Dictionaries are datastructure objects that can store information in a key value pair format, in this post, we will explore some information about dictionaries and their associated methods. The prerequisites are knowledge of boolean, exceptions, float, integers, methods, strings, etc.

Define a Dictionary

We are going to import the datetime module first, and then create a dictionary.

# datetime module in datetime package is required to create datetime objects
from datetime import datetime

# we create a dictionary
dictionary1 = {
    'mainVersion': 3.8,
    'releaseVersion': '3.8.3',

    # strftime is optional and is used to print the date in desired format
    # strtime converts datetime object into string
    # %b for short version of month, % d for date, %Y for full version of year
    'releaseDate': datetime(2020, 5, 13).strftime('%b %d, %Y')
}

The dictionary has 3 items in it, each with a key and a value. The keys are ‘mainVersion’, ‘releaseVersion’ and ‘releaseDate’ and the correspoding values are written after the ‘:’. The 3rd value is a datetime object, which is used to print a date in specified format as mentioned by the strftime method.

We may now print the dictionary, see it’s data type, and view the contents of the ‘dict’ class.

print(dictionary1)
print(type(dictionary1))
print('All the attributes and methods of the dict class')
print(dir(dict))
print('-' * 50)

output:

{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020'}
<class 'dict'>
All the attributes and methods of the dict class
['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']
--------------------------------------------------

Update

using update method

The update method can be used to update the dictionary. Let’s add few more entries.

print('update dictionary')
dictionary1.update ({
    'language' : 'Python',
    'designer': 'Guido Van Rossum',
    'extrakey': 'extravalue',
    'features': ['Interpreted', 'High level'],
    'dummykey': 'dummyvalue'
})
print(dictionary1)
print('-' * 50)

output:

update dictionary
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'extrakey': 'extravalue', 'features': ['Interpreted', 'High level'], 'dummykey': 'dummyvalue'}
--------------------------------------------------

using the key directly

This method is more easy, we could directly add or modify a key as follows.

print('update or modify dictionary using key')                                                                                                       # new key and value
dictionary1['developer'] = 'Python software foundation'
# modify existing key's value
dictionary1['extrakey'] = 'EXTRA Value'
print(dictionary1)
print('-' * 50)

output:

update or modify dictionary using key
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'extrakey': 'EXTRA Value', 'features': ['Interpreted', 'High level'], 'dummykey': 'dummyvalue', 'developer': 'Python software foundation'}
-------------------------

pop

We can remove a key and values using the pop method.

print('pop from dictionary')
poppedValue = dictionary1.pop('extrakey')
print(poppedValue)
print(dictionary1)
print('-' * 25)
# if a key is not found, pop() can return a default value
poppedValue = dictionary1.pop('extrakey', 'no value popped')
print(poppedValue)
print(dictionary1)
print('-' * 50)

output:

pop from dictionary
EXTRA Value
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'features': ['Interpreted', 'High level'], 'dummykey': 'dummyvalue', 'developer': 'Python software foundation'}
-------------------------
no value popped
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'features': ['Interpreted', 'High level'], 'dummykey': 'dummyvalue', 'developer': 'Python software foundation'}
--------------------------------------------------

Length

print('Get the length of dictionary')
# prints the no. of keys
print(len(dictionary1))
print('-' * 50)

output:

Get the length of dictionary
8
--------------------------------------------------

Check for a key

We can use the ‘in’ statement to check if a key exists in a dictionary. The result will be a boolean, either True or False.

print('To check if a specific key exists in the dictionary')
# prints True or False
print('mainVersion' in dictionary1)
print('MainVersion' in dictionary1)
print('-' * 50)

output:

To check if a specific key exists in the dictionary
True
False
--------------------------------------------------

