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Python Dictionaries

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Python dictionaries are data structures that contain elements (items) formed by a key (key) and a value (value).

my_dict = {“key_1”: “value_1”, “key_2”: “value_2”, “key_3”: “value_3”, …}

Let’s see a basic example:


my_dict = {'a': 1, 'b': 2, 'c': 3}
print(my_dict)

The my_dict dictionary contains alphabetic keys and numeric values. The keys of a dictionary must be unique and can be declared to get the corresponding value.

So if for example I want to print a dictionary in Python I can write:

print (my_dict [‘a’])

If the key does not exist, a keyError is returned. To check if a key exists in Python dictionaries we can use the in or not in operator to see if it is not present.


if('a' in student):
   print(my_dict['a'])

Let’s take another example of a Python dictionary that contains student data:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
print(student)
print(student['name'])

Calculating the length of the Python dictionaries

To calculate the length of a dictionary we use the function len (d). This function calculates the number of key-value pairs contained in the dictionary.


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
print(len(student))

Operations with Python dictionaries

Let’s add data to a dictionary:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
student['surname'] = 'Doe'
print(student)

Let’s delete data from a dictionary:


del(student['name'])
print(student)

Methods supported by dictionaries in Python and which we will study in detail:

MethodDescription
d.items()returns all dictionary key-value pairs d
d.keys()returns all dictionary keys d
d.values()returns all dictionary values ​​d
d.pop(key, default)removes the value from the dictionary
d.popitem()delete the last item from the dictionary
d.update(d2)adds elements of a d2 dictionary to the dictionary d
d.copy()creates and returns a copy of the dictionary d
d.clear()empty your dictionary d

Let’s take some examples. We use the d.items() method to display all key-value pairs, as a set of tuples:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
print(student.items())

We use d.keys() and d.values​​() to display the keys and values ​​of the dictionary d:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
print(student.keys())
print(student.values())

We use the d.update(d2) method in Python to merge two dictionaries:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
student_address = {'Country': 'London', 'State': 'UK', 'street': 'coding creativo'}
student.update(student_address)
print(student)

We use the d.clear() method to clear the dictionary:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
student.clear()
print(student)

This way you get an empty dictionary!

We use the d.popitem() method to remove the last element from the dictionary:


student = {'name': 'Paul', 'age': 20, 'mail': 'info@codingcreativo.it'}
student.clear()
print(student)

There are other methods to use on dictionaries in Python which we will cover later.

Some useful links

Python tutorial

Python Compiler

Install Python

Variables

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