Shelve: Can't pickle <class 'method'>: attribute lookup builtins.method failed

Question

I'm using shelve to store some data.

Traceback (most recent call last):
    File "rogue.py", line 312, in <module>
    curses.wrapper(game)
File "/usr/lib/python3.3/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
File "rogue.py", line 289, in game
    save_game(y,x)
File "rogue.py", line 119, in save_game
    file['player'] = player
File "/usr/lib/python3.3/shelve.py", line 124, in __setitem__
    p.dump(value)

_pickle.PicklingError: Can't pickle <class 'method'>: attribute lookup builtins.method failed

I looked around and found stuff about unbound methods could be causing this, but I can't find the problem in my code.

The problem arises at

file['player'] = player

player is class entity:

player = entity(y,x, '@', 200, False)

This is the code for class entity:

class entity:
def __init__(self, y, x, ch, speed, ai=True):
    self.y = y
    self.x = x
    self.ch = ch
    self.speed = speed
    self.ai = ai


    self.ap = 0
    self.current_action = {'action': self.wait, 'cost': self.speed}
    self.my_turn = False

def draw(self):
    world[self.y][self.x].walkable = False
    gamepad.addch(self.y, self.x, self.ch)

def take_turn(self):

    self.my_turn = True
    cost = self.current_action['cost']
    self.current_action['action']()

    return cost
def move(self, dy, dx):

    if self.my_turn == False:
        self.current_action = {'action': partial(self.move, dy,dx), 'cost':200}
        deck.append(self)

    if self.my_turn == True:

        #p = previous
        py = self.y
        px = self.x


        pt = world[py][px]
        #Paint previous ground tile
        pt.walkable = True
        gamepad.addch(pt.y,pt.x,pt.ch)

        if world[dy][dx].walkable == True:
            self.y = dy
            self.x = dx
            world[dy][dx].walkable = False

        if self.ai == False:
            draw_map(self.y,self.x)


        self.my_turn = False
    return self.y,self.x

def wait(self):
    self.current_action = {'action': self.wait, 'cost': self.speed}
def drunk_move(self):
    dy = self.y + random.randint(-1,1)
    dx = self.x + random.randint(-1,1)
    self.move(dy, dx)
def ai_simple(self):

    #Figure out if player is higher or not:
    wherey = self.y - player.y
    if wherey > 0:
        dy = self.y-1
    else:
        dy = self.y+1

    wherex = self.x - player.x
    if wherex > 0:
        dx = self.x-1
    else:
        dx = self.x+1

    self.move(dy, dx)

---

Before posting this I tried to take one final look with pudb - and I'm also getting an error on

file['world'] = world

_

│  File "/usr/lib/python3.3/shelve.py", line 124, in                    │
│__setitem__                                                            │
│    p.dump(value)                                                      │         
│_pickle.PicklingError: Can't pickle <class                             │
│'__main__.tile'>: attribute lookup __main__.tile failed                │

(which does not show when running the code without pudb)

This is world:

world = [[ tile(yy,xx,True,'.')
    for xx in range(WORLD_WIDTH) ]
        for yy in range(WORLD_HEIGHT) ]

this is class tile:

class tile:
def __init__(self,y,x,walkable,ch):
    self.x = x
    self.y = y
    self.walkable = walkable
    self.ch = ch

Finally, this is the complete function that calls shelve:

def save_game():
    file = shelve.open('savegame', 'n')
    file['world'] = world
    file['player']= player

    file.close()

I believe that's all the relevant code.

What's causing these Errors?

Answer 1

The first problem is caused by this line in your entity class:

self.current_action = {'action': self.wait, 'cost': self.speed}

current_action contains references to bound methods, which can't be pickled. You could use __getstate__ and __setstate__ do change your pickling behaviour, so you don't pickle the method but it's name (or name and arguments in the case where have assigned a partial object) when pickling, and restore that values at unpickling time.

I'm not really sure about your second problem, but if it only happens when you run it inside a debugger, it's probably an issue with how the debuger loads the __main__ module. You could try to move your __main__ block into another module and import and lounch your code from there. Sometimes that can fix problems like these.