Check for Keys, Values

>>> shirt = {
... 'Brand': 'Louis Philippe',
... 'Size': '40',
... 'Color': 'White'
... }
>>>
>>> # check for a particular key
>>> if 'Brand' in shirt:
...     print('yes')
... else:
...     print('no')
...
yes
>>> if 'Model' in shirt:
...     print('yes, model is there')
... else:
...     print('no, model is not there')
...
no, model is not there
>>> # check for a particular value
>>> print(shirt.values())
dict_values(['Louis Philippe', '40', 'White'])
>>>
>>> if 'White' in shirt.values():
...     print('yes its a white shirt')
... else:
...     print('no, its not white')
...
yes its a white shirt
>>> if 'Black' in shirt.values():
...     print('yes, its black')
... else:
...     print('no, its not black')
...
no, its not black
>>>

Loop

Iterate over keys

We can iterate over a dictionary’s keys using the ‘for’ statement. And we can retrieve the value of each key either directly calling the key as index or by using the get method.

print("To iterate over the dictionary's keys")
for key in dictionary1:
    print('-' * 25)
    print(type(key))
    print(key)
    print('---')
    # retrieve the value of the key
    value = dictionary1[key]
    print(type(value))
    print(value)
    print('---')
    # retrieve the value of key in an alernate way
    value = dictionary1.get(key)
    print(type(value))
    print(value)

print('-' * 50)

output:

To iterate over the dictionary's keys
-------------------------
<class 'str'>
mainVersion
---
<class 'float'>
3.8
---
<class 'float'>
3.8
-------------------------
<class 'str'>
releaseVersion
---
<class 'str'>
3.8.3
---
<class 'str'>
3.8.3
-------------------------
<class 'str'>
releaseDate
---
<class 'str'>
May 13, 2020
---
<class 'str'>
May 13, 2020
-------------------------
<class 'str'>
language
---
<class 'str'>
Python
---
<class 'str'>
Python
-------------------------
<class 'str'>
designer
---
<class 'str'>
Guido Van Rossum
---
<class 'str'>
Guido Van Rossum
-------------------------
<class 'str'>
features
---
<class 'list'>
['Interpreted', 'High level']
---
<class 'list'>
['Interpreted', 'High level']
-------------------------
<class 'str'>
dummykey
---
<class 'str'>
dummyvalue
---
<class 'str'>
dummyvalue
-------------------------
<class 'str'>
developer
---
<class 'str'>
Python software foundation
---
<class 'str'>
Python software foundation
--------------------------------------------------

Iterate over both keys and values

The items method could be used to refer to both keys and values.

print("To iterate over both keys and values")
for key, value in dictionary1.items():
    print('-' * 25)
    print(key, value)

print('-' * 50)

output:

To iterate over both keys and values
-------------------------
mainVersion 3.8
-------------------------
releaseVersion 3.8.3
-------------------------
releaseDate May 13, 2020
-------------------------
language Python
-------------------------
designer Guido Van Rossum
-------------------------
features ['Interpreted', 'High level']
-------------------------
dummykey dummyvalue
-------------------------
developer Python software foundation
--------------------------------------------------

Iterate over values

Or we can just iterate over values with the values method.

print("To iterate over values")
for value in dictionary1.values():
    print(value)

print('-' * 50)

output:

To iterate over values
3.8
3.8.3
May 13, 2020
Python
Guido Van Rossum
['Interpreted', 'High level']
dummyvalue
Python software foundation
--------------------------------------------------

Same value dictionary

If the values for a set of keys are same, the fromkeys method could be used to create those entries quickly.

print('Create a dictionary with different keys, same value, using fromkeys()')
keys = ('color', 'fruit', 'company', 'extrakey')
value = 'Orange'
# we can initialize a dictionary and use fromkeys method it
dictionary2 = {}
dictionary2 = dictionary2.fromkeys(keys, value)
print(dictionary2)
# we can also use fromkeys() as a direct dict class method
dictionary3 = dict.fromkeys(keys, value)
print(dictionary3)

print('-' * 50)

output:

Create a dictionary with different keys, same value, using fromkeys()
{'color': 'Orange', 'fruit': 'Orange', 'company': 'Orange', 'extrakey': 'Orange'}
{'color': 'Orange', 'fruit': 'Orange', 'company': 'Orange', 'extrakey': 'Orange'}
--------------------------------------------------

