> The only alternative is to manually duplicate tests, these leads to very
> poor test coverage because of the average developer's laziness (json is
> an example).

No, here is another alternative:


==> _example.py <==
def foo():
    print(__name__)

==> example.py <==
def foo():
    print(__name__)
try:
    from _example import *
except ImportError:
    pass

==> test_example.py <==
import sys
sys.modules['_example'] = None
import example
example.foo()
del sys.modules['_example']
import _example as example
example.foo()

With the code above,

$ ./python.exe test_example.py
example
_example


If we move import to setUp(), we can run each test case twice: with and without native code.  Tests that are specific to one implementation can be run once or skipped conditionally on per test method basis.