Garrett Berg <googb...@gmail.com> writes: > However, there are times when I want to do type checking, and the > builtin function *isinstance* is of great use.
I would advise that when you think you want type checking, you are probably being overly restrictive. > However, this function fails to be satisfactory in returning whether > the object is a valid iterator. The call hasattr(obj, '__iter__') also > fails because str and bytes types both have that Yes, because both ‘str’ and ‘bytes’ objects are valid iterables. Using ‘isinstance’ in particular is a code smell: it breaks duck typing (checking not for the object's type, but for how the object behaves, which is usually what matters) and is almost always the wrong choice. Not *always* the wrong choice, which is why it's provided, and which is why I call it a code smell: a strong but not infallible sign something is wrong with the code as written. When you reach for ‘isinstance’ (LBYL, which is un-Pythonic), instead ask yourself: ignoring what type this object is, how do I want it to behave, and should I just use it in the expectation that it will raise an exception if it doesn't support that (EAFP, which is very Pythonic)? -- \ “What is needed is not the will to believe but the will to find | `\ out, which is the exact opposite.” —Bertrand Russell, _Free | _o__) Thought and Official Propaganda_, 1928 | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
