cokofreedom
1/16/2008 12:59:00 PM
On Jan 16, 1:21 pm, "Neil Cerutti" <mr.ceru...@gmail.com> wrote:
> On Jan 15, 2008 10:09 PM, J. Peng <peng....@gmail.com> wrote:
>
> > Hello,
>
> > I saw this statement in Core Python Programming book,
>
> > All arguments of function calls are made by reference, meaning that
> > any changes to these parameters within the function
> > affect the original objects in the calling function.
>
> Yes, that's generally correct. But you must be careful about what is
> meant by "changes to parameters". Assigning a new value to a parameter
> name (inside the function, a parameter is just a local variable) does
> not change the original object--it only rebinds the local variable to
> a new object.
>
> In the following function, a is rebound with an assignment statement,
> while b is mutated, i.e., changed, with an assignment statement.
>
> def f(a, b):
> a = 12
> b.value = 14
>
> Argument a will never be changed, while argument b will be. Python's
> argument passing semantics are extremely simple. It's the assignment
> statement that's tricky: some assignments mutate/change objects, and
> some only rebind names.
>
> > Does this mean there is not pass-values calling to a function in
> > python? only pass-reference calling? Thanks!
>
> Neither is quite true. Values are passed by binding parameter names to
> their corresponding arguments. This is similar to pass-by-reference in
> some cases (when the argument is mutated) but not in others (when the
> argument is not mutated). Thinking of it as pass-by-reference may help
> you to understand it, but bear in mind that Python's "references" may
> be rebound to new objects, which is quite different from the usual
> behavior of references.
>
> --
> Neil Cerutti <mr.cerutti+pyt...@gmail.com>
So basically the scope is the reason for confusion a lot of the time?
def some_name():
alist = [5]
bint = 5
cstring = '5'
ddictionary = {0:5}
def other_name(alist, bint, cstring, ddictionary):
alist = 4
bint = 4
cstring = '4'
ddictionary = 4
print "other_name:",
print alist, bint, cstring, ddictionary
def another_name(alist, bint, cstring, ddictionary):
alist[0] = 3
# bint cannot be changed it is immutable
# cstring cannot be changed it is immutable
ddictionary[0] = 3
print "another_name:",
print alist, bint, cstring, ddictionary
another_name(alist, bint, cstring, ddictionary)
other_name(alist, bint, cstring, ddictionary)
print "our entries are now:",
print alist, bint, cstring, ddictionary