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Guide to the python interp. source?

Terry Reedy tjreedy at udel.edu
Sat Jul 27 13:39:06 EDT 2002 
"Tim Gahnström /Bladerman" <tim at bladerman.com> wrote in message
news:R5x09.17790$p56.5752496 at newsb.telia.net...
> I want to make all variables mutable.

In a real sense, Python does not have variables.  It has objects, each
of which has 1 or more named or unnamed references thereunto.  Which
do *you* think of as variables:  objects, references, or names?  If
objects, you should look to another language since immutability of
certain types of objects is an integral part of Python (the operation
of dicts, for instance.)  I believe most people think of variables as
names, which works best (fewest surprises) within the Python data
model when bound to immutable data types.  For example:

x = 2
...
y = x
... (many lines)
x = x*x
# what should now be the value of y?  2 or 4?
# immutability of ints guarantees 2, which is what most people expect

> When you want to make functions like inc(x) where inc is supposed to
> increase the value of x by one. How is that done in Python?

You 'mutate' names most easily by rebinding them to a new value with
the binding (assignment) operator '=' as in 'x = x+1', 'x+=1', or even
x=inc(x)'.  With work, you could make inc('x') work for global var x
by fiddling with the globals dict (but this cannot be done for local
function vars).  Note that in C, you would also have to 'quote' x,
though with the '&' operator, as in inc(&x).

> That is my view of CBR, maybe that is not the correct definition .

To me, call by reference means that the function gets a reference to
the actual argument instead of an anonymous copy.  However, for Python
immutables, the effect is the same, except for avoiding the creation
and usual destruction of the copy.

> and I don't need CBR

If you work with instead of fighting the Python data model, you can
probably do what you want without completely reworking the
interpreter.

Terry J. Reedy
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