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When using a for loop in Python to iterate over items in a list, will changing item (below) change the corresponding item in items?
for item in items: item += 1 Will each item in items be incremented or remain the same as before the loop?
[Note: I would be interested in Python 2.7 and 3.x]
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Python always uses pass-by-reference values. There isn't any exception. Any variable assignment means copying the reference value.
for loops are used when you have a block of code which you want to repeat a fixed number of times. The for-loop is always used in combination with an iterable object, like a list or a range. The Python for statement iterates over the members of a sequence in order, executing the block each time.
For loops are always inclusive in Python: they always run over all elements of the iterator (other than exceptions such as break , etc.).
Long story short: Python uses pass-by-value, but the things that are passed by value are references. The actual objects have 0 to infinity references pointing at them, and for purposes of mutating that object, it doesn't matter who you are and how you got a reference to the object.
No, variables in Python are not pointers.
They refer to objects on a heap instead, and assigning to a variable doesn't change the referenced object, but the variable. Variables and objects are like labels tied to balloons; assignment reties the label to a different balloon instead.
See this previous answer of mine to explore that idea of balloons and labels a bit more.
That said, some object types implement specific in-place addition behaviour. If the object is mutable (the balloon itself can change), then an in-place add could be interpreted as a mutation instead of an assignment.
So, for integers, item += 1 is really the same as item = item + 1 because integers are immutable. You have to create a new integer object and tie the item label to that new object.
Lists on the other hand, are mutable and lst += [other, items] is implemented as a lst.__iadd__([other, items]) and that changes the lst balloon itself. An assignment still takes place, but it is a reassigment of the same object, as the .__iadd__() method simply returns self instead of a new object. We end up re-tying the label to the same balloon.
The loop simply gives you a reference to the next item in the list on each iteration. It does not let you change the original list itself (that's just another set of balloon labels); instead it gives you a new label to each of the items contained.
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Well, it really depends on the items.
Take the following case:
class test(): pass a = test() a.value = 1 b = test() b.value = 2 l = [a,b] for item in l: item.value += 1 for item in l: print item.value >>> 2 3 and in this case:
l2 = [1,2,3] for item in l2: item += 1 for item in l2: print item >>> 1 2 3 So as you can see, you need to understand the pointers as Martijn said.
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