Prev: Problem with subprocess.Popen wget within a thread
Next: interpretation of special characters in Python
From: ssecorp on 6 Jul 2008 14:45
>>> h = "aja baja"
>>> h += 'e'
>>> h
'aja bajae'
>>>

From: Peter Otten on 6 Jul 2008 14:57
ssecorp wrote:

>>>> h = "aja baja"
>>>> h += 'e'
>>>> h
> 'aja bajae'
>>>>

The inplace-add operator doesn't mutate the lvalue, it just rebinds it:

>>> a = b = "foo"
>>> id(a)
47643036142016
>>> a += "bar"
>>> id(a), a
(47643036142064, 'foobar')
>>> id(b), b
(47643036142016, 'foo')

Peter
From: Mel on 6 Jul 2008 16:36
ssecorp wrote:
>>>> h = "aja baja"
>>>> h += 'e'
>>>> h
> 'aja bajae'
>>>>
What Peter said, or, to put it another way:

Python 2.5.2 (r252:60911, Apr 21 2008, 11:12:42)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a = b = "aja baja"
>>> a += "e"
>>> print a
aja bajae
>>> print b
aja baja

Mutability/immutability makes a difference in programs when different
symbols (or container items) share a value.


Mel.
From: Terry Reedy on 6 Jul 2008 16:50


Peter Otten wrote:
> ssecorp wrote:
>
>>>>> h = "aja baja"
>>>>> h += 'e'
>>>>> h
>> 'aja bajae'
>
> The inplace-add operator doesn't mutate the lvalue, it just rebinds it:

In Python, neither '=' nor members of the 'op=' family are operators.
They all mark *assignment* or *augmented assignment* statements that
*all* bind objects to targets.

Augmented assignments are, rather obviously, restricted to binding one
object to one target. For 'x op= y', if the object originally bound to
x is mutable, the arithmetic operation part of the augmented assignment
can (should) be implemented by an inplace __i<opname>__ special method
that (normally, but not necessarily) mutates and returns self to be
rebound. Otherwise, the interpreter calls the normal __<opname>__
special method (if it exits) that returns a new object to be bound.

Thus, '+=' is neither an operator nor is the indicated operation
necessarily inplace.

Immutable built-in classes do not have __i<opname>__ methods. So given
that name h is bound to a string,
h += 'e'
has exactly the same effect as
h = h + 'e'
which has exactly the same effect as
h = h.__add__('e')
The same is true for immutable instances of other built-in classes.


Terry Jan Reedy

From: ssecorp on 6 Jul 2008 17:33
so if strings were mutable and i did
a = b = "foo"
and then did
a += "bar"
then a and b would be foobar?
 |  Next  |  Last
Pages: 1 2
Prev: Problem with subprocess.Popen wget within a thread
Next: interpretation of special characters in Python