difference between adding lists in python with + and += [duplicate]

Answer 1

+ 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.

Answer 2

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

Answer 3

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.