On 9/25/17 5:32 AM, Antoon Pardon wrote:
Can you explain, what you mean by "Pass-By-Reference" as far a I understand,
pass by reference means that the parameter of the function becomes an alias
of the argument, so that if the entity is mutated through one name that
mutation is visible through the other name.
function(x, y) # allowed
function(namespace.x, module.y) # allowed
function(x + 1, 2) # FORBIDDEN: can't pass expressions or constants
Pass by reference, doesn't imply the above is forbidden. A language can
define, that the "x + 1" will produce an new entity and that a reference
to that entity will be passed.
Wikipedia has the right definition of call by reference
(https://en.wikipedia.org/wiki/Evaluation_strategy#Call_by_reference):
/Call by reference/ (also referred to as /pass by reference/) is an
evaluation strategy where a function receives an implicit reference
<https://en.wikipedia.org/wiki/Reference_%28computer_science%29> to
a variable used as argument, rather than a copy of its value. This
typically means that the function can modify (i.e. assign to
<https://en.wikipedia.org/wiki/Assignment_%28computer_science%29>)
the variable used as argument—something that will be seen by its caller.
The key idea here is that it is a reference to a *variable* that is
passed, not a reference to a value. Python has no references to
variables, so it cannot support call by reference. Because Python
passes references to values, it is easy to call it "call by reference,"
but that is not what the term means.
The Wikipedia definition unfortunately includes "rather than a copy of
its value," as if those are the only two options (a common
misunderstanding).
Elsewhere in this thread, someone asserted that to be call by reference,
you have to be able to write a swap(x,y) function. True
call-by-reference would make this possible. Python cannot do it.
--Ned.
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