 0 else
       0)

Limiting the scope is interesting, for "with" the only limitation in that
the body must have exactly one assignment, like in the ternary if case?

a = ([l.strip() for l in f.readlines()] with open ('name') as f)

By cutting the edge cases, "with something():" is not possible, only "with
... as" being possible?

But the "scope limitation" for try except in expression concept would be
not to have "else" or "finally"? The else and finally clauses do not make
sense in Expression style assignment anyway.

a = (int('stuff') except ValueError: 5)


> On Fri, Aug 3, 2018 at 12:56 PM, Todd <toddrjen at gmail.com> wrote:
>
>> On Thu, Aug 2, 2018 at 5:35 AM, Ken Hilton <kenlhilton at gmail.com> wrote:
>>
>>> Hi, I don't know if someone has already suggested this before, but here
>>> goes:
>>>
>>> With expressions allow using the enter/exit semantics of the with
>>> statement inside an expression context. Examples:
>>>
>>>     contents = f.read() with open('file') as f #the most obvious one
>>>     multiplecontents = [f.read() with open(name) as f for name in names]
>>> #reading multiple files
>>>
>>> I don't know if it's worth making the "as NAME" part of the with
>>> mandatory in an expression - is this a valid use case?
>>>
>>>     data = database.selectrows() with threadlock
>>>
>>> Where this would benefit: I think the major use case is `f.read() with
>>> open('file') as f`. Previous documentation has suggested
>>> `open('file').read()` and rely on garbage collection; as the disadvantages
>>> of that became obvious, it transitioned to a method that couldn't be done
>>> in an expression:
>>>
>>>     with open('file') as f:
>>>         contents = f.read()
>>>
>>> Therefore `f.read() with open('file') as f`, I think, would be much
>>> welcomed as the best way to read a file in an expression.
>>>
>>> For those wondering about the scope semantics of the "as NAME", I think
>>> they would be identical to the scope semantics of the "for" expression -
>>> i.e. these are legal:
>>>
>>>     contents = f.read() with open('file') as f
>>>     grid = [[i] * 4 for i in range(4)]
>>>
>>> But these are not:
>>>
>>>     contents = f.read() with open('file') as f
>>>     f.seek(0)
>>>     grid = [[i] * 4 for i in range(4)]
>>>     grid[i][i] = 4
>>>
>>> Is this a good idea? Are there some subtleties I've failed to explain?
>>> Please let me know.
>>>
>>> Sharing,
>>> Ken Hilton
>>>
>>>
>> If this is a common enough operation for you, it would be trivially easy
>> to just write a function that does this.  There is already a module on pypi
>> that has this function: read_and_close.
>>
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