[Python-Dev] str with base

[Guido van Rossum]

On 1/16/06, Alex Martelli <aleaxit at gmail.com> wrote:
> Is it finally time in Python 2.5 to allow the "obvious" use of, say,
> str(5,2) to give '101', just the converse of the way int('101',1)
[I'm sure you meant int('101', 2) here]
> gives 5?  I'm not sure why str has never allowed this obvious use --
> any bright beginner assumes it's there and it's awkward to explain
> why it's not!-).  I'll be happy to propose a patch if the BDFL
> blesses this, but I don't even think it's worth a PEP... it's an
> inexplicable though long-standing omission (given the argumentative
> nature of this crowd I know I'll get pushback, but I still hope the
> BDFL can Pronounce about it anyway;-).

I wish you had an argument better than "every bright beginner assumes
it exists". :-)

But (unlike for some other things that bright beginners might assume)
I don't think there's a deep reason why this couldn't exist.

The only reasons I can come up with is "because input and output are
notoriously asymmetric in Python" and "because nobody submitted a
patch". :-)

There are some corner cases to consider though.

- Should repr() support the same convention? I think not.
- Should str(3.1, n) be allowed? I think not.
- Should str(x, n) call x.__str__(n)? Neither.
- Should bases other than 2..36 be considered? I don't think so.

Unfortunately this doesn't obsolete oct() and hex() -- oct(10) returns
'012', while str(10, 8) soulc return '12'; hex(10) returns '0xa' while
str(10, 16) would return 'a'.

I do think that before we add this end-user nicety, it's more
important to implement __index__() in Python 2.5. This behaves like
__int__() for integral types, but is not defined for float or Decimal.
Operations that intrinsically require an integral argument, like
indexing and slicing, should call __index__() on their argument rather
than __int__(), so as to support non-built-in integral arguments while
still complaining about float arguments. This is currently implemented
by explicitly checking for float in a few places, which I find
repulsing. __index__() won't be requested by bright beginners, but it
is important e.g. to the Numeric Python folks, who like to implement
their own integral types but are suffering from that their integers
aren't usable everywhere.

--
--Guido van Rossum (home page: http://www.python.org/~guido/)