Pickles are for Delis, not for Software

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Pickles are for Delis, Not Software Alex Gaynor

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About me • Django, PyPy, OpenStack, CPython, etc. • Director of the Python Software Foundation • Software engineer at Rackspace

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These are a few of my favorite things…

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These are a few of my least favorite things…

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>>> import pickle!

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What is pickle? • Serialization library • Take a random python object, turn it into bytes • Take some bytes, turn them back into an object later

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Use cases • Sending things between two Python processes which are both running (e.g. multiprocessing) • Storing random Python objects in your database (e.g. memcached)

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So how’s it work?

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>>> pickle.dumps([1, "a", None])! "(lp0\nI1\naS'a'\np1\naNa."!

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>>> pickle.dumps([1, "a", None])! "(lp0\nI1\naS'a'\np1\naNa."! Random nonsense!

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def dump(self, obj):! # Check the type dispatch table! t = type(obj)! f = self.dispatch.get(t)! if f:! f(self, obj)! return! ...!

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def dump_list(self, obj):! self.write(EMPTY_LIST)! for x in obj:! self.dump(x)! self.write(APPEND)!

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def dump_int(self, obj):! self.write(INT + repr(obj) + '\n')!

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def dump_none(self, obj):! self.write(NONE)!

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>>> import pickletools! >>> pickletools.dis("(lp0\nI1\naS'a'\np1\naNa.")! 1: l LIST (MARK at 0)! 5: I INT 1! 8: a APPEND! 9: S STRING 'a'! 17: a APPEND! 18: N NONE! 19: a APPEND! 20: . STOP

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So what about your classes?

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def dump(self, obj):! # Check the type dispatch table! t = type(obj)! f = self.dispatch.get(t)! if f:! f(self, obj)! return! ...!

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def dump(self, obj):! # ...! func, args = obj.__reduce__()! if reduce:! rv = reduce()! self.save(func)! self.save(args)! self.write(REDUCE)!

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>>> pickletools.dis(pickle.dumps(object()))! 0: c GLOBAL 'copy_reg _reconstructor'! 29: c GLOBAL '__builtin__ object'! 55: N NONE! 56: t TUPLE! 60: R REDUCE! 64: . STOP!

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>>> class X(object):! ... def __init__(self):! ... self.my_cool_attr = 3! ...! >>> x = X()!

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>>> pickletools.dis(pickle.dumps(x))! 0: c GLOBAL 'copy_reg _reconstructor'! 29: c GLOBAL '__main__ X'! 44: c GLOBAL '__builtin__ object'! 67: N NONE! 68: t TUPLE! 72: R REDUCE! 77: d DICT! 81: S STRING 'my_cool_attr'! 100: I INT 3! 103: s SETITEM! 104: b BUILD! 105: . STOP!

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What if I want to have custom pickle logic?

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>>> class FunkyPickle(object):! ... def __reduce__(self):! ... return (str, ('abc',),)! ...!

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>>> pickle.loads(pickle.dumps(FunkyPickle()))! 'abc'!

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Unpickling

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Stack machines

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>>> [1, 'a', None]! [1, 'a', None]!

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LIST INT 1 APPEND STRING APPEND None APPEND

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LIST INT 1 APPEND STRING APPEND None APPEND []!

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LIST INT 1 APPEND STRING APPEND None APPEND []! 1!

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LIST INT 1 APPEND STRING APPEND None APPEND [1]

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LIST INT 1 APPEND STRING APPEND None APPEND [1] 'a'

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LIST INT 1 APPEND STRING APPEND None APPEND [1, 'a']!

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LIST INT 1 APPEND STRING APPEND None APPEND [1, 'a']! None!

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LIST INT 1 APPEND STRING APPEND None APPEND [1, 'a', None]!

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STOP [1, 'a', None]!

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def loads(self):! try:! while True:! key = self.read(1)! self.dispatch[key](self)! except _Stop as exc:! return exc.value!

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def load_empty_list(self):! self.stack.append([])! dispatch[EMPTY_LIST] = load_empty_list! ! def load_append(self):! stack = self.stack! value = stack.pop()! list = stack[-1]! list.append(value)! dispatch[APPEND] = load_append!

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def load_reduce(self):! stack = self.stack! args = stack.pop()! func = stack[-1]! value = func(*args)! stack[-1] = value! dispatch[REDUCE] = load_reduce!

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Security

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>>> class WhatEvenIsSecurity(object):! ... def __reduce__(self):! ... return (os.listdir, ('.',),)! ...! >>> p = pickle.dumps(WhatEvenIsSecurity())! >>> pickle.loads(p)! ['Applications', 'Desktop', 'Documents', 'Downloads', ...]!

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You cannot safely unpickle untrusted data. https://blog.nelhage.com/2011/03/exploiting-pickle/

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Do you, Programmer, take this Object to be part of the persistent state of your application, to have and to hold, through maintenance and iterations, for past and future versions, as long as the application shall live? ! - Erm, can I get back to you on that?

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Let me tell you a story…

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>>> class X(object):! ... pass! ...! >>> x = X()! >>> p = pickle.dumps(x)! >>> del X! >>> pickle.loads(p)! Traceback (most recent call last):! ...! AttributeError: 'module' object has no attribute 'X'!

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>>> class X(object):! ... def __init__(self):! ... self.x = 4! ... def f(self):! ... return self.x! ...! >>> p = pickle.dumps(X())! >>> pickle.loads(p)! <__main__.X object at 0x107f80d90>!

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>>> class X(object):! ... def __init__(self):! ... self.y = 4! ... def f(self):! ... return self.y! ...! >>> x = pickle.loads(p)! >>> x.f()! Traceback (most recent call last):! File "", line 1, in ! File "", line 5, in f! AttributeError: 'X' object has no attribute 'y'!

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Alternatives (Also known as, write some functions)

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class Table(object):! def __init__(self, size):! self.size = size! ! def dump(self):! return json.dumps({! "version": 1,! "size": self.size! })!

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@classmethod! def load(cls, data):! assert data["version"] == 1! return cls(data["size"])!

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class Table(object):! def __init__(self, height, width):! self.heigh = heigh! self.width = width! ! def dump(self):! return json.dumps({! "version": 2,! "height": self.height,! "width": self.width,! })!

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@classmethod! def load(cls, data):! if data["version"] == 1:! h = w = sqrt(data["size"])! return cls(h, w)! elif data["version"] == 2:! return cls(data["height"], data["width"])! else:! raise ValueError!