Memory.clear() exception when cachedir=None

[bmcfee]

[note: I hit this on 0.8.4, but I suspect it affects earlier versions as well.]

Creating a dummy Memory object (using cachedir=None) isn't as transparent as it could be. Trying to clear the cache causes a TypeError:

In [1]: import joblib

In [2]: M = joblib.Memory(cachedir=None)

In [3]: M.clear()
WARNING:root:[Memory(cachedir=None)]: Flushing completely the cache
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-f5283b9ae328> in <module>()
----> 1 M.clear()

/usr/local/lib/python2.7/dist-packages/joblib/memory.pyc in clear(self, warn)
    880         if warn:
    881             self.warn('Flushing completely the cache')
--> 882         rm_subdirs(self.cachedir)
    883 
    884     def eval(self, func, *args, **kwargs):

/usr/local/lib/python2.7/dist-packages/joblib/disk.pyc in rm_subdirs(path, onerror)
     81     names = []
     82     try:
---> 83         names = os.listdir(path)
     84     except os.error as err:
     85         if onerror is not None:

TypeError: coercing to Unicode: need string or buffer, NoneType found

I would expect clear() on a dummy Memory object to be a silent no-op, so this seems like a bug.

[lesteve]

Confirmed, seems like a bug indeed.