Python Dictionary Pop vs Popitem

In python, the dictionary functions pop() and popitem() are used to remove items from a dictionary. In this tutorial we’ll look at these functions, their syntax and use-cases along with some examples.

Before we proceed, here’s a quick refresher on dictionaries in python – Dictionaries are a collection of items used for storing key to value mappings. They are mutable and hence we can update the dictionary by adding new key-value pairs, removing existing key-value pairs, or changing the value corresponding to a key. For more, check out our guide on dictionaries and other data structures in python.

How to remove items from a dictionary?

The python dictionary functions pop() and popitem() are used to remove items from a dictionary. The pop() function removes the key-value pair of the key passed whereas the popitem() function removes the last (based on LIFO order) key-value pair from the dictionary.

The pop() function

The pop() dictionary function is used to remove a key from a dictionary and return its value. The following is the syntax:

sample_dict.pop(key, default)

Here, sample_dict is the dictionary from which you want to remove the items.

Parameters:

• key: The key corresponding to the key-value pair you want to remove from the dictionary.
• default (optional): The value to be returned if the key is not present in the dictionary.

Returns:

• If the key is in the dictionary, it removes the key and returns its value.
• If the key is not in the dictionary and the default is not provided, it raises a KeyError.
• If the key is not in the dictionary and the default is provided, it returns the default.

Example 1: Key present in the dictionary

# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50, 'MSFT': 200}

# print the dictionary
print("Shares original:", shares)

# remove 'GOOG' from shares
returned_val = shares.pop('GOOG')

# print the dictionary
print("Shares after removing GOOG:", shares)
print("Return value from pop:", returned_val)

Output:

Shares original: {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
Shares after removing GOOG: {'APPL': 100, 'MSFT': 200}
Return value from pop: 50

In the above example, we remove the key 'GOOG' from the dictionary shares using the pop() function. It removed the key from the dictionary and returned 50, the value corresponding to the removed key.

Example 2: Key not present in the dictionary and default not specified.

# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50, 'MSFT': 200}

# print the dictionary
print("Shares original:", shares)

# remove 'GOOG' from shares
returned_val = shares.pop('TSLA')

# print the dictionary
print("Shares after removing TSLA:", shares)
print("Return value from pop:", returned_val)

Output:

Shares original: {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-5-3d73afad69c6> in <module>
      6 
      7 # remove 'GOOG' from shares
----> 8 returned_val = shares.pop('TSLA')
      9 
     10 # print the dictionary

KeyError: 'TSLA'

In the above example, we try to remove 'TSLA' which is not present in the dictionary using the pop() function. Since no default return value is specified, it resulted in a KeyError.

Example 3: Key not present but default specified.

# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50, 'MSFT': 200}

# print the dictionary
print("Shares original:", shares)

# remove 'GOOG' from shares
returned_val = shares.pop('TSLA', "Not present in dictionary")

# print the dictionary
print("Shares after removing TSLA:", shares)
print("Return value from pop:", returned_val)

Output:

Shares original: {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
Shares after removing TSLA: {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
Return value from pop: Not present in dictionary

In the above example, we again try to remove 'TSLA' which is not present in the dictionary but this time we provide the default value to be returned. The dictionary shares remains unchanged before and after the pop() function and we get our custom "Not present in dictionary" as the return value.

The popitem() function

The dictionary function popitem() removes and returns the last key-value pair from the dictionary. That is, it removes items based on the Last-In-First-Out (LIFO) order. The following is the syntax:

sample_dict.popitem()

Here, sample_dict is the dictionary from which you want to remove the items.

Parameters: The popitem() function does not take any parameters.

Returns: It removes and returns the last (key, value) pair from the dictionary. If the dictionary is empty, it raises a KeyError.

Note: From Python version 3.7, the LIFO order is guaranteed. In prior versions, the popitem() function would return an arbitrary key-value pair from the dictionary.

Example 1: Removing the last dictionary item

# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
# print the dictionary
print("Shares:", shares)

# remove item
print("Removing item using popitem")
returned_val = shares.popitem()
print("Return Value:", returned_val)
print("Shares:", shares)

# add a new item to the dictionary
print("Adding a new item to shares")
shares.update({"TSLA": 80})
print("Shares:", shares)

# remove item
print("Removing item using popitem")
returned_val = shares.popitem()
print("Return Value:", returned_val)
print("Shares:", shares)

Output:

Shares: {'APPL': 100, 'GOOG': 50, 'MSFT': 200}
Removing item using popitem
Return Value: ('MSFT', 200)
Shares: {'APPL': 100, 'GOOG': 50}
Adding a new item to shares
Shares: {'APPL': 100, 'GOOG': 50, 'TSLA': 80}
Removing item using popitem
Return Value: ('TSLA', 80)
Shares: {'APPL': 100, 'GOOG': 50}

In the above example, the popitem() function is used to remove items from the dictionary shares. It removes the last (key, value) pair from the dictionary and returns it.

Example 2: Using popitem() on an empty dictionary

# empty dictionary
shares = {}
# print the dictionary
print("Shares:", shares)

# remove item
print("Removing item using popitem")
returned_val = shares.popitem()
print("Return Value:", returned_val)
print("Shares:", shares)

Output:

Shares: {}
Removing item using popitem
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-9-6dcf5946355a> in <module>
      6 # remove item
      7 print("Removing item using popitem")
----> 8 returned_val = shares.popitem()
      9 print("Return Value:", returned_val)
     10 print("Shares:", shares)

KeyError: 'popitem(): dictionary is empty'

In the above example, a KeyError is raised on using the popitem() function on an empty dictionary.

For more on python dictionary functions, refer to the python docs.