Removing Items From a Dictionary

The clear and pop dictionary methods mutate (change) the original collection.

clear Example

The general syntax for this method is:

dictionary_name.clear()

clear removes all key/value pairs from a dictionary. The dictionary still exists, but it is empty.

No arguments go inside the ().

Example

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phone_book = {
   'Mom' : '555-5555',
   'Work' : '555-5556',
   'Home' : '123-456-7890',
   'BFF' : '555-5557'
}

print(phone_book)

phone_book.clear()
print(phone_book)

print(len(phone_book))  # Print the length of the dictionary

Output

{'Mom': '555-5555', 'Work': '555-5556', 'Home': '123-456-7890', 'BFF': '555-5557'}
{}
0

pop Examples

The general syntax for this method is:

dictionary_name.pop(key)

pop removes the key indicated inside the (), along with its linked value. Even though only the key goes inside the (), the key/value pair gets removed from the dictionary.

In addition to removing a key/value pair from the dictionary, pop also returns the value. This means dictionary_name.pop(key) can be printed or assigned to a variable.

Example

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phone_book = {
   'Mom' : '555-5555',
   'Work' : '555-5556',
   'Home' : '123-456-7890',
   'BFF' : '555-5557'
}

# Remove the 'Home' key/value pair, but the returned value is lost.
phone_book.pop('Home')
print(phone_book)

# Remove the 'BFF' key/value pair and assign the returned value to a variable.
returned_value = phone_book.pop('BFF')
print(returned_value)
print(phone_book)

Output

{'Mom': '555-5555', 'Work': '555-5556', 'BFF': '555-5557'}
555-5557
{'Mom': '555-5555', 'Work': '555-5556'}

Missing Key

Unlike the list method with the same name, the pop() method for dictionaries MUST include a key inside the (). If we call the method without a key, or if the key does not exist in the dictionary, Python throws an error message.

Example

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phone_book = {
   'Mom' : '555-5555',
   'Work' : '555-5556',
   'Home' : '123-456-7890',
   'BFF' : '555-5557'
}

phone_book.pop()
phone_book.pop('Beyonce')

  1. Line 8 throws TypeError: pop expected at least 1 argument, got 0. Python expected to find a key argument, but we did not provide one.

  2. Line 9 throws KeyError: 'Beyonce'. The key Beyonce does not exist in phone_book.

Set a Default Return Value

As good coders, we can build our pop statements to only use known keys. However, if we allow outside users to change a dictionary, then we cannot guarantee that calling pop(key) remains safe.

Fortunately, we can set a default return value to avoid throwing runtime errors! The general syntax is:

dictionary_name.pop(key, default_value)

If Python cannot find the given key inside a dictionary, it returns the default_value instead. The dictionary itself remains unchanged, but we do avoid crashing the program.

Try It!

Run the following program several times and enter different key names. Be sure to try:

  1. A key that exists in phone_book,

  2. A key that does NOT exist in phone_book,

  3. Just hitting return to enter the empty string, ''.