Python JSON encoding

I'm trying to encode data to JSON in Python and I been having a quite a bit of trouble. I believe the problem is simply a misunderstanding.

I'm relatively new to Python and never really got familiar with the various Python data types, so that's most likely what's messing me up.

Currently I am declaring a list, looping through and another list, and appending one list within another:

import simplejson, json
data = [['apple', 'cat'], ['banana', 'dog'], ['pear', 'fish']]

x = simplejson.loads(data)
# >>> typeError: expected string or buffer..

x = simplejson.dumps(stream)
# >>> [["apple", "cat"], ["banana", "dog"], ["pear", "fish"]]
# - shouldn't JSON encoded strings be like: {{"apple":{"cat"},{"banana":"dog"}}

So I either:

• I don't understand JSON Syntax
• I don't understand the Pythons JSON module(s)
• I'm using an inappropriate data type.

Python lists translate to JSON arrays. What it is giving you is a perfectly valid JSON string that could be used in a Javascript application. To get what you expected, you would need to use a dict:

>>> json.dumps({'apple': 'cat', 'banana':'dog', 'pear':'fish'})
'{"pear": "fish", "apple": "cat", "banana": "dog"}'

I think you are simply exchanging dumps and loads.

>>> import json
>>> data = [['apple', 'cat'], ['banana', 'dog'], ['pear', 'fish']]

The first returns as a (JSON encoded) string its data argument:

>>> encoded_str = json.dumps( data )
>>> encoded_str
'[["apple", "cat"], ["banana", "dog"], ["pear", "fish"]]'

The second does the opposite, returning the data corresponding to its (JSON encoded) string argument:

>>> decoded_data = json.loads( encoded_str )
>>> decoded_data
[[u'apple', u'cat'], [u'banana', u'dog'], [u'pear', u'fish']]
>>> decoded_data == data
True

In simplejson (or the library json in Python 2.6 and later), loads takes a JSON string and returns a Python data structure, dumps takes a Python data structure and returns a JSON string. JSON string can encode Javascript arrays, not just objects, and a Python list corresponds to a JSON string encoding an array. To get a JSON string such as

{"apple":"cat", "banana":"dog"}

the Python object you pass to json.dumps could be:

dict(apple="cat", banana="dog")

though the JSON string is also valid Python syntax for the same dict. I believe the specific string you say you expect is simply invalid JSON syntax, however.

The data you are encoding is a keyless array, so JSON encodes it with [] brackets. See www.json.org for more information about that. The curly braces are used for lists with key/value pairs.

From www.json.org:

> JSON is built on two structures: > > A collection of name/value pairs. In > various languages, this is realized as > an object, record, struct, dictionary, > hash table, keyed list, or associative > array. An ordered list of values. In > most languages, this is realized as an > array, vector, list, or sequence. > > An object is an unordered set of > name/value pairs. An object begins > with { (left brace) and ends with } > (right brace). Each name is followed > by : (colon) and the name/value pairs > are separated by , (comma). > > An array is an ordered collection of > values. An array begins with [ (left > bracket) and ends with ] (right > bracket). Values are separated by , > (comma).

JSON uses square brackets for lists ( [ "one", "two", "three" ] ) and curly brackets for key/value dictionaries (also called objects in JavaScript, {"one":1, "two":"b"}).

The dump is quite correct, you get a list of three elements, each one is a list of two strings.

if you wanted a dictionary, maybe something like this:

x = simplejson.dumps(dict(data))
>>> {"pear": "fish", "apple": "cat", "banana": "dog"}

your expected string ('{{"apple":{"cat"},{"banana":"dog"}}') isn't valid JSON. A

So, simplejson.loads takes a json string and returns a data structure, which is why you are getting that type error there.

simplejson.dumps(data) comes back with

'[["apple", "cat"], ["banana", "dog"], ["pear", "fish"]]'

Which is a json array, which is what you want, since you gave this a python array.

If you want to get an "object" type syntax you would instead do

>>> data2 = {'apple':'cat', 'banana':'dog', 'pear':'fish'}
>>> simplejson.dumps(data2)
'{"pear": "fish", "apple": "cat", "banana": "dog"}'

which is javascript will come out as an object.

Try:

import simplejson
data = {'apple': 'cat', 'banana':'dog', 'pear':'fish'}
data_json = "{'apple': 'cat', 'banana':'dog', 'pear':'fish'}"

simplejson.loads(data_json) # outputs data
simplejson.dumps(data) # outputs data_joon

NB: Based on Paolo's answer.