Python enumerate inside for loop skips alternate items

Question

I have a situation where a for loop outside a class appears to be interacting with an enumerate inside the class.

Example code:

class MyContainerClass(object):
    def __init__(self):
        self.collection = []
    def __str__(self):
        return (len(self.collection) == 0 and 'Empty' or
                ', '.join(str(item) for item in self.collection))
    def __iadd__(self,item):
        print "adding {item}".format(item=str(item))
        self.collection.append(item)
        return self
    def __iter__(self):
        return iter(self.collection)
    def discard(self,item):
        print "discarding {item}".format(item=str(item))
        for index, value in enumerate(self.collection):
            if value == item:
                print "found {value}=={item}".format(value=value,item=item)
                return self.collection.pop(index)
        return False

class MyItemClass(object):
    def __init__(self,value):
        self.value=value
    def __str__(self):
        return '{value}'.format(value=self.value)
    def __eq__(self,other):
        if self.value == other.value:
            return True
        else:
            return False

c1 = MyContainerClass()
c2 = MyContainerClass()

c2 += MyItemClass('item1')
c2 += MyItemClass('item2')
c2 += MyItemClass('item3')
c2 += MyItemClass('item4')
c2 += MyItemClass('item5')
print "c1 is : {c1}".format(c1=str(c1))
print "c2 is : {c2}".format(c2=str(c2))

for item in c2:
    print "for got {item}".format(item=str(item))
    c1 += c2.discard(item)

print "c1 is : {c1}".format(c1=str(c1))
print "c2 is : {c2}".format(c2=str(c2))

Produces this output:

adding item1
adding item2
adding item3
adding item4
adding item5
c1 is : Empty
c2 is : item1, item2, item3, item4, item5
for got item1
discarding item1
found item1==item1
adding item1
for got item3
discarding item3
found item3==item3
adding item3
for got item5
discarding item5
found item5==item5
adding item5
c1 is : item1, item3, item5
c2 is : item2, item4

I'm sure it's something really obvious, possibly to do with the iter function, but I can't see it at the moment.

Answer 1

You are modifying c2.collection (in discard) at the same time as iterating over it (for item in c2:). Don't do that.