使用这些“内置”条件进行for循环的pythonic方法是什么？

Question

In a class method I have the following:

if self.zot == 1:
        for foo in self.bar:
            if self.zot != 1:
                break
            #code that may or may not change self.zot

bar is a property that I'd rather not access and populate unless absolutely necessary, so if zot isn't 1 I'd still like to prevent that first iteration.

Python being python I'm guessing there's still a way cleaner way to do this.

Something like the following?

for foo in self.bar while self.zot == 1:

Bonus points for an answer compatible with python 2.7.x

EDIT: Whether zot is modified during the iteration is partially independent of bar 's contents so takewhile is impractical. Either way bar needs to not be accessed in the first place if zot isn't 1.

Answer 1

You can convert the for into a while loop and use iter() and next() , although it is not very pythonic and would require using a try except .

iterator = None
while self.zot == 1:
    iterator = iterator or iter(self.bar)
    try:
        current = iterator.next()
        #your code
    except StopIteration:
        break

This will start using self.bar only when you go into the while for the first time.

You may create a proxy function to delegate the check.

def check_and_iterate(iterable, predicate):
    i = None
    while predicate():
        i = i or iter(iterable)
        yield i.next()

for foo in check_and_iterate(self.bar, lambda: self.zot == 1):
    #your code

EDIT: As @user2357112 pointed out, self.bar is accessed anyway. You can defer it by using another lambda:

def check_and_iterate(getter, predicate):
    i = None
    while predicate():
        i = i or iter(getter())
        yield i.next()

for foo in check_and_iterate(lambda: self.bar, lambda: self.zot == 1):
    #your code

Answer 2

something like this, with property bar handling the zot checking

@property
def bar(self):
    itr = iter(self._bar)
    while self.zot == 1:
        yield next(itr)