What is the difference between dump and dumps from the Pickle module in Python?

Question

I have read the documentation of Python and also of Pickle himself, but I could not assimilate the content (lack of examples). On the Web I only found information about using "dump + load" and nothing about "dumps + loads"

Answer 1

dump, and load - each take as parameter an open file (or other object with the file interface) - and save the serialized content of the object in that file; (or load it, in the case of load).

The "dumps" does not have the parameter equivalent to the file and returns the serialized object as a byte string. Loads takes a byte string as parameter and returns the rebuilt object. They are used for when you will not write the result of the pickle to a file immediately, but for example you are Serializing objects to transmit them over the network, or to another process.

The standard Python library uses dumps and loads a lot internally, just in modules like multiprocessing, to pass objects transparently to other processes.

>>> a = {"b": ["c", "d", {1,2,3}, ({"e": "f"})]}
>>> print a
{'b': ['c', 'd', set([1, 2, 3]), {'e': 'f'}]}
>>> import pickle
>>> b = pickle.dumps(a)
>>> repr(b)
'"    (dp0\\nS\'b\'\\np1\\n(lp2\\nS\'c\'\\np3\\naS\'d\'\\np4\\nac__builtin__\\nset\\np5\\n((lp6\\nI1\\naI2\\naI3\\natp7\\nRp8\\na(dp9\\nS\'e\'\\np10\\nS\'f\'\\np11\\nsas."'
>>> c = pickle.loads(b)
>>> c == a
True
>>> c is a
False

Other serialization modules mimic the pickle interface, and have the 4 methods: dump, dumps, load and loads - this is the case of the "json" and "Marshall" module. json creates a serialized object as specified ECMA-404, which is syntactically valid Javascript, almost syntactically valid Python and interchangeable with several languages - however it can only serialize a subset of native Python data types (Unicode strings, integers and floats, booleans, None, lists and dictionaries - other strings are converted to lists) - Already Marshall can serialize all Python native data types: lists, dictionaries, sets (sets), complex numbers, etc... but will falahr with objects defined in pure Python classes - even if they are in the standard library, such as Ordereddict, namedtuple, and several others.

Pickle in turn will serialize almost everything in front of it - including class objects defined in its own code, and functions - (with a few remarks: whoever deserializes has to "know" the names of functions and classes of serialized objects)

Finally, if you really want to serialize crazy things - including functions with your content ("code" object), there’s a module in Pypi that works on top of Pickle and supports this: the Dill (and guess what: it has load, Lods, dump and dumps)

(Beware of "load" and "dump": they must use open files in binary mode, never in text mode - especially in Python 3.x)