[Python-ideas] "While" suggestion

[Talin]

Guido van Rossum wrote:
> On Thu, Jul 3, 2008 at 8:21 PM, Fred Drake <fdrake at acm.org> wrote:
>> On Jul 3, 2008, at 10:58 PM, Raymond Hettinger wrote:
>>> I co-authored a PEP for a do-while construct and it died solely because we
>>> couldn't find a good pythonic syntax.  The proposed while-as construct will
>>> neatly solve some of the use cases -- the rest will have to live with what
>>> we've got.
>> I actually like the "do: ... while <expr>: ..." syntax better.
> 
> Same here. The while ... as ... syntax introduces a special case that
> doesn't handle enough cases to be worth the cost of a special case.
> Zen of Python: "Special cases aren't special enough to break the
> rules."
> 
> To summarize why while...as... doesn't handle enough cases: it only
> works if the <setup> can be expressed a single assignment and the
> value assigned is also the value to be tested. Like it or not, many
> uses of assignment expressions in C take forms like these:

I'm actually more interested in the "as" clause being added to "if" 
rather than "while". The specific use case I am thinking is matching 
regular expressions, like so:

    def match_token(s):
       # Assume we have regex's precompiled for the various tokens:
       if re_comment.match(s) as m:
          # Do something with m
          process(TOK_COMMENT, m.group(1))
       elif re_ident.match(s) as m:
          # Do something with m
          process(TOK_IDENT, m.group(1))
       elif re_integer.match(s) as m:
          # Do something with m
          process(TOK_INT, m.group(1))
       elif re_float.match(s) as m:
          # Do something with m
          process(TOK_COMMENT, m.group(1))
       elif re_string.match(s) as m:
          # Do something with m
          process(TOK_STRING, m.group(1))
       else:
          raise InvalidTokenError(s)

In other words, the general design pattern is one where you have a 
series of tests, where each test can return either None, or an object 
that you want to examine further. Of course, if it's just a single 
if-statement, it's easy enough to assign the variable in one statement 
and test in the next.

There are a number of workarounds, of course - put the tests in a 
function and use return instead of elif; Use 'else' instead of elif and 
put the subsequent 'if' in the else block (and deal with indentation 
crawl) and so on. So it's not a huge deal, I'm just pointing out one 
possible use case where it might make the code a bit prettier.

(Also, there's a much faster way to do the above example, which is to 
smush all of the patterns into a single massive regex, and then check 
the last group index of the match object to see which one matched.)

-- Talin