On Monday, August 14, 2017 at 4:26:06 AM UTC+5:30, Gregory Ewing wrote:
> Steve D'Aprano wrote:
>
> > Python's comprehensions are inspired by Haskell's, but we made different
> > choices
> > than they did: we make the fact that a comprehension is a loop over values
> > explicit, rather than implicit, and we use words instead of cryptic symbols.
>
> The presence of the word "for" in the comprehension syntax doesn't
> by itself imply that the values are generated by a sequential loop.
>
> Comprehensions in Python are intended to be read declaratively.
> The definition in terms of an expansion into nested loops may
Thanks for speaking up Greg [Creator of comprehensions in python 2??]
Here's a bunch of different ways in which a mapping comprehension could be
implemented:
def sq(x): return x*x
Just a trivial example
def simple(f, l):
r = []
for i in range(0,len(l)):
r.append(f(l[i]))
return r
def backwards(f,l):
r = []
for i in range(len(l)-1,-1,-1):
r.insert(0,f(l[i]))
return r
def randmap(f,l):
""" Map in random order """
from random import shuffle
r = []
index = list(range(0,len(l)))
shuffle(index)
r = [None] * len(l)
for i in index:
r[i] = f(l[i])
return r
def inplace(f, l, lb, le, res): #, rb):
""" Helper function: Maps subslice (lb,le) of input l onto (into) output
res, with
subslice rb,re"""
for i in range(lb,le):
res[i] = f(l[i])
return None # Side-effecting into res
def parprequel0(f,l):
"""Illustrative trivial prequel to parallelizing version"""
r = [None] * len(l)
inplace(f,l,0,len(l),r)
return r
def parprequel1(f,l):
"""Illustrative prequel to parallelizing version
with 2-way split"""
r = [None] * len(l)
inplace(f,l, 0, len(l)//2, r)
inplace(f,l, len(l)//2, len(l), r)
return r
def par(f,l):
"""Illustrative prequel to parallelizing version
with 2-way split"""
from threading import Thread
r = [None] * len(l)
t1 = Thread(target=inplace, args = (f,l, 0, len(l)//2, r))
t2 = Thread(target=inplace, args = (f,l, len(l)//2, len(l), r))
t1.start()
t2.start()
t1.join()
t2.join()
return r
--
https://mail.python.org/mailman/listinfo/python-list
