Difference between + and += operator in Python

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Hi Hari,

The rule for augmented assignment operators such as += is that, like

other operators, they call a special “dunder” (double-underscore) method

to allow classes to customise what the operation does.

If the special dunder operation doesn’t exist, then the augmented

assignment falls back on the ordinary operator.

So the operation:

obj += x

turns into something like this in the interpreter:

  1. Does obj have the special method __iadd__?

  2. If so, then run obj = obj.__iadd__(x).

  3. If not, then run obj = obj + x.

For integers, there is no special __iadd__ method, and so the +=

operation is just syntactic sugar for n = n + m. Same for strings.

But lists do define a special __iadd__ method, making it an inplace

concatenation.

For lists, + does list concatenation:

mylist + x

requires x to be a list, and it returns a new list formed by

concatenating all the items of mylist and the items of x.

mylist = [10, 20]

new = mylist + [30, 40]  # Can only add list to list.

print(new)

prints a new list, [10, 20, 30, 40] while leaving the original mylist

untouched.

But augmented assignment += does in-place concatenation:

mylist += x

and x can be any sequence, not just a list, and it concatenates the

items of x to mylist in-place. It does not create a new list.

mylist = [10, 20]

mylist += [30, 40]  # mylist is now [10, 20, 30, 40]

mylist += (50, 60)  # tuples are okay, not just lists

mylist += "xyz"  # strings are sequences too!

print(mylist)

will print [10, 20, 30, 40, 50, 60, 'x', 'y', 'z'].

So + and += for lists are still concatenation, just slightly

different versions:

  • + is list-to-list concatenation, and returns a new list

  • += concatenates the items of any sequence or iterator to the

    existing list, in-place.

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