On 09/06/2016 12:19, Antoon Pardon wrote:
Op 09-06-16 om 12:53 schreef BartC:
On 09/06/2016 10:46, Antoon Pardon wrote:
Op 09-06-16 om 09:36 schreef Steven D'Aprano:
Your example demonstrates object mutation, not assignment.
Generally assignment and mutation don't contradict each other.
So IMO the cause is the same, a mutation. In some languages you
can mutate your variable through an assignment and in others you
can't.
I think this is what is confusing you.
Mutation is like repairing or customising my car.
Full assignment is like buying a new car.
I think you are confused. Full assignment of C struct or
Pascal record is the same as mutating all individual
attributes. You don't get a new record/instance as in
Python. You replace the value of the attributes with
something else.
As far a language semantics are concerned, you get the
(new) car at scope entry, all assigments later are
mutations.
You may think of assignment as like buying a new car,
that is not how the semantics of languages like pascal
and C are defined.
Static languages like C and Pascal don't make for a good comparison. C
is just too low level: anything is possible so can be twisted any way. A
bit like assembly.
C variables have a fixed type, which you can't change by assignment (so
a Ford can't change to a different marque). But being low-level,
assignment in C is just copying N bytes from A to B. Which is also what
happens when, instead of copying a whole struct, you mutate all of it by
assigning the fields one by one.
Python assignments are bit more sophisticated, and there are real
differences between full assignment (where a variable ends up with a
different object reference) and mutation (where it might be the same,
but modified, object).
--
Bartc
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