On 08/16/2017 03:39 PM, Chris Angelico wrote:
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.
ChrisA
Here's another explanation, same concepts just stated differently, perhaps giving a different
perspective.
In a "traditional" programming language, variables are created/assigned by:
Setting aside a chunk of memory to hold the data and associating the variable name with that
memory. And since this chunk is a fixed size, the data type that can be stored there is also
fixed at that time as well. This is static typing.
Assignment is done by storing the new data in that memory location, overwriting what was already
there. But it has to be the proper data type so it doesn't overrun the memory allocated for it.
Python does it in the opposite order:
The data is stored in memory (or already exists) and the variable name is associated with that
memory. Python calls this name binding. It is possible, and common, that this memory (data)
has multiple names bound to it.
Assignment is done by associating the name to a different data value (memory location). This
data may be newly created or it can be previously existing data. (The old data may or may not
be 'garbage collected' — depending on other factors.) A key point here is that the name has no
memory size/data type itself and can be bound to ANY data — this is dynamic typing.
Where things get confusing is when we consider mutable/immutable data.
If it is immutable (unchangeable: ints, floats, strings, tuples...) the _practical_ result is
that although the implementation details are different, the result of running the program is the
same.
But for mutable data (changeable IN PLACE: lists, dictionaries...) the results can be
surprising from the viewpoint of other languages. Since this (mutable) data can have multiple
variable names bound to it, changing the value(s) through ANY of the names changes this data for
ALL of the names bound to it.
The OP said...
>> I suppose it's good
>> for any practical programming language to minimize the cases of
>> surprises for those that come from other programming languages.
I see your point, but it's irrelevant. Part of learning any new language is to learn where/how
the new language handles things differently from your previous experience. You MUST learn the
new ways and forget (or at least modify) your previous knowledge. It's simply a necessary part
of learning a new language. Expect it and live with it!
I hope this description gives you another viewpoint to consider. Good luck on your journey to
becoming a Pythonista!
<For any pedants out there, my description here is meant strictly to describe these underlying
concepts in general terms. The details may be (and probably are) quite different.>
--
-=- Larry -=-
--
https://mail.python.org/mailman/listinfo/python-list
