Chaining "is" operators

Yes. Any operators classified as comparisons can be chained. From the language reference:

Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.

The comparison operators are <, >, ==, >=, <=, <> (a little-used synonym for !=, gone in Python 3), !=, is, is not, in, and not in.

Yes. See comparison docs.

Comparisons can be chained arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except that y is evaluated only once (but in both cases z is not evaluated at all when x < y is found to be false).

Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once.

What the is comparison operator does:

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

Referencing the Python grammar documentation, which is read by Python to parse source files (so this is the source):

comparison: expr (comp_op expr)*
comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'

expr (comp_op expr)* should read, in plain English, "any number of expressions separated by a comparison operator," of which is is one. This means that yes, you can chain any number of is comparisons together.

To demonstrate that the comparisons are chained:

>>> a = b = c = 'foo'
>>> a is b
True
>>> a is b is c
True
>>> True is c
False

Yes, is is a comparison operator, and the formal description of chaining is in the reference manual.

https://docs.python.org/2/reference/expressions.html#not-in