What does += mean in Python?

Solution 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], [])

Solution 2:

a += b

is in this case the same as

a = a + b

In this case cnt += 1 means that cnt is increased by one.

Note that the code you pasted will loop indefinitely if cnt > 0 and len(aStr) > 1.

Edit: quote Carl Meyer: ``[..] the answer is misleadingly mostly correct. There is a subtle but very significant difference between + and +=, see Bastien's answer.''.

Solution 3:

Google 'python += operator' leads you to http://docs.python.org/library/operator.html

Search for += once the page loads up for a more detailed answer.

Solution 4:

FYI: it looks like you might have an infinite loop in your example...

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

• a condition of entering the loop is that cnt is greater than 0
• the loop continues to run as long as cnt is greater than 0
• each iteration of the loop increments cnt by 1

The net result is that cnt will always be greater than 0 and the loop will never exit.