Purpose of Python Type Checking?

[Terry Reedy]

"Mongryong" <Mongryong at sympatico.ca> wrote in message
news:mailman.1043633328.27598.python-list at python.org...
> Why do people use type checking in Python?

Possibilities (not necessarily disjoint):
1. Habit carried over or based on experience with other languages.
2. Situation-specific need such as changing behavior depending on
type.
3. Application of belief in "look-before-you-leap" rather than
"try-it-and-see".
4. Being careful in life- or mission-critical application.

> Yes, I know its to "ensure" that parameters are valid but doesn't
that
> defeat the whole purpose of one of Python's main feature?
Polymorphism
> and code reuse.  Who cares if a parameter is not of type(X), as long
as
> it has the methods and member variables necessary.  If the parameter
is
> missing stuff, then an exception is thrown.
>
> The reason I'm asking is because I don't use type checking at all
but I
> ses a lot of other code use it.

There have also been past postings advising against it, especially by
Alex Martelli, citing just the reason you gave above.

>  So, is there a good reason I should be using it too?

Your decision.

Terry J. Reedy