How do I serialize a Python dictionary into a string, and then back to a dictionary?

Question

How do I serialize a Python dictionary into a string, and then back to a dictionary? The dictionary will have lists and other dictionaries inside it.

Answer 1

If you are trying to only serialize then pprint may also be a good option. It requires the object to be serialized and a file stream.

Here's some code:

from pprint import pprint
my_dict = {1:'a',2:'b'}
with open('test_results.txt','wb') as f:
    pprint(my_dict,f)

I am not sure if we can deserialize easily. I was using json to serialize and deserialze earlier which works correctly in most cases.

f.write(json.dumps(my_dict, sort_keys = True, indent = 2, ensure_ascii=True))

However, in one particular case, there were some errors writing non-unicode data to json.

Answer 2

Use Python's json module, or simplejson if you don't have python 2.6 or higher.

Answer 3

One thing json cannot do is dict indexed with numerals. The following snippet

import json
dictionary = dict({0:0, 1:5, 2:10})
serialized = json.dumps(dictionary)
unpacked   = json.loads(serialized)
print(unpacked[0])

will throw

KeyError: 0

Because keys are converted to strings. cPickle preserves the numeric type and the unpacked dict can be used right away.

Answer 4

If you fully trust the string and don't care about python injection attacks then this is very simple solution:

d = { 'method' : "eval", 'safe' : False, 'guarantees' : None }
s = str(d)
d2 = eval(s)
for k in d2:
    print k+"="+d2[k]

If you're more safety conscious then ast.literal_eval is a better bet.

Answer 5

Pickle is great but I think it's worth mentioning literal_eval from the ast module for an even lighter weight solution if you're only serializing basic python types. It's basically a "safe" version of the notorious eval function that only allows evaluation of basic python types as opposed to any valid python code.

Example:

>>> d = {}
>>> d[0] = range(10)
>>> d['1'] = {}
>>> d['1'][0] = range(10)
>>> d['1'][1] = 'hello'
>>> data_string = str(d)
>>> print data_string
{0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], '1': {0: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], 1: 'hello'}}

>>> from ast import literal_eval
>>> d == literal_eval(data_string)
True

One benefit is that the serialized data is just python code, so it's very human friendly. Compare it to what you would get with pickle.dumps:

>>> import pickle
>>> print pickle.dumps(d)
(dp0
I0
(lp1
I0
aI1
aI2
aI3
aI4
aI5
aI6
aI7
aI8
aI9
asS'1'
p2
(dp3
I0
(lp4
I0
aI1
aI2
aI3
aI4
aI5
aI6
aI7
aI8
aI9
asI1
S'hello'
p5
ss.

The downside is that as soon as the the data includes a type that is not supported by literal_ast you'll have to transition to something else like pickling.

Answer 6

While not strictly serialization, json may be reasonable approach here. That will handled nested dicts and lists, and data as long as your data is "simple": strings, and basic numeric types.