Introduction to Programming

Creating dictionaries in python

A dictionary is a collection of key/value pairs. The keys must be unique, and must be of an immutable type (such as int, str, float, tuple, but not list). The dictionary itself is mutable.

One way to define a dictionary in python is to use a dictionary literal. We list the items separated by commas, and surround the list with curly brackets {}. Each item is defined as key:value. For example, here's a dictionary that stores phone numbers associated with each unique name:

phonebook = {'Jane': '213-665-1234', 
             'Milagros': '720-933-5418', 
             'Kebede': '847-439-4312',
             'Desta': '847-439-4312' }

Each name is the key, and the phone numbers are the associated values. The key type is str, which is immutable as required. Here, the value is also a str, but note that the value is allowed to be mutable. For example, the value could be a list of associated phone numbers. Duplicate names are not allowed, but duplicate values are allowed (in this example, two people could share the same phone number).

Another way to create a dictionary is with the dict() function. Pass a list of tuples representing the key/value pairs. The phonebook above could equally have been created with this line:

phonebook = dict([('Jane', '213-665-1234'), 
             ('Milagros', '720-933-5418'), 
             ('Kebede', '847-431-1729'),
             ('Desta', '847-439-4312')])

Read the above definition of phonebook carefully: it has a pair of outermost parentesis () for the funtion call to dict(), it has a pair of square brackets [] for the beginning and end of the list of tuples, and each tuple in the list is defined with a pair of parentheses (), and consists of a key/value pair.

You can create a new empty dictionary in either of these two ways:

empty_dictionary1 = {}
empty_dictionary2 = dict()

Accessing, modifying and adding items to a dictionary

We can access individual elements of a dictionary using the same square-bracket syntax we use for an index in lists and strings. Put the key in the square brackets.

print(phonebook['Desta']) # outputs 847-439-4312
milagros_number = phonebook['Milagros'] # look up Milagro's number, then store in a variable

Think of this as a lookup. The expression phonebook['Desta'] searches in the dictionary for an item with key Desta, producing the value associated with the key Desta.

That same syntax can be used to add a new item to the dictionary or to modify an existing element

phonebook['HuanHuan'] = '602-654-3210' # adds a new item to the dictionary with key 'HuanHuan'
phonebook['Jane'] = '808-647-1234' # changes phone number for key 'Jane'

If you try to access a key that does not exist in the dictionary, a KeyError is produced.

# The program crashes on this line with a KeyError
# since there is no item in phonebook with key "Kai"
print(f"Kai's number: {phonebook['Kai']}")

Deleting items from a dictionary

To remove an item from a dictionary, we can use the del operator:

print(phonebook['Kebede']) # outputs '847-431-1729'
del phonebook['Kebede']    # removes item with key `Kebede`
print(phonebook['Kebede']) # crashes with a KeyError

Checking for membership in a dictionary

To check if a key exists in a dictionary, use the in operator. The expression <key> in <dictionary> is a boolean expression that evaluates to True if the key matches an item in the dictionary, False otherwise.

This gives us a way to avoid accessing a non-existent element, thus avoiding a KeyError.

if `Kebede` in phonebook:
    print(f"Kebede's number is {phonebook['Kebede']}")
else:
    print("No  known phone number for Kebede.")

Note: you could alternately handle key errors by accessing the dictionary within a try block and handling KeyErrors with the except block.

Practice exercises

# Exercise 1: create an empty phonebook and print it:
phonebook = dict()
print(f"empty: {phonebook}")

# Exercise 2: Reinitialize the phonebook with two number in 2 different ways
#first way:
phonebook = {"Alice": "123-145-2541", "Bob": "145-145-7514"}
print(f"Phonebook with 2 items: {phonebook}")
# second way:
phonebook = dict([("Alice", "123-145-2541"), ("Bob", "145-145-7514")])
print(f"Same phonebook with 2 items: {phonebook}")

# Exercise 3: Add two more names to phonebook and output it
phonebook["Hong"] = "145-134-5614"
phonebook["Emily"] = "145-146-7316"
print(f"Phonebook after adding two more items: {phonebook}")

# Exercise 4: Print a phone number from the phonebook, then modify it, then print it again
print(f"Before: {phonebook['Alice']}")
phonebook["Alice"] = "111-111-1111"
print(f"After: {phonebook['Alice']}")

# Exercise 5: Try printing a phone number for a key that doesn't exist (producing a KeyError)
print(phonebook["Marcelo"]) # crashes with a KeyError

# Exercise 6: Fix the code above to check for membership before accessing a nonexistent key
if "Marcelo" in phonebook:
    print(phonebook["Marcelo"]) # output Marcelo's number if found
else:
    print("Marcelo is not in the phonebook")

# Exercise 7: Delete an item and print the dictionary
del phonebook["Alice"]
print(phonebook)