On 16/02/18 02:06, Nick Coghlan wrote:
The recent thread on variable assignment in comprehensions has
prompted me to finally share
https://gist.github.com/ncoghlan/a1b0482fc1ee3c3a11fc7ae64833a315 with
a wider audience (see the comments there for some notes on iterations
I've already been through on the idea).
== The general idea ==
The general idea would be to introduce a *single* statement local
reference using a new keyword with a symbolic prefix: "?it"
[snip]
If we did pursue this, then PEPs 505, 532, and 535 would all be
withdrawn or rejected (with the direction being to use an it-reference
instead).
I don't think that follows.
== Examples ==
`None`-aware attribute access:
value = ?it.strip()[4:].upper() if (?it=var1) is not None else None
`None`-aware subscript access:
value = ?it[4:].upper() if (?it=var1) is not None else None
`None`-coalescense:
value = ?it if (?it=var1) is not None else ?it if (?it=var2) is
not None else var3
`NaN`-coalescence:
value = ?it if not math.isnan((?it=var1)) else ?it if not
math.isnan((?that=var2)) else var3
>
Conditional function call:
value = ?it() if (?it=calculate) is not None else default
I have to say I don't find this an improvement on "value = var if var is
not None else None" and the like. In fact I think it's markedly harder
to read. It has the same repetition problem as the current situation,
just with added glyphs.
Avoiding repeated evaluation of a comprehension filter condition:
filtered_values = [?it for x in keys if (?it=get_value(x)) is not None]
Definite win here. It doesn't read particularly naturally, but then
list comprehensions don't read that naturally either. I would still
prefer something that read better.
Avoiding repeated evaluation for range and slice bounds:
range((?it=calculate_start()), ?it+10)
data[(?it=calculate_start()):?it+10]
Avoiding repeated evaluation in chained comparisons:
value if (?it=lower_bound()) <= value < ?it+tolerance else 0
Avoiding repeated evaluation in an f-string:
print(f"{?it=get_value()!r} is printed in pure ASCII as {?it!a}
and in Unicode as {?it}"
While these are wins, they don't read nicely at all. I still don't see
what's wrong with
start = calculate_start()
values = range(start, start+10)
which beats everything I've seen so far for clarity.
--
Rhodri James *-* Kynesim Ltd
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