Adding item in front of a list

Alex Martelli aleaxit at
Thu Apr 10 09:25:46 CEST 2003

Tim Peters wrote:

> [Andrew Koenig, starting with l=[2, 3, 4]]
>> ...
>> I would have thought that after l.insert(-1, 1), l would be
>> [2, 3, 1, 4], but it doesn't work that way.
> Alas, list.insert() existed before sequence indices were generalized to
> give
> a "count from the right end" meaning to negative index values.  When the
> generalization happened, it appears that list.insert() was just
> overlooked.
> I'd like to change this.  If I did, how loudly would people scream?

If you presented it as a *bug fix* I wouldn't scream -- I'd PURR...!!!


>>> def ins1(al, where, what):
...     al.insert(where, what)
>>> def ins2(al, where, what):
...     al[where:where] = [what]

are these two functions equivalent?  It sure seems so...:

>>> a=list('lopkan'); ins1(a, 2, 'i'); print a
['l', 'o', 'i', 'p', 'k', 'a', 'n']
>>> a=list('lopkan'); ins2(a, 2, 'i'); print a
['l', 'o', 'i', 'p', 'k', 'a', 'n']

BUT the bug breaks the equivalence when where<0...!

>>> a=list('lopkan'); ins2(a, -2, 'i'); print a
['l', 'o', 'p', 'k', 'i', 'a', 'n']
>>> a=list('lopkan'); ins1(a, -2, 'i'); print a
['i', 'l', 'o', 'p', 'k', 'a', 'n']

Yeah, I know, the docs now carefully guard this assertion of equivalence
with an "if i >= 0" and just as carefully specify that if i < 0 then the
item being "inserted" is actually prepended to the sequence.  So I'd say
there's a bug in the DOCS, too...;-)

> Guido says he also wishes list.insert() had been defined with the
> arguments in the opposite order, so that list.insert(object) could have a
> natural
> default index argument of 0.  I'd like to change that too, but it's
> clearly too late for that one.

Yep.  This has to go into a list of "tiny warts that will never be
fixed because we DO need backwards compatibility, sigh".


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