ValueError: wrapper loop when unwrapping

Question

Python3 test cases (doctests) are failing with my sample code. But the same is working fine in Python2.

test.py

class Test(object):
    def __init__(self, a=0):
        self.a = a

    def __getattr__(self, attr):
        return Test(a=str(self.a) + attr)

tst.py

from test import Test

t = Test()

Run test cases: python3 -m doctest -v tst.py

Error:

Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/lib/python3.6/doctest.py", line 2787, in <module>
    sys.exit(_test())
  File "/usr/lib/python3.6/doctest.py", line 2777, in _test
    failures, _ = testmod(m, verbose=verbose, optionflags=options)
  File "/usr/lib/python3.6/doctest.py", line 1950, in testmod
    for test in finder.find(m, name, globs=globs, extraglobs=extraglobs):
  File "/usr/lib/python3.6/doctest.py", line 933, in find
    self._find(tests, obj, name, module, source_lines, globs, {})
  File "/usr/lib/python3.6/doctest.py", line 992, in _find
    if ((inspect.isroutine(inspect.unwrap(val))
  File "/usr/lib/python3.6/inspect.py", line 513, in unwrap
    raise ValueError('wrapper loop when unwrapping {!r}'.format(f))
ValueError: wrapper loop when unwrapping <test.Test object at 0x7f6e80028550>

Can anyone please help in how to overcome this error.

Thanks.

Answer 1

For unittest.mock try importing items as

from unittest import mock

instead of

from unittest.mock import patch

This worked around the bug for me.

Answer 2

This is arguably a bug in doctest. What's happening is that doctest is searching for functions/methods/callables with a docstring, and while doing so it's unwrapping any decorators it finds. Why it does this, I have no idea. But anyway, doctest ends up calling inspect.unwrap(t) (where t is a Test instance), which is essentially equivalent to doing this:

while True:
   try:
       t = t.__wrapped__
   except AttributeError:
       break

Because t is a Test instance, accessing t.__wrapped__ calls __getattr__ and returns a new Test instance. This would go on forever, but inspect.unwrap is smart enough to notice that it's not getting anywhere, and throws an exception instead of entering an infinite loop.

---

As a workaround, you can rewrite your __getattr__ method to throw an AttributeError when __wrapped__ is accessed. Even better, throw an AttributeError when any dunder-attribute is accessed:

def __getattr__(self, attr):
    if attr.startswith('__') and attr.endswith('__'):
        raise AttributeError
    return Test(a=str(self.a) + attr)