More random python observations from a perl programmer

Tom Christiansen tchrist at
Thu Aug 19 21:18:37 CEST 1999

     [courtesy cc of this posting mailed to cited author]

In comp.lang.python, 
    duncan at (Duncan Booth) writes:
:>GOTCHA: (high)
:>    Lists don't autoallocate the way dictionaries do.  So while this works
:>    on two dictionaries:
:>     new = {}
:>     for key in old.keys:
:>         new[key] = old[key]
:>    This fails on two lists:
:>     new = []
:>     for i in old:
:>         new[i] = old[i]
:This is meaningless and nothing to do with autoallocating. i is iterating 
:over values not indexes. you get the effect you want here by writing
:    	for i in old:
:    	    	new.append(i)

You're right.  I should have written

    new = []
    for i in range(0,len(old)):
	new[i] = old[i]

And that certainly is parallel to the dictionary case.  Dictionary
auto-allocate.  Lists don't.  It's a gotcha to a Perl programmer,
because he's using to having both hashes and arrays autoallocate.  
Not, of course, that we need to copy things that way, since arrays and
hashes are firstclassy. :-)  But you know what I mean.

     "... an initial underscore already conveys strong feelings of
      magicalness to a C programmer."
	--Larry Wall in <1992Nov9.195250.23584 at>

More information about the Python-list mailing list