duck-type-checking?

[Steven D'Aprano]

On Wed, 12 Nov 2008 08:06:31 -0700, Joe Strout wrote:

> Let me preface this by saying that I think I "get" the concept of duck-
> typing.
> 
> However, I still want to sprinkle my code with assertions that, for
> example, my parameters are what they're supposed to be -- too often I
> mistakenly pass in something I didn't intend, and when that happens, I
> want the code to fail as early as possible, so I have the shortest
> possible path to track down the real bug.


I'm surprised nobody has pointed you at Alex Martelli's recipe here:

http://code.activestate.com/recipes/52291/

While the recipe is great, it can be tiresome to apply all the time. I 
would factor out the checks into a function, something like this:

def isstringlike(obj, methods=None):
    """Return True if obj is sufficiently string-like."""
    if isinstance(obj, basestring):
        return True
    if methods is None:
        methods = ['upper', 'lower', '__len__', '__getitem__']
    for method in methods:
        if not hasattr(obj, method):
            return False
    # To really be string-like, the following test should pass.
    if len(obj) > 0:
        s = obj[0]
        if s[0] != s:
            return False
    return True





-- 
Steven