output.sh

$ python3 shelve_test.py
Opening handle
No cached results, calling function
outside function
Opening handle
Traceback (most recent call last):
  File "shelve_test.py", line 46, in <module>
    other_expensive_calculation()
  File "shelve_test.py", line 29, in wrapper
    _check_cache(c, key, user_function, args, kwds)
  File "shelve_test.py", line 10, in _check_cache
    result = func(*args, **kwargs)
  File "shelve_test.py", line 44, in other_expensive_calculation
    return expensive_calculation()
  File "shelve_test.py", line 27, in wrapper
    with shelve.open(filename, writeback=True) as c:
  File "/usr/local/Cellar/python3/3.4.1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/shelve.py", line 239, in open
    return DbfilenameShelf(filename, flag, protocol, writeback)
  File "/usr/local/Cellar/python3/3.4.1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/shelve.py", line 223, in __init__
    Shelf.__init__(self, dbm.open(filename, flag), protocol, writeback)
  File "/usr/local/Cellar/python3/3.4.1/Frameworks/Python.framework/Versions/3.4/lib/python3.4/dbm/__init__.py", line 94, in open
    return mod.open(file, flag, mode)
_gdbm.error: [Errno 35] Resource temporarily unavailable

shelve_test.py

import shelve
import functools

def _check_cache(cache_, key, func, args, kwargs):
    if key in cache_:
        print("Using cached results")
        return cache_[key]
    else:
        print("No cached results, calling function")
        result = func(*args, **kwargs)
        cache_[key] = result
        return result

def cache(filename):
    def decorating_function(user_function):
        def wrapper(*args, **kwds):
            key = str(hash(functools._make_key(args, kwds, typed=False)))
            handle_name = "{}_handle".format(filename)
            if (hasattr(cache, handle_name) and
                not hasattr(getattr(cache, handle_name).dict, "closed")
               ):
                print("Using open handle")
                return _check_cache(cache._handle, key, 
                                    user_function, args, kwds)
            else:
                print("Opening handle")
                with shelve.open(filename, writeback=True) as c:
                    cache._handle = c  # Save a reference to the open handle
                    _check_cache(c, key, user_function, args, kwds)

        return functools.update_wrapper(wrapper, user_function)
    return decorating_function


@cache(filename='cache')
def expensive_calculation():
    print('inside function')
    return


@cache(filename='cache')
def other_expensive_calculation():
    print('outside function')
    return expensive_calculation()

other_expensive_calculation()
print("Again")
other_expensive_calculation()