3.5 Dictionary Basics

Creating a dictionary

• A dictionary is a Python container used to describe associative relationships.
• A dictionary is represented by the dict object type
• A dict object is created using curly braces { } to surround the key:value pairs
  • Ex: players = {'Lionel Messi': 10, 'Cristiano Ronaldo': 7}
• An empty dictionary is created with the expression players = { }
• Why use a dictionary rather than a list?
  • Ex: If a program contains a collection of anonymous student test scores, those scores should be stored in a list. However, if each score is associated with a student name, a dictionary could be used to associate student names to their score.
  • Other examples of associative relationships include last names and addresses, car models and price, or student ID number and university email address.

Accessing dictionary entries

• Dictionaries maintain a left-to-right ordering, dictionary entries cannot be accessed by indexing
• To access an entry, the key is specified in brackets [ ].
  • If no entry with a matching key exists in the dictionary, then a KeyError runtime error occurs and the program is terminated.

Adding, modifying, and removing dictionary entries

• A new dictionary entry is added by using brackets to specify the key: prices['banana'] = 1.49
• The del keyword is used to remove entries from a dictionary: del prices['papaya'] removes the entry whose key is 'papaya'.
  • If the requested key to delete does not exist then a KeyError occurs.

Adding new entries to a dictionary:

• dict[k] = v: Adds the new key-value pair k-v, if dict[k] does not already exist.
  • Example: students['John'] = 'A+'

Modifying existing entries in a dictionary:

• dict[k] = v: Updates the existing entry dict[k], if dict[k] already exists.
  • Example: students['Jessica'] = 'A+'

Removing entries from a dictionary:

• del dict[k]: Deletes the entry dict[k].
  • Example: del students['Rachel']