[Python-ideas] PEP 484: Generating test inputs from type hints

[Terry Reedy]

PEP 484 proposes "a standard syntax for type annotations", principally 
for static analysis (and other static uses), with runtime checking or 
optimization mentioned as a possibility.

Another (real) runtime use is generating unit test inputs.  Today on 
python-list, David MacIver announced Hypothesis, a testing library that 
does just that. (The thread title is "Hypothesis 1.0: A production 
quality property-based testing library for Python".)  Here is an example 
from http://hypothesis.readthedocs.org/en/latest/quickstart.html

from hypothesis import given
@given([int])
def test_reversing_twice_gives_same_list(xs):
     assert xs == list(reversed(reversed(xs)))

When a test runner such as pytest calls the wrapper, it calls the real 
function with a variety of int lists (including  [], but details not 
important here). @given() also works with unittest TestCase methods.

When I mentioned PEP 484 and the alternative syntax List(int), he 
replied "Having something like that as standard would be great for 
Hypothesis and I intend to support it once it becomes available."

I presume 'support' could and would include a version of given() that 
accessed .__annotations__.  Of course, if every 'test_xyz' were to be 
decorated with the same decorator, then the decorator could be omitted 
and the fuzzing moved into the test runner.

-- 
Terry Jan Reedy