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Python Done Right

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Python Done Right · 100 lessons

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Modifying a dictionary

Updating a dictionary entry

Explanation

To update an existing dictionary entry, we assign a new value to an existing key using the assignment operator =:

Syntax

dictionary[key] = new_value

For example:

# Simple dictionary representing player high scores
high_scores = {
    'player1': 1500,
    'player2': 3000,
    'player3': 2200
}

# Updating the high score of player2
high_scores['player2'] = 3200
print(high_scores)

Output:

{'player1': 1500, 'player2': 3200, 'player3': 2200}

Practice questions

4 questions

The scores for a high-stakes gaming tournament where players earn points each round are stored in the following dictionary:

scores = {
    "dragonSlayer": 500,
    "DragonMaster": 750,
    88: 1200
}

Which of the following options updates the scores for player "dragonSlayer" to 550 and for player 88 to 1300?

Select the correct answer:

+ 3 more questions

Adding a new entry to a dictionary

Explanation

To add a new entry, we use the same syntax as updating - but with a key that doesn't yet exist:

Syntax

dictionary[key] = new_value

For example:

# Simple dictionary representing fruit prices
fruit_prices = {
    'apple': 0.50,
    'banana': 0.30,
    'cherry': 1.25
}

# Adding the price of an orange
fruit_prices['orange'] = 0.8

# Checking the updated dictionary
print(fruit_prices)

Output:

{'apple': 0.5, 'banana': 0.3, 'cherry': 1.25, 'orange': 0.8}

Warning

Since the same syntax updates existing keys and creates new ones, we need to be careful not to unintentionally overwrite existing entries. We can use membership testing to check first.

Consider tracking player top scores - we only want to update if the new score is higher:

top_scores = {"player1": 1500, "player2": 1800, "player3": 1700}

new_player = "player1"
new_score = 2000

# Update or add new score if it's higher
if (
    new_player not in top_scores
    or new_score > top_scores[new_player]
):
    top_scores[new_player] = new_score
    print(f"{new_player}'s score updated to {new_score}.")
else:
    print(
        f"{new_player} already has a higher score of {top_scores[new_player]}."
    )

print("Updated player top scores:", top_scores)

Output:

player1's score updated to 2000.
Updated player top scores: {'player1': 2000, 'player2': 1800, 'player3': 1700}

The condition checks if the player is new (not in top_scores) or has beaten their previous score - if either is true, we update their entry.

Practice questions

4 questions

In the Python Console there's a dictionary defined called singer_to_song. Add the following entries to it:

  • "Elton John" as the singer, "Rocket Man" as the song.
  • "Lady Gaga" as the singer, "Poker Face" as the song.

Once done, assign the result to a variable named answer and press submit.

+ 3 more questions

Removing an entry from a dictionary

Explanation

We can remove a dictionary entry using the del statement:

Syntax

del dictionary[key]

For example:

inventory = {'hammers': 10, 'screws': 50, 'nails': 100}

# Removing an entry with key 'screws'
del inventory['screws']

print(inventory)  # Output: {'hammers': 10, 'nails': 100}

If the key is not found, attempting to delete it using del will raise a KeyError exception.

inventory = {'hammers': 10, 'screws': 50, 'nails': 100}

# Try removing an entry with key 'wrenches'
del inventory['wrenches']  # Will raise a KeyError

Practice questions

4 questions

In the Python Console, a dictionary employee_access is defined, where the keys represent employee IDs and the values represent their current access level to the system.

Some employees have left the company, and their access needs to be removed from the dictionary:

  • "emp123"
  • "emp456"
  • "emp789"

Once you've updated the employee_access dictionary to remove these entries, assign the updated dictionary to a variable named answer and press submit.

+ 3 more questions

Dictionary keys

Explanation

What can be a dictionary key?

There are a couple of constraints on what can be a dictionary key:

  1. Hashability: A dictionary key must be hashable, meaning it can be converted into a fixed-size hash code that uniquely identifies it during lookups. Typically, only immutable types (like strings, integers, floats, and tuples*) are hashable and suitable as dictionary keys. Mutable types, such as lists, sets or dictionaries, cannot be used as keys.
  2. Uniqueness: Each key within a dictionary must be unique. If a key is assigned multiple times, the last assigned value will overwrite previous ones.

*tuples can only be used as dictionary keys if all elements within the tuple are themselves immutable.

Warning

Whilst keys must be unique, duplicate keys will not raise an error - the latter value simply overwrites the first:

my_dict = {"a": 1, "b": 2, "a": 3}
print(my_dict)  # Output: {'a': 3, 'b': 2}

Practice questions

4 questions

Which of the following statements about dictionary keys is true?

Select the correct answer:

+ 3 more questions

Part of

Dictionaries

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