In the section on isinstance() in the Python Standard Library documentation Chapter 2 (https://docs.python.org/3.6/library/functions.html#isinstance) , it says that classinfo "may be a tuple of type objects", but it doesn't explain what the semantics are in that case (e.g., that it will return true iff it is an instance of any of the types in the tuple).

I don't see any ambiguity here.  There are other Python APIs with the same behavior, and all we say is that it can be a tuple (eg: str.startswith).  That is, I don't see any other way a tuple could be interpreted that would make any sense.

Since Python has multiple inheritance, it could be misconstrued as a conjunctive test. For example, if c is an instance of C, which subclasses both A and B, then someone might think isinstance(c, (A, B)) requires c to be an instance of both A and B. The description could clarify that it's a disjunctive test with short circuiting.

    class MetaA(type):
        def __instancecheck__(self, other):
            print('MetaA.__instancecheck__')
            return False

    class A(metaclass=MetaA): pass

    >>> isinstance(1, (A, int))
    MetaA.__instancecheck__
    True
    >>> isinstance(1, (int, A))
    True

But without using the word "disjunctive".

Perhaps add after third sentence (ending with "not accepted)."), "If classinfo is a tuple, isinstance(x, classinfo) is the same as any(isinstance(x, C) for C in flatten(classinfo))".

I think that the following part could replace the part of the other text that's between ###. It's more objective and easier to understand. 

classinfo may could be a tuple of type objects, or may contains others tuples that contains type objects (other sequence types are not accepted).

Return true if the object argument is an instance of the classinfo argument, or of a (direct, indirect or virtual) subclass thereof. If object is not an object of the given type, the function always returns false. ### If classinfo is not a class (type object), it may be a tuple of type objects, or may recursively contain other such tuples (other sequence types are not accepted). ### If classinfo is not a type or tuple of types and such tuples, a TypeError exception is raised.

What do you think?

The fact that nested tuples are accepted is just bizarre, IMO :(.

It could say something like "If classinfo is a (possibly nested) list of types, return True if any of the types match."

r.david.murray: I liked your way.

David, if you are suggesting that the initial negative clause of "If classinfo is not a class (type object)" is not needed, I agree.  If classinfo is a tuple, is is not a class.  'list of types' has to be 'tuple of types'. I think 'match' needs to be expanded to 'is an instance of'. How about

"If classinfo is a tuple of type objects (or recursively, other such tuples), return true if object is an instance of any of the types.

This leaves out the left to right ordering of the tests, but I think that is implied by how Python generally works.  Short circuiting certainly is.

Sure, that's fine with me.  I was trying to minimize words, but clarity is good.

Doing this now.

New changeset aaf8a25133ff by Terry Jan Reedy in branch '2.7':
Issue #25432: Explain isinstance behaviour when type is a tuple.
https://hg.python.org/cpython/rev/aaf8a25133ff

New changeset 8b1418e5dcc8 by Terry Jan Reedy in branch '3.4':
Issue #25432: Explain isinstance behaviour when type is a tuple.
https://hg.python.org/cpython/rev/8b1418e5dcc8