Python - Change Dictionary Items

Dictionaries store key-value pairs, making it easy to organize and access data efficiently. But what if you need to modify a dictionary?

In this section, we’ll explore different methods to change dictionary items.

Updating Single Dictionary Value

The simplest way to change a value in your dictionary is just like updating an entry in your phone's contact list. You can directly assign a new value to an existing key.

✳️ Syntax:

dictionary[key] = value

For example, if we (hypothetically) needed to update Italy's capital, we would write:

You can change values using a loop. In this example, we transform the values in the capitals dictionary to uppercase by iterating over it and modifying each value.

👣 Explanation:

• We define a dictionary capitals that maps country names to their capital cities.
• The for loop iterates over the keys in the dictionary capitals.
• The variable country takes on each key in the dictionary sequentially ("Greece", "Italy", and "Spain").
• The upper() method is applied to convert the string value to uppercase.
• The key's value in the dictionary is updated with this uppercase string.

But what happens if you try to modify a value for a key that doesn't exist? Let's try to change the capital of France when France isn't even in our dictionary:

Plot twist: Instead of raising an error, Python just adds France to the dictionary! While this flexibility can be useful, it might not always be what you want.

Here's a safer approach that checks before making changes:

Remember, when modifying dictionary items, Python doesn't distinguish between updating an existing value and creating a new one. It's up to you to implement any necessary checks if you want to prevent accidental additions to your dictionary.

Updating Multiple Dictionary Values

You can also use the update() method to modify existing values. This method is particularly useful when you need to change multiple items at once:

✳️ Syntax:

dictionary.update(other_dictionary)

The update() method adds the key-value pairs from the provided dictionary or iterable to the original dictionary, overwriting any existing keys with the new values if they already exist in the original dictionary.

FAQs on Change Dictionary Items

How do I update the value of a specific key in a dictionary?

Simply use the bracket notation with the key and assign the new value, for example:

my_dict["key"] = new_value

This will overwrite the existing value associated with that key.

Is there a way to bulk-update multiple keys at once?

Absolutely! You can use the update() method to merge key-value pairs from another dictionary or from a list of tuples. For example:

my_dict.update({"key1": new_value1, "key2": new_value2})

If a key exists in both dictionaries, the value in your dictionary will be overwritten by the value from the other dictictionary.

How do I change a dictionary value based on a condition?

Just wrap your update in an if statement:

if "key" in my_dict and my_dict["key"] < 10:
    my_dict["key"] = 10

If a dictionary is updated with an empty key/value will it erase the contents of the dictionary?

No, calling update() with no parameters or with an empty dictionary will not change any of the existing key/values in the dictionary. The following code example shows both types of empty updates. The existing dictionary remains unchanged.

Empty updates are like no-ops - they won't affect your existing data at all.