Python in 30 Days: Day 8 – Dictionaries

Day 8

Dictionaries

A dictionary is a collection of unordered, modifiable (mutable), paired (key: value) data types.

Creating a Dictionary

To create a dictionary, we use curly brackets, {} or the dict() built-in function.

# syntax
empty_dict = {}
# Dictionary with data values
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }

The dictionary above shows that a value could be any data type: string, boolean, list, tuple, set, or dictionary.

Dictionary Length

It checks the number of ‘key: value’ pairs in the dictionary.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(len(dct)) # 4

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }
print(len(person)) # 7

Accessing Dictionary Items

We can access Dictionary items by referring to their key name.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dct['key1']) # value1
print(dct['key4']) # value4

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }
print(person['first_name']) # Tech
print(person['country'])    # India
print(person['skills'])     # ['JavaScript', 'React', 'Node', 'MongoDB', 'Python']
print(person['skills'][0])  # JavaScript
print(person['address']['street']) # Space street
print(person['city'])       # Error

Accessing an item by key name raises an error if the key does not exist. To avoid this error first, we have to check if a key exists or if we can use the get method. The get method returns None, which is a NoneType object data type if the key does not exist.

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }
print(person.get('first_name')) # Tech
print(person.get('country'))    # India
print(person.get('skills')) #['HTML','CSS','JavaScript', 'React', 'Node', 'MongoDB', 'Python']
print(person.get('city'))   # None

Adding Items to a Dictionary

We can add new key and value pairs to a dictionary

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dct['key5'] = 'value5'

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
        }
}
person['job_title'] = 'Instructor'
person['skills'].append('HTML')
print(person)

Modifying Items in a Dictionary

We can modify items in a dictionary

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dct['key1'] = 'value-one'

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }
person['first_name'] = 'Pravin'
person['age'] = 25

Checking Keys in a Dictionary

We use the in operator to check if a key exists in a dictionary

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print('key2' in dct) # True
print('key5' in dct) # False

Removing Key and Value Pairs from a Dictionary

pop(key): removes the item with the specified key name:
popitem(): removes the last item
del: removes an item with a specified key name

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dct.pop('key1') # removes key1 item
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dct.popitem() # removes the last item
del dct['key2'] # removes key2 item

Example:

person = {
    'first_name':'Tech',
    'last_name':'G',
    'age':25,
    'country':'India',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'110034'
    }
    }
person.pop('first_name')        # Removes the firstname item
person.popitem()                # Removes the address item
del person['is_married']        # Removes the is_married item

Changing Dictionary to a List of Items

The items() method changes the dictionary to a list of tuples.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dct.items()) # dict_items([('key1', 'value1'), ('key2', 'value2'), ('key3', 'value3'), ('key4', 'value4')])

Clearing a Dictionary

If we don’t want the items in a dictionary we can clear them using clear() method

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
print(dct.clear()) # None

Deleting a Dictionary

If we do not use the dictionary we can delete it completely

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
del dct

Copy a Dictionary

We can copy a dictionary using a copy() method. Using copy we can avoid mutation of the original dictionary.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
dct_copy = dct.copy() # {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}

Getting Dictionary Keys as a List

The keys() method gives us all the keys of a dictionary as a list.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
keys = dct.keys()
print(keys)     # dict_keys(['key1', 'key2', 'key3', 'key4'])

Getting Dictionary Values as a List

The values method gives us all the values of a dictionary as a list.

# syntax
dct = {'key1':'value1', 'key2':'value2', 'key3':'value3', 'key4':'value4'}
values = dct.values()
print(values)     # dict_values(['value1', 'value2', 'value3', 'value4'])

Now do some exercises for your brain and muscles.

Exercises: Python in 30 Days: Day 8 – Dictionaries

Create an empty dictionary called dog
Add name, color, breed, legs, and age to the dog dictionary
Create a student dictionary and add first_name, last_name, gender, age, marital status, skills, country, city, and address as keys for the dictionary
Get the length of the student dictionary
Get the value of skills and check the data type, it should be a list
Modify the skills values by adding one or two skills
Get the dictionary keys as a list
Get the dictionary values as a list
Change the dictionary to a list of tuples using items() method
Delete one of the items in the dictionary
Delete one of the dictionaries