Am 17.08.2017 um 00:39 schrieb Chris Angelico:
On Thu, Aug 17, 2017 at 8:29 AM, Mok-Kong Shen
<mok-kong.s...@t-online.de> wrote:
I have earlier learned some other (older) programming languages. For
these the formal parameters are either "by reference" or "by value".
In the first case, any modification of the formal parameter inside
a function affects the corresponding actual parameter of a function
call, while in the second case a copy of the actual parameter is
passed into the function so that any modification of the formal
parameter inside the function has no effect at all outside. This is
extremely clear-cut in comparison to Python, isn't it? Anyway, while
any new user of a programming language certainly can be expected to
take good efforts to learn a lot of new stuffs, I suppose it's good
for any practical programming language to minimize the cases of
surprises for those that come from other programming languages.
Python has a data model that is neither of the above, but it's simpler
in that you have one pattern for everything. Whether you're looking at
function parameters, return values, assignment, loops, function
definitions, or anything else, the model is exactly the same. And that
model is: objects exist independently of names, and names refer to
objects. If you do "x = y", you're saying "figure out which object 'y'
means, and make the name 'x' refer to it". If you do "x[1] = y",
you're saying "figure out which object 'y' means, and tell the object
that 'x' means that it should make [1] refer to that object". So if
you have multiple names referring to the same object, any change you
ask that object to do will be seen by every other name that also
refers to it - because it's all about the object.
I may have misunderstood you. But I don't think what you wrote
above would explain why the program below produces the output:
[1, 2, 3]
[3, 6, 9]
M. K. Shen
-----------------------------------------------------
def test2(alist):
alist[0],alist[1],alist[2]=3,6,9
alist=[30,60,90]
return
def test3(alist):
alist=[30,60,90]
alist[0],alist[1],alist[2]=3,6,9
return
ss=[1,2,3]
test3(ss)
print(ss)
test2(ss)
print(ss)
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