On Wed, 10 Aug 2005 12:40:54 +0200, Jiri Barton <[EMAIL PROTECTED]> wrote:
>Hi everyone,
>
>I have a problem with initialization.
>>>> a, b = [[]]*2
>>>> a.append(1)
>>>> b
>[1]
>
>Why is this? Why does not this behave like the below:
>
>>>> a, b = [[], []]
>>>> a.append(1)
>>>> b
>[]
In your 1st example a and b point to copies of the same object, in
the second example the point to two separate objects that happen to
have the same repr() representation.
Here's another example of the same thing:
>>> c = []
>>> d = [c,c]
>>> e = [[],[]]
>>> d
[[], []]
>>> e
[[], []]
(d) and (e) appear to be the same (i.e. they have the same repr()
form), but they are not. Consider:
>>> e[1].append("spam")
>>> e
[[], ['spam']]
But:
>>> d[1].append("spam")
>>> d
[['spam'], ['spam']]
The point is that in Python an object's repr() -- which stands for
"representation" -- doesn't actually tell you how the object is
represented inside the innards to the Python system.
Incidently I am currently working on a programming language that
fixes this problem. In my language "Unify", the equivalent of repr
is called "storage manager format" (or SM-format) because if two
values have the same SM-format it is guaranteed that the system will
treat them identiacally. The SM-format can thus be used for
serialisation, and incidently it is also the same format in which
literals are written in a program.
In Unify to specify an Array containing two separate empty Arrays
would look like this:
e := #(() ())
The code to add a value to one sub-array, and print the result is:
e[1] @= 'spam'. // means same as Python d[1].append("spam")
out << sm(e). // convert to SM-string, output it to stdout
This would print this text:
#(() (spam))
An array containing two copies of the same sub-array might look like
this:
d := #( () &[0] )
Where "&[0]" means "pointer to the 0th array subscript".
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