python nose --with-doctest adding extra test - python-3.x

Question

I am needing to defer the import of some libraries unless they are needed so I have created have an object to defer the import.
class Importer(object):
def __init__(self, name):
self.name = name
def __bool__(self):
try:
mod = importlib.import_module(self.name)
return True
except ImportError:
return False
def __getattr__(self, name):
try:
mod = importlib.import_module(self.name)
attr = getattr(mod, name)
setattr(self, name, attr)
return attr
except ImportError as e:
msg = self.name + " must be installed"
raise ImportError(msg)
I have run nose with my tests and everything passes. However, when I run --with-doctest it runs an extra test (that I have not defined) that generates an error in any environment where a given library isn't installed.
Testing the code below recreates the issue I am experiencing. In an environment without numpy, nosetests --with-doctest is running a second test which I have not defined.
numpy = Importer('numpy')
def mean(array):
"""
Mean of an array.
Examples
--------
>>> mean([1, 2, 3])
2.0
>>> mean([1, -1])
0.0
"""
if numpy:
return numpy.mean(array)
return float(sum(array) / len(array))
Clearly, there is only one doctest available to test. Also if numpy is installed only 1 test is run. So why then when numpy is not installed am I getting a second test? Here is the output.
.E
======================================================================
ERROR: Failure: ImportError (numpy must be installed)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/devin/Dropbox/Devin_S_+_Spark_Wave/nimble/zissue/issue.py", line 16, in __getattr__
mod = importlib.import_module(self.name)
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/importlib/__init__.py", line 126, in import_module
return _bootstrap._gcd_import(name[level:], package, level)
File "<frozen importlib._bootstrap>", line 994, in _gcd_import
File "<frozen importlib._bootstrap>", line 971, in _find_and_load
File "<frozen importlib._bootstrap>", line 953, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'numpy'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/site-packages/nose/failure.py", line 39, in runTest
raise self.exc_val.with_traceback(self.tb)
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/site-packages/nose/plugins/manager.py", line 154, in generate
for r in result:
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/site-packages/nose/plugins/doctests.py", line 228, in loadTestsFromModule
tests = self.finder.find(module)
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/doctest.py", line 933, in find
self._find(tests, obj, name, module, source_lines, globs, {})
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/doctest.py", line 992, in _find
if ((inspect.isroutine(inspect.unwrap(val))
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/inspect.py", line 512, in unwrap
while _is_wrapper(func):
File "/Users/devin/anaconda3/envs/issueEnv/lib/python3.6/inspect.py", line 503, in _is_wrapper
return hasattr(f, '__wrapped__')
File "/Users/devin/Dropbox/testing/issue/issue.py", line 22, in __getattr__
raise ImportError(msg)
ImportError: numpy must be installed
----------------------------------------------------------------------
Ran 2 tests in 0.003s

Answer 1

Well, this is fun!
The core difficulty is that the doctest module "tricked" you into attempting a numpy import.
That is, it probed for numpy.__wrapped__, and you took the bait.
You need to distinguish between a probe call,
for which returning a dummy value is acceptable,
and a real call, for which you must import or raise.
Perhaps a heuristic of testing for __wrapped__ suffices.
But you may find it necessary to inspect the call stack to find the caller,
notice it is e.g. autodoc,
and change behavior based on that.