I've noticed when experimenting with lists that p= p+i is different then p += i For example:
test = [0, 1, 2, 3,] p = test test1 = [8] p = p + test1 print test In the above code test prints out to the original value of [0, 1, 2, 3,]
But if I switch p = p + test1 with p += test1 As in the following
test = [0, 1, 2, 3,] p = test test1 = [8] p += test1 print test test now equals [0, 1, 2, 3, 8]
What is the reason for the different value?
p = p + test1 assigns a new value to variable p, while p += test1 extends the list stored in variable p. And since the list in p is the same list as in test, appending to p also appends to test, while assigning a new value to the variable p does not change the value assigned to test in any way.
tobias_k explained it already.
In short, using + instead of += changes the object directly and not the reference that's pointing towards it.
To quote it from the answer linked above:
When doing foo += something you're modifying the list foo in place, thus you don't change the reference that the name foo points to, but you're changing the list object directly. With foo = foo + something, you're actually creating a new list.
Here is an example where this happens:
>>> alist = [1,2] >>> id(alist) 4498187832 >>> alist.append(3) >>> id(alist) 4498187832 >>> alist += [4] >>> id(alist) 4498187832 >>> alist = alist + [5] >>> id(alist) 4498295984 In your case, test got changed since p was a reference to test.
>>> test = [1,2,3,4,] >>> p = test >>> id(test) 4498187832 >>> id(p) 4498187832 
+ and += represent two different operators, respectively add and iadd
From http://docs.python.org/2/reference/datamodel.html#object.iadd: the methods iadd(self,other), etc
These methods are called to implement the augmented arithmetic assignments (+=, -=, =, /=, //=, %=, *=, <<=, >>=, &=, ^=, |=). These methods should attempt to do the operation in-place (modifying self) and return the result
p += test1 uses the iadd operator and hence changes the value of p while p = p + test1 uses the add which does not modify any of the two operands.
