Pointers

[Remco Gerlich]

Peter Halliday wrote in comp.lang.python:
> I just started learning python.  I just want to make sure that I understand the
> object interaction you were discussion here.  To use your statement.  So x + 1
> is an object, x is an object, and y is an object.  Y and x happen to both be
> pointing to x + 1.  Is this right? 

Sort of. x can never point to x+1 :). Rather, x = x+1 computes the result of
the expression x+1, makes a new object with that value, and makes x point to
it.

> If you then did something to y would you be doing something to x + 1?

No, not if this is about integers anyway.

> I would say, no because it is a seperate object right!

Actually, that's not the reason. y and x are both exactly the same object.
Thing is, if it's an integer, then you can't change it! You can only make a
new object and assign it to y.

> >         x = x + 1
> >         y = x

Now y and x are the same object. But the only way to change y, say with y=y+1,
makes a new object (the result of y+1) and lets y refer to it.

It's different with mutable objects like lists, they can be changed in place:
>>> x = [1,2]
>>> y = x  # They're both the same list now
>>> x
[1, 2]
>>> y.append(3)
>>> y
[1, 2, 3]
>>> x
[1, 2, 3]
>>> y = y+[4] # This still creates a new object
>>> y
[1, 2, 3, 4]
>>> x
[1, 2, 3]

Assignment never changes an object, except when it's an assignment to some
internal of the object (like y[1] = 3).

Try playing around in the interpreter for a while, this is a key thing to
understand about Python in my opinion. You can test objects to see if
they're the same object with 'x is y', and test their values with 'x == y'.
-- 
Remco Gerlich,  scarblac at pino.selwerd.nl
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