Lecture 15


MCS 260 Fall 2021
Emily Dumas


  • We'll talk about Project 2 a bit today
  • Homework 5 due tomorrow at 10am
  • Worksheet 6 posted
  • Hints

    • In the terminal:
      • Tab = autocomplete
      • ↑ = previous command
      • ls lists files in current directory
    • You can download sample scripts from Github using the "Raw" button, then File>Save.


    JSON stands for JavaScript object notation. It is a format for storing various types of data in text files. Many languages can read and write this format.

    Python has a module for reading and writing JSON.

    By using that module, you can move various types of data in and out of files cleanly, without needing to handle the read/write details yourself.

    JSON types

    Supported basic types:

    • string — must use double quotes.
    • number — float, int, other? Up to reader.
    • boolean — lower case names true, false.
    • null — similar to Python's None.

    JSON types

    Supported composite types:

    • array — ordered sequence of values like Python list. Surrounded by square brackets, values separated by commas.
    • object — associative array like Python dict. Surrounded by curly braces, comma separator. Keys must be strings.

    A JSON file must contain a single value. Most often it is an object or array.

    Writing JSON

    Import the module json to make JSON function available.

    You'll need to open a file yourself; JSON functions expect a file object.

    Use json.dump(val,f) to write val to file object f as JSON.

    Python to JSON type conversion table:

    • dictobject
    • list or tuplearray
    • int or floatnumber
    • boolboolean
    • Nonenull

    JSON string

    If you want to handle writing yourself, you can ask the JSON module to convert a value to a JSON string, e.g.

    json.dumps(val) # make JSON string out of val

    Reading JSON

    Use json.load(f) to interpret contents of file object f as JSON and return the decoded result.

    json.loads(text) will instead process string text as JSON.

    Not supported in JSON

    • Complex numbers
    • Date/time types
    • Distinctions between:
      • int and float*
      • tuple and list
    • Comments

    * But Python's json module will try to guess when reading.


    Let's modify wordstats3file.py so that it writes a dictionary of statistics to a JSON file.

    That way, another program can read and use the report easily without concern for formatting details.


    A list of "awesome" JSON datasets.

    One of the items in that list is a data file listing episodes of the Netflix TV show Stranger Things. The link to it is actually broken, but the list is available at:


    Let's download it and poke around using Python.


    Revision history

    • 2021-09-27 Initial publication
    • 2021-09-27 Add episode list link