syntax - What does += mean in Python?

Question

I see code like this for example in Python:

    if cnt > 0 and len(aStr) > 1:
        while cnt > 0:                  
            aStr = aStr[1:]+aStr[0]
            cnt += 1

What does the += mean?

Answer 1

a += b is essentially the same as a = a + b, except that:

• + always returns a newly allocated object, but += should (but doesn't have to) modify the object in-place if it's mutable (e.g. list or dict, but int and str are immutable).

• In a = a + b, a is evaluated twice.

• Python: Simple Statements

  • A simple statement is comprised within a single logical line.

---

If this is the first time you encounter the += operator, you may wonder why it matters that it may modify the object in-place instead of building a new one. Here is an example:

# two variables referring to the same list
>>> list1 = []
>>> list2 = list1

# += modifies the object pointed to by list1 and list2
>>> list1 += [0]
>>> list1, list2
([0], [0])

# + creates a new, independent object
>>> list1 = []
>>> list2 = list1
>>> list1 = list1 + [0]
>>> list1, list2
([0], [])