[Python-Dev] find_first (and relatives)

[Josiah Carlson]

> I was a bit surprised to find out that python doesn't seem to have builtin
> functors, such as find_first.  Although there are ways to simulate such
> functions, it would be good to have an expanded set of functional
> programming tools which are coded in C for speed.

I think I've been here long enough, and it is getting to be my turn to
do this kind of thing once, so here it goes...

Address your concerns of "Python should or should not have this thing"
on python-list first (available as a newsgroup as comp.lang.python if
you so prefer).  Python-dev is about developing the core language, not
about fielding requests without background, support, salutation, and
"thank you for your consideration".

As for "find_first", the first mention of such things I found on the net
were the Boost C++ libraries for searching strings.  Considering your
recent posts on the C++ sig with regards to Boost::Python, this seems
like what you were referring to.  Searching strings are generally done
via "string literal".find("substring literal") or
string_variable.find(substring_variable), and any other such variations
you would care to use.  There also exists a find_all mechanism in the
regular expression module re, which comes standard with Python. Lists
also include find methods, though they call them index().  If your list
is sorted, you may want to consider the bisect module.

If you desire your finding methods to return an iteratble through the
sequence of positions of the item, perhaps this little throwaway
generator would be sufficient (which I'm sure a python-list user could
have helped you with)...

def find_first(str_or_list, item):
    if type(str_or_list) in (str, unicode):
        f = str_or_list.find(item)
        while f != -1:
            yield f
            f = str_or_list.find(item, f)
    elif type(str_or_list) is list:
        try:
            f = str_or_list.index(item)
        except ValueError:
            return
        while 1:
            yield f
            try:
                f = str_or_list.index(item, f)
            except ValueError:
                return
    else:
        raise ValueError,\
          "type %s is not supported for searching"%type(str_or_list)

 - Josiah