class008-Dictionaries.py

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

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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':25,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
The dictionary above shows that a value could be any data types:string, boolean, list, tuple, set or a 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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_married':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
print(len(person)) # 7
Accessing Dictionary Items
We can access Dictionary items by referring to its key name.

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

person = {
    'first_name':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
print(person['first_name']) # Asabeneh
print(person['country'])    # Finland
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 exist or 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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
print(person.get('first_name')) # Asabeneh
print(person.get('country'))    # Finland
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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
        }
}
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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
person['first_name'] = 'Eyob'
person['age'] = 252
Checking Keys in a Dictionary
We use the in operator to check if a key exist 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 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':'Asabeneh',
    'last_name':'Yetayeh',
    'age':250,
    'country':'Finland',
    'is_marred':True,
    'skills':['JavaScript', 'React', 'Node', 'MongoDB', 'Python'],
    'address':{
        'street':'Space street',
        'zipcode':'02210'
    }
    }
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 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 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 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'])
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