Implicit lists

holger krekel pyth at devel.trillke.net
Fri Jan 31 18:37:04 CET 2003


Donnal Walter wrote:
> 
> "Alex Martelli" wrote:
> > Dale Strickland-Clark wrote:
> >
> > > In many places, often small utility functions, I find myself using the
> > > form:
> > >
> > > lst = maybeAnArg
> > > if type(lst) not in (list, tuple)
> > >     lst = [lst]
> > > for x in lst:
> > >     whatever
> > >
> > > or
> > >
> > > lst = maybeAnArg
> > > if type(lst) not in (list, tuple)
> > >     lst = [lst]
> > > mangle = [x for x in lst if whatever]
> > >
> > >
> > > In other words, I want to be able to treat an argument of arbitrary
> > > type as a list of 1 if it isn't already a list.
> > >
> > > Has anyone got a neater way of doing this?
> >
> > Sure:
> >
> > for x in aslist(list):
> >     whatever(x)
> >
> >
> > How aslist is written is obviously quite secondary (at least,
> > I _hope_ that's obvious...) -- all of its USES will be just as
> > neat as the above example.
> > ...
> 
> I have followed this thread with interest because I need the same kind of
> functionality. The iteron definition using generators doesn't really work
> for me for some reason. (I confess I don't fully understand generators.) I
> would settle for a function aslist that would meet the following unit tests:
> 
> class MyObject(object): pass
> 
> class test_aslist(unittest.TestCase):
> 
>     def test_string(self):
>         x = 'spam'            # arg
>         y = aslist(x)         # test function
>         z = [x]               # target
>         self.assertEquals(y, z)
> 
>     def test_tuple(self):
>         x = ('spam', 'eggs')  # arg
>         y = aslist(x)         # test function
>         z = ['spam', 'eggs']  # target
>         self.assertEquals(y, z)
> 
>     def test_list(self):
>         x = ['spam', 'eggs']  # arg
>         y = aslist(x)         # test function
>         z = x                 # target
>         self.assertEquals(y, z)
> 
>     def test_instance(self):
>         x = MyObject()        # arg
>         y = aslist(x)         # test function
>         z = [x]               # target
>         self.assertEquals(y, z)
> 
> if __name__ == '__main__':
>     unittest.main()

that's a nice approach to ask for coding. Here is
one possibility to satisfy the four tests.

def _isatom(arg):
    if isinstance(arg, (str, unicode)):
        return 1

def aslist(arg, isatom=_isatom):
    if not isatom(arg):
        try:
            return list(arg)
        except TypeError:
            pass
    return [arg]

You can also pass in an Martellian 'isstringlike'
check to determine 'atomicity'.  See earlier posts.

regards,

    holger





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