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From: jepler at inetnebr.com (Jeff Epler)
Date: 21 Feb 1999 18:21:29 GMT
Subject: New (?) suggestion to solve "assignment-in-while" desire
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We all know what the problem looks like:
while 1:
x=sys.stdin.readline()
if not x: break
....
well, someone can write an "xreadlines" which permits
for i in xreadlines(sys.stdin):
....
but next, who knows what "x"-function we will need.
And, at the same time, "for" embodies a test (for IndexError) and an
assignment (to the loop variable). So what we need is a nice, generic
class to embody this sort of functionality, with the ability to use an
arbitrary test on the assigned value, as well as accept an arbitrary
exception as an "end of loop" marker.
This is an implementation of the "lazy" class, which does what I've
discussed:
--------------------------------------------------------------------------
class lazy:
def __init__(self, function, test=lambda x: not x, exception=None,
index=0):
self.f=function
self.t=test
self.e=exception
self.i=index
def __getitem__(self, i):
try:
if self.i: ret=self.f(i)
else: ret=self.f()
except self.e:
raise IndexError
if self.t(ret):
raise IndexError
return ret
--------------------------------------------------------------------------
here are some uses of it: xreadlines, and "xrange1" a limited
reimplementation of xrange.
--------------------------------------------------------------------------
xreadlines=lambda x: lazy(x.readline, exception=EOFError)
xrange1=lambda min, max, inc: lazy(lambda x, min=min, inc=inc: min+inc*x,
lambda y, max=max: y>=max, index=1)
--------------------------------------------------------------------------
the basic
for i in lazy(f):
body
is the same as:
while 1:
i=f()
if not i: break
body
but you can embellish with more complicated tests, exception tests, or
whatever.
The class assumes it will be called in a "for-like way" so please refrain
from taunting it.
Jeff