slice notation as values?
duncan.booth at invalid.invalid
Sat Dec 10 17:03:03 CET 2005
Antoon Pardon wrote:
> In general I use slices over a tree because I only want to iterate
> over a specific subdomain of the keys. I'm not iterested in make
> a tree over the subdomain. Making such a subtree would be an
> enormous waste of resources.
Probably not unless you have really large data structures. If you have
something like a dbhash which would be inefficient to copy then both the
original view of the data structure and the slices could share the same
data. Creating a slice doesn't *have* to copy anything just so long as the
semantics are clear.
> With slice notation you could have the following two cases:
> for key in tree.iterkeys('a':)
> for key in tree.iterkeys(:'b')
x['a':] is short for x['a':None]
x[:'b'] is short for x[None:'b']
> But you can't do
> for key in tree.iterkeys('a',)
> or more regrettably, you can't do:
> for key in tree.iterkeys(,'b')
If your datatype defines iterkeys to accept start and end arguments then
you can do:
for key in tree.iterkeys('a',None)
for key in tree.iterkeys(None,'b')
which is directly equivalent to the slices, or you can do:
for key in tree.iterkeys(start='a')
for key in tree.iterkeys(stop='b')
which is more explicit.
> I also think that other functions could benefit. For instance suppose
> you want to iterate over every second element in a list. Sure you
> can use an extended slice or use some kind of while. But why not
> extend enumerate to include an optional slice parameter, so you could
> do it as follows:
> for el in enumerate(lst,::2)
'Why not'? Because it makes for a more complicated interface for something
you can already do quite easily.
More information about the Python-list