Clear

We can clear a specific key’s value, by just setting it to an empty string.

print("clear a particular key's value")
dictionary1['dummykey'] = ''
print(dictionary1)
# using ''
dictionary2['extrakey'] = ''
print(dictionary2)

print('-' * 50)

output:

clear a particular key's value
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'features': ['I>{'color': 'Orange', 'fruit': 'Orange', 'company': 'Orange', 'extrakey': ''}
--------------------------------------------------

Copy

The copy method can be used to copy a dictionary.

print('copy dictionary')
dict1Copy = dictionary1.copy()
print(dict1Copy)
print('-' * 50)

output:

copy dictionary
{'mainVersion': 3.8, 'releaseVersion': '3.8.3', 'releaseDate': 'May 13, 2020', 'language': 'Python', 'designer': 'Guido Van Rossum', 'features': ['I>--------------------------------------------------

Empty

print('Empty the dictionary')
# using clear()
dictionary1.clear()
# or using {}
dictionary2, dictionary3 = {}, {}

print(dictionary1, dictionary2, dictionary3)
print('-' * 50)

output:

Empty the dictionary
{} {} {}
--------------------------------------------------

Delete

We can delete the objects completely using ‘del’ statement.

print('Delete the dictionary variables')

del dictionary1
del dictionary2
del dictionary3
del dict1Copy

for i in 'dictionary1', 'dictionary2', 'dictionary3':
    try:
        # eval is used to see if a variable exists with the given name
        eval(i)
    except NameError as n:
        print(n)
print('-' * 50)

output:

Delete the dictionary variables
name 'dictionary1' is not defined
name 'dictionary2' is not defined
name 'dictionary3' is not defined
--------------------------------------------------

Nested dictinoary

A dictionary can also have a sub dictionary inside it.

Initialize and check length

# nested dictionary

mydetails = {
    'name': 'Peter',
    'surname': 'England',
    'address': {
        'door' : '23',
        'postcode' : '100098',
        'street no': '35',
        'county' : 'Berkshire'
    }
}


# its only 3 coz it has 3 keys - name, surname, and address
print(len(mydetails))

output:

3

Access the values of keys

The values of a sub key can also be accesses by calling the sub key appropriately. The get method may also be used alternatively to retrieve values.

print(mydetails['address'])
print(mydetails['address']['county'])
print(mydetails['address']['postcode'])
print(mydetails.get('address').get('door'))
print(mydetails['address']['door'])

output:

{'door': '23', 'postcode': '100098', 'street no': '35', 'county': 'Berkshire'}
Berkshire
100098
23
23

Loop over a sub dictionary

We can iterate through the ‘address’ sub dictionary.

>>> for k, v in mydetails['address'].items():
...     print(k, v)
...
door 23
postcode 100098
street no 35
county Berkshire

Delete this dictionary

del mydetails

dict class

We can also create a dictionary using the dict class, this method refers to creating an instance of a class, as however it belongs to the ‘dict’ class.

# alternate way of creating a dictionary
# no quotes in keys
# use parantheses
# use the dict keyword
shirt = dict(
Brand = 'Louis Philippe',
Size = '40',
Color = 'White'
)

print(shirt)
del shirt

output:

{'Brand': 'Louis Philippe', 'Size': '40', 'Color': 'White'}

Another example

One more example for practice.

$ cat phone-dictionary.py
emptyDictionary = {}
print(type(emptyDictionary))

sampleDictionary = {
'model': 'iphone7',
'manufacturer': 'apple',
'color': 'space grey'
}

print(type(sampleDictionary))
print(sampleDictionary)

print(sampleDictionary['model'])
print(sampleDictionary['manufacturer'])
print(sampleDictionary['color'])

# change the color to rose gold
sampleDictionary['color'] = 'rose gold'

print(sampleDictionary['color'])

$ python3 phone-dictionary.py
<class 'dict'>
<class 'dict'>
{'model': 'iphone7', 'manufacturer': 'apple', 'color': 'space grey'}
iphone7
apple
space grey
rose gold

–end-of-post–