If you encounter a custom class that explicitly raises an exception in __str__ (or __repr__ if __str__ is not defined). Or, for example a class that returns a bytes object from __str__:
class Foo:
def __str__(self):
return b''
str(Foo()) # TypeError: __str__ returned non-string (type bytes)
But personally, I have never seen this and I’m pretty sure no one has; it would be daft to do it. Likewise, a silly mistake in the implementation of __str__ or edge cases might create another Exception. It is interesting to see how you could push Python to these edge cases (look at @user2357112 answer here).
Other than that case, no built-ins generally raise an exception in this case since it is defined for all of them in Py2 and Py3.
For user defined classes str will use object.__str__ by default if not defined in Python 3 and, in Python 2, use it if a class is a new style class (inherits from object).
If a class is an old style class I believe it is classobj.__str__ that is used for classes and instance.__str__ for instances.
In general, I would not catch this, special cases aren’t special enough for this.