Can not pickle instances of classes having methods decorated with `lru_cache`

[simonepozzoli]

I detected that is impossible to pickle an instance of a class having a method decorated with lru_cache after the latter method is called. If the method is never called, the pickle succeeds.

Example:

import pickle
from methodtools import lru_cache

class MyClass(object):
    @lru_cache(maxsize=None)
    def decorated_method(self):
        return True

c = MyClass()
c.decorated_method()  # without this call the pickle succeeds

pickle.dumps(c)

Traceback (most recent call last):
  File "test_lru_cache_pickle.py", line 12, in <module>
    pickle.dumps(c)
  File "/usr/lib/python2.7/pickle.py", line 1380, in dumps
    Pickler(file, protocol).dump(obj)
  File "/usr/lib/python2.7/pickle.py", line 224, in dump
    self.save(obj)
  File "/usr/lib/python2.7/pickle.py", line 331, in save
    self.save_reduce(obj=obj, *rv)
  File "/usr/lib/python2.7/pickle.py", line 425, in save_reduce
    save(state)
  File "/usr/lib/python2.7/pickle.py", line 286, in save
    f(self, obj) # Call unbound method with explicit self
  File "/usr/lib/python2.7/pickle.py", line 655, in save_dict
    self._batch_setitems(obj.iteritems())
  File "/usr/lib/python2.7/pickle.py", line 669, in _batch_setitems
    save(v)
  File "/usr/lib/python2.7/pickle.py", line 306, in save
    rv = reduce(self.proto)
  File "/usr/lib/python2.7/copy_reg.py", line 77, in _reduce_ex
    raise TypeError("a class that defines __slots__ without "
TypeError: a class that defines __slots__ without defining __getstate__ cannot be pickled

[youknowone]

Oh, the cached value must be ignored for pickle.