> Changing `Foo a = new Foo()` to `Bar a = new Bar()` doesn't seem that bad... except when you have to propagate that change through the code base. Half of Java

> Changing `Foo a = new Foo()` to `Bar a = new Bar()` doesn't seem that bad... except when you have to propagate that change through the code base. Half of Java's tooling is dedicated to solving a problem that doesn't exist in good duck-typed languages or those with type inference. A sweeping change like that is hard in Java, but it's nearly impossible in Python. That is, unless you're willing throw up your hands and assume your new duck looks enough like the old one, and cross your fingers that you don't find out in production one day. With a modern IDE like Eclipse, it's a fairly trivial matter to find all references to the class, navigate call hierarchies and make the necessary changes to adapt to the change in type. In a sizable Python program, good luck. You're pretty much forced to rely on string searches - hope you picked a distinctive function name - or interactive debugging. Ease of refactoring is a huge win for static types IMHO.