x += ... is not the same than x = x + ... if x is mutable

[bartolome.sintes]

Hi,

I thought that x += ... was the same than x = x + ..., but today I haverealized it is not true when operating with mutable objects.

In Python 3.3 or 2.7 IDLE (Windows) compare:

a = [3]
b = a
a = a + [1]
b

[3]

and

a = [3]
b = a
a += [1]
b

[3, 1]

Is this behaviour explained in the Python documentation?

Thanking you in advance,
Bartolomé Sintes

 

[Jussi Piitulainen]

Hi,

I thought that x += ... was the same than x = x + ..., but today I
have realized it is not true when operating with mutable objects.

In Python 3.3 or 2.7 IDLE (Windows) compare:

a = [3]
b = a
a = a + [1]
b

[3]

and

a = [3]
b = a
a += [1]
b

[3, 1]

Is this behaviour explained in the Python documentation?

Yes, it's documented in the language reference, specifically in the
latter half of the paragraph quoted below.

<http://docs.python.org/3/reference/simple_stmts.html#assignment-statements>

# An augmented assignment expression like x += 1 can be rewritten as x
# = x + 1 to achieve a similar, but not exactly equal effect. In the
# augmented version, x is only evaluated once. Also, when possible,
# the actual operation is performed in-place, meaning that rather than
# creating a new object and assigning that to the target, the old
# object is modified instead.

 

[Nobody]

I thought that x += ... was the same than x = x + ..., but today I have
realized it is not true when operating with mutable objects.

It may or may not be the same. x += y will invoke x.__iadd__(y) if x has
an __iadd__ method, otherwise x + y will be evaluated and the result
assigned to x.

In the first case, x will always continue to refer to the same object
(i.e. id(x) won't change). In the second case, x will typically (but not
always) refer to a different object.

 

[Colin J. Williams]

It may or may not be the same. x += y will invoke x.__iadd__(y) if x has
an __iadd__ method, otherwise x + y will be evaluated and the result
assigned to x.

Does this depend on whether Py27 or Py32 is used?

Colin W.

 

[Steven D'Aprano]

Does this depend on whether Py27 or Py32 is used?

No. It is equally true for all versions of Python.