A dictionary in Python is a scrambled collection of objects. Unlike other data types such as a list or a set which has a single value field, the dictionary type stores a key along with its value.
The keys pair with values using a colon (:) while the commas work as a separator for the elements. Also, the entire dictionary object uses curly braces to enclose itself.
Below is an example of a barren dictionary object which doesn’t have any elements.
#The empty Python dictionary object
{}Please remember that only keys inside a dictionary can’t have duplicates, but their values can repeat themselves.
Next, the value in a dictionary can be of any valid Python data type. But the keys have a constraint to be of any immutable data types such as a string, a number, or a tuple.
Dictionary in Python
Create Dictionaries
If you wish to create a Python dictionary with fixed keys and values, then it’s quite easy to do so. Just combine all the “key1:value1, key2:value2,…” pairs and enclose with curly braces.
The combination of a key and its value, i.e. “key: value” represents a single element of a dictionary in Python.
While defining static dictionary objects, you must be careful to use unique values for keys. However, they can derive from any valid Python data type.
Also, remember to use only immutable data types for the values while they can have duplicates.
# Define a blank dictionary with no elements
blank_dict = {}
# Define a dictionary with numeric keys
num_dict = {1: 'soccer', 2: 'baseball'}
# Define a dictionary with keys having different types
misc_dict = {'class': 'senior secondary', 1: [1, 2, 3,4,5]}
# Create a dictionary with dict() method
get_dict_from_func = dict({1:'veg', 2:'non-veg'})
# Create a dictionary from a sequence of tuples
make_dict_from_seq = dict([(1,'jake'), (2,'john')])Learn about – Python Strings
Dictionary Operations
Access Elements
Python dictionary has the key as an additional parameter. We can access the elements of a dictionary with the help of keys by using them as Index to the array operator ([]) or passing as Argument to the get() method.
Both methods will work the same, but the former will return a KeyError while the latter returns None if the element is not available.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("dict['Student Name']: ", dict['Student Name'])
print("dict['Roll No.']: ", dict['Roll No.'])
print("dict['Subject']: ", dict['Subject'])Running the above coding snippet will result in the following output.
dict['Student Name']: Berry
dict['Roll No.']: 12
dict['Subject']: EnglishAccessing an element with a non-existent key will return an error. Check out the below code.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("dict['Name']: ", dict['Name'])The above example uses the “Name” key which doesn’t exist. Running the same would produce a “KeyError.”
Traceback (most recent call last):
File "C:/Python/Python35/test_dict.py", line 2, in <module>
print("dict['Name']: ", dict['Name'])
KeyError: 'Name'In the next example, we’ll access the elements in a Python dictionary using the get() function.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print(dict.get('Student Name'))
print(dict.get('Roll No.'))
print(dict.get('Subject'))This code will yield the following result.
Berry
12
EnglishAppend in a Dictionary
You can append a new item to an existing dict object with elements in the following two ways.
Assignment to Append Elements
>>> dict_append = {"1" : "Python", "2" : "Java"}
>>> print(dict_append)
{'1': 'Python', '2': 'Java'}
>>> dict_append["3"] = "CSharp"
>>> print(dict_append)
{'1': 'Python', '3': 'CSharp', '2': 'Java'}
>>>Update Method to Append Elements
You can call the dictionary’s update method to append a new element to it. We are extending the above example, see below:
>>> dict_append.update({"4" : "JavaScript"})
>>> print(dict_append)
{'1': 'Python', '3': 'CSharp', '2': 'Java', '4': 'JavaScript'}
>>>Update a Dictionary
Python allowed the dictionary object to be mutable. Hence, the add or update operations are permissible. You can push a new item or modify any existing with the help of the assignment operator.
Whenever you add an element whose key already exists, it’s value will get changed to the new value. On adding a fresh “key: value” pair, a new element will get added to the dictionary.
Use Assignment
In the below example, we are demonstrating both the update and the addition operations on the existing dictionary object.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("The student who left:", dict.get('Student Name'))
dict['Student Name'] = 'Larry'
print("The student who replaced: [Update]", dict.get('Student Name'))
dict['Student Name'] = 'Jarry'
print("The student who joined: [Addition]", dict.get('Student Name'))The execution of the above code will produce the following output.
The student who left: Berry
The student who replaced: [Update] Larry
The student who joined: [Addition] JarryUse the Update Method
You can alternatively call the update method to modify the value of any existing element. See the example below.
Let’s resume the previous example. It has a dict object which contains a key as ‘Student Name’. We’ll update its value.
>>> sample_dict = {"python" : 1, "csharp" : 2, "javascript" : 3}
>>> sample_dict.update( javascript = 2 )
>>> print(sample_dict)
{'python': 1, 'javascript': 2, 'csharp': 2}
>>> sample_dict.update( csharp = 3 )
>>> print(sample_dict)
{'python': 1, 'javascript': 2, 'csharp': 3}Remove Elements
Python lends us a no. of methods to remove elements from the dictionary.
The most common of them is the “pop()” method. It takes the key as the input and deletes the corresponding element from the Python dictionary. It returns the value associated with the input key.
Another method is “popitem()”. It removes and returns a random element (key, value) from the dictionary.
If you like to drop all the elements from the dictionary, then use the “clear()” method to flush everything.
