Semantics of the empty list

Dave Opstad dave.opstad at agfamonotype.com
Thu Jul 15 19:12:21 CEST 2004


I fully understand why this happens:

----------------------------
>>> a = [[], [], [], [], []]
>>> b = [[]] * 5
>>> a
[[], [], [], [], []]
>>> b
[[], [], [], [], []]
>>> a == b
True
>>> id(a[0]) == id(a[1])
False
>>> id(b[0]) == id(b[1])
True
>>> a[1].append(42)
>>> a
[[], [42], [], [], []]
>>> b[1].append(42)
>>> b
[[42], [42], [42], [42], [42]]
----------------------------

What is curious to me is the implication that there are multiple, 
distinct empty list objects (whose ids are not equal). Is this the case 
for all mutable objects?

I wonder if it would be useful to have some symbol whose semantics are 
"distinct mutable." For the sake of discussion, let's say the symbol \\ 
means the same as [] (i.e. it's the empty list), but with this "distinct 
mutable" semantic. Then I could safely do something like:

>>> b = [\\] * 5
>>> b
[[], [], [], [], []]
>>> id(b[0]) == id(b[1])
False

to safely initialize b as a list of distinct empty lists. As it is now, 
I have to do something like this:

>>> b = [[] for x in range(5)]

in order to get the semantic I want. It's not any great hardship, of 
course, but I have been bitten by the [[]] * 5 case and its cousins on 
several occasions, and it might be nice if there were a separate 
semantic as described above to make it more clear what's going on.

Just some random thoughts...

Dave



More information about the Python-list mailing list