``Type-checking'' with dir()

[Harry Pehkonen]

In order to leave my classes open to receiving objects that are
string-like, list-like, dictionary-like, etc, and not necessarily
_exactly_ the built-in string, list, dictionary, etc types, I have the
desire to just check if the necessary methods exist. Instead of:

if type(a) == type(""):
...

.. . . I like:

wanted_methods = ["__getslice__", "__len__"]
if len([ m
for m in dir(a)
if m in wanted_methods ]) == len(wanted_methods):
...

In the above example, I might want to get a slice of variable a if
it's length is appropriate. The above code is a simplified in-line
version of, say, has_methods(). Any thoughts?

Thanks!
Harry.

 

[Raymond Hettinger]

In order to leave my classes open to receiving objects that are
string-like, list-like, dictionary-like, etc, and not necessarily
_exactly_ the built-in string, list, dictionary, etc types, I have the
desire to just check if the necessary methods exist. Instead of:

if type(a) == type(""):
...

. . . I like:

wanted_methods = ["__getslice__", "__len__"]
if len([ m
for m in dir(a)
if m in wanted_methods ]) == len(wanted_methods):
...

In the above example, I might want to get a slice of variable a if
it's length is appropriate. The above code is a simplified in-line
version of, say, has_methods(). Any thoughts?

The code becomes especially clean if you use the sets module:

if not Set(["__getslice__", "__len__"]) < Set(dir(a)):
raise InterfaceError(a)


Raymond Hettinger

 

[Alex Martelli]

Raymond Hettinger wrote:
...

The code becomes especially clean if you use the sets module:

if not Set(["__getslice__", "__len__"]) < Set(dir(a)):
raise InterfaceError(a)

I think you want a <= here -- otherwise, an InterfaceError would
be raised in the (unlikely, sure;-) case in which dir(a) has
ONLY the __len__ and __getslice__ entries.

(BTW, of course: __getslice__ is obsolete, and properly substituted
by __getitem__ taking a slice object as an 'index' in today's
well-implemented sequences -- so, this is not a good example
[not Raymond's fault, of course, as it was what the OP used]).


Alex

 

[Asun Friere]

oops, I meant "is going to be passed list-like objects etc ..."