I am all in for that proposal.
Let the construction:
for element in iterable if condition:
loop_body
mean:
for element in iterable:
if not condition: continue
loop_body
However this construction may be abused (in a good way) to cause the
`condition` be evaluated on every loop iteration. And this is exactly
what we need for loop invariants.
class InvariantError(AssertionError):
pass
def loop_invariant(condition:bool):
if not condition:
raise InvariantError
return True
for element in iterable if loop_invariant(condition):
loop_body
We are killing 3 birds with 1 stone:
1. We are unifying normal loop syntax with list comprehension syntax.
2. We are adding a syntactic sugar for a very useful feature (filtering).
3. We are adding a means of specifying loop invariants where they belong.
To specify invariants on `while` loops we could do something like:
while loop_invariant(condition) and running:
loop_body
so no new syntax is needed.
With this syntax, annotatin loops with invariants would be possible
without changing indentation.
haael
Hello Everyone,
If I've done this incorrectly, please let me know so that I can
improve/revise. I'm new to the Python community and quite enjoy the more
functional features of Python 3, but have I have a peeve about it. I'd
like to propose and discuss the following enhancement to Python 3:
*
Consider the following trivial for-loop:*
chars= "abcaaabkjzhbjacvb"
seek= {'a','b','c'}
count= 0
for ain chars:
if ain seek:
count+= 1
Gross. Twice nested for a simple count.
/We could refactor the block like so:/
chars= "abcaaabkjzhbjacvb"
seek= {'a','b','c'}
count= 0
for ain filter(lambda c: cin seek, chars): count+= 1
Which is all well and good, but doesn't quite read like English. It's
verbose, too.
It also uses advanced concepts new programmers may not understand.
/We could do this:/
chars= "abcaaabkjzhbjacvb"
seek= {'a','b','c'}
count= sum([1 for ain charsif ain seek])
However, this changes important semantics by creating an entire new list
before summing.
Also, adding just one more expression to the most nested block thwarts
that refactor.
I propose the following enhancement:
chars= "abcaaabkjzhbjacvb"
seek= {'a','b','c'}
count= 0
for ain charsif ain seek: count+= 1
*What happened there?*
I've allowed a condition to follow the "for" construct without a colon
or newline between.
/To be clear, this remains incorrect:/
chars = "abcaaabkjzhbjacvb" seek = {'a','b','c'} count = 0 for a in
chars # No colon prior to the newlineifa inseek:count +=1
*Value proposal:*
I assert that the inlined 'if' condition pattern is superior to the
alternative refactors.
Right now, the way to acquire an invariant in a more nasty loop would be:
for ain iterable:
if condition:
continue
But this is messy and not particularly Pythonic.
The filter approach uses concepts that should not be necessary for this
task.
The comprehension approach has different, undesirable semantics.
*Conclusion:*
I wanted to submit my thoughts here before getting too deep into this.
Any input would be appreciated!
Thanks everyone.
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