Dictionaries

• Suggested video Python Dictionaries by Socratica

Python dictionaries are similar to lists. Where a list uses a number to keep track of the individual elements contained within it, a dictionary (generally) uses a word.

Create an empty dictionary

person = { }    # Curly braces instead of the square brackets used for lists

Set values to your dictionary

# Once the dictionary is created we then revert to using square brackets
person["given_name"] = "Paul"   
person["family_name"] = "Baumgarten"

Getting elements from the dictionary

print( person["given_ame"] )
print( person["family_name"] )

Adding / modifying elements in the dictionary

person["email"] = "pbaumgarten@isl.ch"
person["website"] = "https://pbaumgarten.com"

Remove an element from the dictionary

del person["website"]

Loop through all the elements of the dictionary

for key,val in person.items():
   print(f"field {key} has value {val}")

Read/write JSON files

A JSON file is a convienent file format for storing dictionary data. It is derived from the world of Javascript.

The following are two example functions that can be used for reading and writing JSON files.

import json

def read_json( filename ):
    with open(filename, "r", encoding="utf-8") as f:
        content = json.load(f)
    return content

def write_json( filename, data ):
    with open(filename, "w", encoding="utf-8") as f:
        json.dump(f, data)

I have placed some common JSON data sets on Github you can test this with, https://github.com/paulbaumgarten/data-sets.

The following example uses the countries.json data set.

import files

countries = read_json("countries.json")
for val in countries:
    country = val["name"]
    code = val["code"]
    continent = val["continent"]
    print(f"The country {country} has code {code} and is in the continent of {continent}.")

Requests

JSON is frequently used as the medium of transferring data between web servers and applications, as such knowledge of Python dictionaries can come in particularly useful for retrieving data from the internet. Take a look at my Web requests notes for more detail.

Problem set

You better believe these are coming soon! :-)

• Dictionary questions
• JSON file read/write questions
• Requests questions