# [Python-Dev] New PEP

Huan Do doboy0 at gmail.com
Thu Mar 22 00:39:48 CET 2012

```*Hi,

I am a graduating Berkeley student that loves python and would like to
propose an enhancement to python. My proposal introduces a concept of
slicing generator. For instance, if one does x[:] it returns a list which
is a copy of x. Sometimes programmers would want to iterate over a slice of
x, but they do not like the overhead of constructing another list. Instead
we can create a similar operator that returns a generator. My proposed
syntax is x(:). The programmers are of course able to set lower, upper, and
step size like the following.

x(1::-1)

This would make code much cleaner in a lot of instances, one example lets
say we have a very large list x and we want to sum all the numbers but the
last 20, and we only want to loop through the even indices.

We would have to do something like this.

sum(x[:-20:2])

or we can do a workaround to save space for time and do something like this.

sum( value for i, value in enumerate(x) if i < -20 and not i % 2 )

But with my proposal we are able do the following.

sum(x(:-20:2))

Which affords space without sacrificing expressiveness.

For another example lets say we have a problem that we want to check a
condition is true for every pairwise element in a list x.

def allfriends(x):

for i in range(len(x)):

for j in range(i+1, len(x)):

if not friends(x[i], x[j]):

return False

return True

A more pythonic way is to actually loop through the values instead of the
indices like this.

def allfriends(x):

for i, a in enumerate(x):

for j, b in enumerate(x[i+1:]):

if not friends(a, b):

return False

return True

This however bring a lot of overhead because we have to construct a new
list for every slice call. With my proposal we are able to do this.

def allfriends(x):

for i, a in enumerate(x):

for j, b in enumerate(x(i+1:)):

if not friends(a, b):

return False

return True

This proposal however goes against one heuristic in the zen of python,
namely “Special cases aren’t special enough to break the rules.” The way
that the proposal breaks this rule is because the syntax x(:), uses a
function call syntax but would be a special case here. I chose using
parenthesis because I wanted this operation to be analogous to the
generator syntax in list comprehensions.

ListGeneratorsComprehension[ x for x in L ]( x for x in L )SlicingL[a:b:c]
L(a:b:c)

Tell me what you guys think.

Thanks!*
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