[EMAIL PROTECTED] wrote:
> On 21 fév, 23:19, John Henry <[EMAIL PROTECTED]> wrote:
>> On Feb 21, 2:06 pm, Jeff Schwab <[EMAIL PROTECTED]> wrote:
>>
>>
>>
>>> John Henry wrote:
>>>> On Feb 21, 1:48 pm, John Henry <[EMAIL PROTECTED]> wrote:
>>>>> On Feb 21, 1:43 pm, mrstephengross <[EMAIL PROTECTED]> wrote:
>>>>>> Hi all. In C, an assignment statement returns the value assigned. For
>>>>>> instance:
>>>>>> int x
>>>>>> int y = (x = 3)
>>>>>> In the above example, (x=3) returns 3, which is assigned to y.
>>>>>> In python, as far as I can tell, assignment statements don't return
>>>>>> anything:
>>>>>> y = (x = 3)
>>>>>> The above example generates a SyntaxError.
>>>>>> Is this correct? I just want to make sure I've understood the
>>>>>> semantics.
>>>>>> Thanks,
>>>>>> --Steve
>>>>> That's true, and I am happy that they decided to make that a syntax
>>>>> error.
>>>> BTW: The less obvious issues when coming from the C world are Python
>>>> syntax like these:
>>>> y = x = 3
>>>> a = 4
>>>> y = x = a
>>>> print x,y
>>>> a = 5
>>>> print x,y
>>> That's the same behavior I would expect in C, on the grounds that C
>>> assignments do bit-wise copies. What I found confusing at first was
>>> that the same variable will either directly store or merely refer to an
>>> object, depending on the type of the object:
>>> >>> a = [ 'hello' ]
>>> >>> y = x = a
>>> >>> a += [ 'world' ]
>>> >>> print x, y
>>> ['hello', 'world'] ['hello', 'world']
>> Yep. Took me a while to realize there is mutable objects, and non-
>> mutable objects. To be honest, I am still not too comfortable about
>> it. For instance, I still get nervous for code like:
>>
>> def invoke_some_fct(parent):
>> y = parent.x
>> y += [ 'world' ]
>> print y, parent.x
>>
>> class abc:
>> def __init__(self):
>> self.x=[ 'hello' ]
>> invoke_some_fct(self)
>> print self.x
>>
>
> Explicitely using list.extend would make things clearer:
>
> def invoke_some_fct(parent):
> parent.x.extend(['world'])
Whether you use += or extend has nothing to do with it. You omitted the
relevant part. Using extend, it would look like:
y = parent.x
y.extend(['world'])
The confusing part is that performing an operation on y may or may not
alter parent.x, depending on whether the initial type of parent.x is
immutable. If parent.x is immutable, y is a copy of the value
represented by parent.x, and modifying y has not effect on the value of
parent.x. If (OTOH) parent.x is mutable, then x and y are really
references to the same object, and modifications to that object via y
can be observed via x. In C, you use pointers to get this effect.
> Now there's no reason to feel nervous about this. All you have to
> remember is that Python never copy anything unless explicitely asked
> for.
It's not that simple. After a statement like:
a = b
Whether a and b denote the same object depends on what kind of object b
represented in the first place.
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