PEP 308: Erik's alternatives list - another suggestion
lannert at lannert.rz.uni-duesseldorf.de
Wed Feb 19 18:30:25 CET 2003
Erik, thank you for compiling this list of proposals! I'd vote against
the introduction of any new special characters into Python syntax,
as I think there could come up better reasons for using "?" or "@"
for some other new concept, which would be more compelling than a
ternary operator ... which, as a separate concept, I don't find
I hope the following suggestion hasn't already been beaten to
death somewhere; pardon me if it has.
I'd like to suggest a syntactical construct which I'd call
a "generator comprehension". It would resemble the list
comprehension, but instead of building a static list object,
it evaluates its components lazily when being asked for the next
For example, an iteration like
x = 99.
for value in [from a(x), b(x), c(x)]:
if not value: break
evaluates a(99.) first, tests/prints its value, then evaluates
b(99.) -- which happens to be False -- and terminates. Thus it
doesn't call c. The [from ...] would act very much like
The "generator comprehension" could select and modify its values
on the fly, with nearly the same syntax that list comprehensions
[v for v from a(x), b(x), c(x)]
produces the same values as
[v for v in a(x), b(x), c(x)]
does today, just evaluates its components lazily.
Then we could actually use the already-proposed bool.choose()
(x < 3).choose([from a(x), b(x)])
Or let the generator comprehension, for instance, pick its first
[from a(x), a(x+1), a(x+2)].firsttrue()
If such a beast was indexed, it wouldn't need to evaluate all
its components up to the index:
[from a(x), b(x), c(x)]
would just evaluate c(x), as a and b are not needed to determine
Pros and cons that I can (or believe to) see myself:
+ Supports a PEP-308-like "ternary operator".
+ May also be useful for very different purposes.
+ Doesn't introduce new keyword nor special character.
+ Doesn't actually introduce new concepts into the language;
generators are already there (albeit not for long ;).
+ Symmetry with list comprehension.
- Symmetry with list comprehension: could be confused.
- Doesn't come to mind if a C programmer is looking for an
equivalent of x ? y : z.
More pros and cons? Probably.
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