Nonetheless, one more way to remove an element from a dictionary is to use the del keyword. It can help you delete individual items and consequently the whole dictionary object.
# Create a Python dictionary
sixMonths = {1:31, 2:28, 3:31, 4:30, 5:31, 6:30}
# Delete a specific element
print(sixMonths.pop(6))
print(sixMonths)
# Delete an random element
print(sixMonths.popitem())
print(sixMonths)
# Remove a specific element
del sixMonths[5]
print(sixMonths)
# Delete all elements from the dictionary
sixMonths.clear()
print(sixMonths)
# Finally, eliminate the dictionary object
del sixMonths
print(sixMonths)Here is the result of running the above coding snippet.
30
{1: 31, 2: 28, 3: 31, 4: 30, 5: 31}
(1, 31)
{2: 28, 3: 31, 4: 30, 5: 31}
{2: 28, 3: 31, 4: 30}
{}
Traceback (most recent call last):
File "C:/Python/Python35/test_dict.py", line 22, in <module>
print(sixMonths)
NameError: name 'sixMonths' is not definedIn the above example, the second last statement removes all the elements from the Python dictionary, leaving it empty.
And the subsequent call to the “del” operator on the dictionary object removes it altogether. Hence, the last print statement fails with the “NameError.”
Learn about – Tuples in Python
Dictionary Attributes
Python doesn’t impose any constraints on the “Values” of a dictionary object. You can form them using the standard Python or any custom data types. But, as we said earlier, the “Keys” aren’t the same as “Values” and have altogether different handling.
Hence, it is vital for you to memorize the following two facts about the dictionary “Keys.”
The same key can’t have another value in the dictionary. It confirms that a duplicate key can’t exist. However, even if you try to supply a duplicate key, it’ll only modify the existing key with the value provided in the last assignment.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English','Student Name': 'Kerry'}
print("dict['Student Name']: ", dict['Student Name'])Executing the above code will show that the key “Student Name” will keep the last assigned value, i.e., “Kerry.”
dict['Student Name']: KerryPython says that the dictionary Keys should derive from the immutable data types. You can infer that only allowed types are strings, numbers or tuples. Check out a standard example below.
dict = {['Student Name']: 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("dict['Student Name']: ", dict['Student Name'])In the above example, the key is using a list type. Since Python doesn’t support this, hence the statement will generate a “TypeError.”
Here is the outcome of the above example.
Traceback (most recent call last):
File "C:/Python/Python35/test_dict.py", line 1, in <module>
dict = {['Student Name']: 'Berry', 'Roll No.': 12, 'Subject': 'English'}
TypeError: unhashable type: 'list'Iterate Dictionary
Similar to the lists, you can use a “for in” loop to traverse through a dictionary. In general, it’s the keys which enable iteration.
Let’s understand the concept with a simple example. It prints all the keys of a dictionary in a loop.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("Keys are:")
for key in dict:
print(key)The output of the above Python code is as follows.
Keys are:
Student Name
Roll No.
SubjectFollow one more example below which prints keys along with values.
dict = {'Student Name': 'Berry', 'Roll No.': 12, 'Subject': 'English'}
print("{Keys}:{Values}")
for key, value in dict.items():
print({key},":",{value})The result of the above Python code is as follows.
{Keys}:{Values}
{'Student Name'} : {'Berry'}
{'Roll No.'} : {12}
{'Subject'} : {'English'}Dictionary Comprehension
Just like a Python list, the dictionary also allows comprehension to create new objects from an iterable in a loop.
You can specify a dictionary comprehension by using an expression including a “key: value” pair followed by for loop and enclosed with curly braces {}.
Follow the below examples to create a dictionary using the comprehension syntax.
Let’s make a dictionary object with the string type for keys and the number format for values.
>>> {str(iter):iter for iter in [11,22,33,44,55]}
{'44': 44, '33': 33, '55': 55, '11': 11, '22': 22}Another example of creating a dictionary from the list of weekdays as keys and the length of each week as the values.
>>> weekdays = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat']
>>> {w:len(w) for w in weekdays}
{'fri': 3, 'tue': 3, 'wed': 3, 'sat': 3, 'thu': 3, 'mon': 3, 'sun': 3}We can use the “enumerate()” function in a dictionary comprehension. It helps us save the index of a specific element that we could use later. Also, in a for loop, the position of a list element isn’t visible without “enumerate().”
Such Python dictionaries which have element index can be useful in many situations.
>>> {w : i for i, w in enumerate(weekdays)}
{'fri': 5, 'tue': 2, 'wed': 3, 'sat': 6, 'thu': 4, 'mon': 1, 'sun': 0}Membership Test
We can quickly confirm whether a Key is present in a dictionary or not using the keyword “in.” Also, please note that the membership test is only applicable to Keys, not for Values.
weekdays = {'fri': 5, 'tue': 2, 'wed': 3, 'sat': 6, 'thu': 4, 'mon': 1, 'sun': 0}
# Output: True
print('fri' in weekdays)
# Output: False
print('thu' not in weekdays)
# Output: True
print('mon' in weekdays)The output of the above snippet is as follows.
True
False
TrueQuick wrap up – Python Dictionary
In this tutorial, we covered one of the most significant topics in Python. In whatever domain of Python, you choose to work, knowing dictionaries will surely help.
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