Dictionaries

Modifying Python Dictionaries

In this lesson, you will learn how to modify existing dictionaries by updating, adding, and deleting key-value pairs.

Let's create a new dictionary for a guy named Bob.

person = {
  'name': 'Bob',
  'age': 30
}

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Now assume we learned that Bob is a data scientist and want to add this information to our dictionary.

New key-value pairs can be added to a dictionary through direct assignment, like so:

dict[key] = value

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Let's update our dictionary with the new information about Bob:

person = {
  'name': 'Bob',
  'age': 30
}
 
person['job'] = 'data scientist'
 
print(person.get('job'))

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Output

Another way to add new information to an existing dictionary is by using the dict.update() method:

person = {
  'name': 'Bob',
  'age': 30
}
 
person.update({'job': 'data scientist'})
 
print(person.get('job'))

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Output

Keep in mind that you should pass a separate dictionary as an argument to the dict.update() method.

With dict.update(), you can also add multiple key-value pairs at once:

person = {
  'name': 'Bob',
  'age': 30
}
 
person.update({'job': 'data scientist', 'gender': 'male'})
 
print(person.get('job'))
print(person.get('gender'))

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Output

We have added Bob's gender and his job at the same time.

What will be the output?

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

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What will be the output?

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

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What will be the output?

my_dict = { 'a': 1, 'b': 2 }
 
my_dict.update({'c': 2})
 
print(my_dict['c'])

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Back to Bob.

person = {
  'name': 'Bob',
  'age': 30
}

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Let's assume that Bob just turned 31, and we need to update his age in our dictionary.

We can update a key-value pair in the same way as we add a new one, through direct assignment:

person = {
  'name': 'Bob',
  'age': 30
}
 
person['age'] = 31
 
print(person.get('age'))

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Output

dict.update() can also update a key-value pair:

person = {
  'name': 'Bob',
  'age': 30
}
 
person.update({'age': 31})
 
print(person.get('age'))

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Output

That's because the dictionary that you pass as argument to dict.update() takes precedence over the existing one.

What will be the output?

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

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What will be the output?

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

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Awesome. Now we just need to learn how to delete existing key-value pairs.

Key-value pairs can be deleted using the del keyword:

person = {
  'name': 'Bob',
  'age': 31
}
 
# Remove the key-value pair with key 'age'
del person['age']
 
print(person)

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Output

You can also use the pop() method, which we already know from its usage with strings and lists.

dict.pop(key) removes the key-value pair associated with the specified key and returns the value:

person = {
  'name': 'Bob',
  'age': 31
}
 
# Remove and return the value of 'age'
age = person.pop('age')
 
print(person)
print(age)

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Output

What will be the output?

my_dict = { 'a': 1, 'b': 2 }
del my_dict['b']
print(my_dict)

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What will be the output?

my_dict = { 'a': 1, 'b': 2 }
del my_dict['b']
print(my_dict['b'])

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