On Apr 24, 11:08 am, Thomas Nelson <t...@mail.utexas.eduwrote:
Occasionally someone posts to this group complaining about the lack of
"repeat ... until" in python. I too have occasionally wished for such
a construct, and after some thinking, I came up with the class below.
I'm hoping to get some feedback here, and if people besides me think
they might use it someday, I can put it on the python cookbook. I'm
pretty happy with it, the only ugly thing is you have to use a
lambda. Ideally i'd like to just see
while Until(i<3)
but that doesn't work.
Please tell me what you think, and thanks for your time.
Tom
class Until:
"""
>>i = 0
>>while Until(lambda: i<3):
... print "hello"
... i += 1
hello
hello
hello
>>while Until(lambda: i<2): #note i still equals 3 here
... print "hello"
hello
"""
yet = True
def __init__(self, mybool):
if self.__class__.yet or mybool():
self.__class__.yet = False
self.ans = True
else:
self.__class__.yet = True
self.ans = False
def __nonzero__(self):
return self.ans
Class-level "yet" variable not a good idea. Loops can be nested:
i = 0
# this should loop once, since test is logically at the end of the
loop
while Until(lambda : i < 0 ):
print "hello"
i += 1
j = 0
while Until(lambda : j < 0 ):
print "byebye"
j += 1
This loops forever.
After tinkering with this a bit, I can see why you need a class
variable. In your outer loop, "Until(lambda : i < 0)" is evaluated
every time around the loop, creating a new Until object, so you can't
preserve looping state in the object, you have to do it in the class.
I used your idea to create an instance-level Until that survives
looping, but the requirements for construction are still onerous. In
essence, the instance-level util defers the break-on-condition by one
loop iteration.
class Until:
"""
>>i = 0
while Until(lambda: i<3):
... print "hello"
... i += 1
hello
hello
hello
>>while Until(lambda: i<2):
... print "hello"
hello
"""
def __init__(self, mybool):
self.lastTest = True
self.mybool = mybool
def __nonzero__(self):
ret,self.lastTest = self.lastTest,self.mybool()
return ret
i = 0
uCond1 = Until(lambda : i < 0 )
uCond2 = Until(lambda : j < 0 )
while uCond1:
print "hello"
i += 1
j = 0
while uCond2:
print "byebye"
j += 1
-- Paul
