Json Module

Posted on with tags: python

The JSON module is often used to transmit data between web servers and applications. It is time to take a close look at this module after discussing several difficult modules.


Realpython website has a nice introductory article on Json. Here is the link,

Working With JSON Data in Python

The official Python document for Json module is good.

Official JSON Module Documentation

Source Code

Like many other Python modules, the best way to learn is to read the source code. The json directory has five files. Here is a list of the files.

$ cd ~/.pyenv/versions/3.9.7/lib/python3.9/json
$ find . -maxdepth 1 -name '*.py' -exec wc -l '{}' + | sort -n
   73 ./scanner.py
   78 ./tool.py
  356 ./decoder.py
  359 ./__init__.py
  442 ./encoder.py
 1308 total

The files in this module is quite short comparing with other modules. The total line count is 1,308. The reason is that some of the source code is written in C and the code is in _json C module. But we are not going to discuss C code in this post.

The __init__ module defines several functions like dump, dumps, load, and loads. For function dumps, it calls JSONEncoder to do the actual encoding work. The code looks like this,

# simplified code 
from .encoder import JSONEncoder
_default_encoder = JSONEncoder(...)

def dumps(obj, ....):
    return _default_encoder.encode(obj)

The encoder.py module only defines one class JSONEncoder and several functions. The encode method is easy to understand.

# encode method of JSONEncoder
def encode(self, o):
    """Return a JSON string representation of a Python data structure.
    if isinstance(o, str):
        if self.ensure_ascii:
            return encode_basestring_ascii(o)
            return encode_basestring(o)

    chunks = self.iterencode(o, _one_shot=True)
    if not isinstance(chunks, (list, tuple)):
        chunks = list(chunks)
    return ''.join(chunks)

The encode method calls iterencode method to do the work. The iterencode method in turn calls c_make_encoder or _make_iterencode function. The c_make_encoder function is imported from C module. In simple cases, the Json module will call the C module to do the encoding.

# in module scope
    from _json import encode_basestring as c_encode_basestring
except ImportError:
    c_encode_basestring = None

# inside JSONEncoder class
def iterencode(self, o, _one_shot=False):


    if (_one_shot and c_make_encoder is not None
            and self.indent is None):
        _iterencode = c_make_encoder(...)
        _iterencode = _make_iterencode(...)
    return _iterencode(o, 0)

The _make_iterencode function shows how to convert a Python object to a JSON iterable. There are recursive calls to do the conversion. The code is not easy to write.

The decoder.py module is the opposite of encoder.py module. It defines a JSONDecoder class to do the decoding work. The decoding is more complicated than the encoding, but most of the implementation is in C code. The scanner.py module defines a helper function make_scanner for decoder.py module.

The tool.py module defines a command line tool, so we can use the module in bash like this.

$ echo '{"json": "obj"}' | python -m json.tool
$ python -m json.tool infile outfile
$ python -m json.tool --help   # to find more about the command line tool