How to prettyprint a JSON file?

Use the indent= parameter of json.dump() or json.dumps() to specify how many spaces to indent by:

>>> import json
>>> your_json = '["foo", {"bar": ["baz", null, 1.0, 2]}]'
>>> parsed = json.loads(your_json)
>>> print(json.dumps(parsed, indent=4))
[
    "foo",
    {
        "bar": [
            "baz",
            null,
            1.0,
            2
        ]
    }
]

To parse a file, use json.load():

with open('filename.txt', 'r') as handle:
    parsed = json.load(handle)

You can do this on the command line:

python3 -m json.tool some.json

(as already mentioned in the commentaries to the question, thanks to @Kai Petzke for the python3 suggestion).

Actually python is not my favourite tool as far as json processing on the command line is concerned. For simple pretty printing is ok, but if you want to manipulate the json it can become overcomplicated. You'd soon need to write a separate script-file, you could end up with maps whose keys are u"some-key" (python unicode), which makes selecting fields more difficult and doesn't really go in the direction of pretty-printing.

You can also use jq:

jq . some.json

and you get colors as a bonus (and way easier extendability).

Addendum: There is some confusion in the comments about using jq to process large JSON files on the one hand, and having a very large jq program on the other. For pretty-printing a file consisting of a single large JSON entity, the practical limitation is RAM. For pretty-printing a 2GB file consisting of a single array of real-world data, the "maximum resident set size" required for pretty-printing was 5GB (whether using jq 1.5 or 1.6). Note also that jq can be used from within python after pip install jq.

Pygmentize is a powerful tool for coloring the output of terminal commands.

Here is an example of using it to add syntax highlighting to the json.tool output:

echo '{"foo": "bar"}' | python -m json.tool | pygmentize -l json

The result will look like:

In a previous Stack Overflow answer, I show in detail how to install and use pygmentize.

Use this function and don't sweat having to remember if your JSON is a str or dict again - just look at the pretty print:

import json

def pp_json(json_thing, sort=True, indents=4):
    if type(json_thing) is str:
        print(json.dumps(json.loads(json_thing), sort_keys=sort, indent=indents))
    else:
        print(json.dumps(json_thing, sort_keys=sort, indent=indents))
    return None

pp_json(your_json_string_or_dict)