Python - Adding Items to Dictionaries

This section will guide you through the various methods and best practices for adding items to dictionaries in Python.

The Simple Way: Using Square Brackets

The easiest way to add a new item to your dictionary is to use square brackets with the new key, like this:

✳️ Syntax:

dictionary[key] = value

📌 Example:

Your dictionary now knows that Paris is the capital of France. It's that simple! This method is perfect when you want to add just one item and you're sure you want to add it.

Using the update() Method: Adding Multiple Items at Once

Sometimes you might want to add several items at once. Perhaps you just learned about more European capitals and want to add them all. The update() method comes to the rescue.

✳️ Syntax:

dictionary.update([other])

📌 Example:

Here we are updating a dictionary using the update() method and passing another dictionary to it as parameters.

The update() method takes either a dictionary or an iterable object of key/value pairs (generally tuples) as parameters. It doesn’t return any value but updates the dictionary with elements from a dictionary object or an iterable object of key/value pairs.

In the next example, instead of using another dictionary, we passed an iterable value to the update() function.

👣 Explanation:

• We start with a dictionary capitals that contains a single key-value pair: "Italy": "Rome"
• Then we pass a list of tuples: [("Spain", "Madrid"), ("Greece", "Athens")]. Each tuple contains a key and its corresponding value.
• After this line, we add the keys "Germany" and "Portugal" with their respective values "Berlin" and "Lisbon"
• This prints the final contents of the capitals dictionary: print(capitals)

So, the update() method is a convenient way to expand dictionaries, whether by adding data from another iterable or using keyword arguments.

What Happens with Duplicate Countries?

Here's something interesting: if you try to add a capital for a country that's already in your dictionary, Python will simply update the existing entry. For example:

Python doesn't create a duplicate entry; it just updates the existing one. This is useful when you need to correct information, but be careful not to accidentally overwrite something you want to keep!

A Practical Example

Let's put it all together with a real example:

This code shows how your dictionary grows step by step, just like your knowledge of European geography might grow over time.

Using the |= Operator

There's a cool new way to add items to your dictionary using the |= operator. Think of it as mixing two dictionaries together, like combining two collections of postcards. Here's how it works:

✳️ Syntax:

dictionary |= other

📌 Example:

The |= operator is actually a shorter way of writing .update(). It's particularly nice because it makes your code look cleaner and more mathematical. You can think of it as saying "combine these dictionaries and update the original one."

It follows the same pattern as other Python operators like += for numbers. Just as number += 5 adds 5 to your number, capitals |= new_capitals adds new items to your dictionary.

One thing to note: just like with other methods, if there are any matching keys, the new values will replace the old ones:

This modern syntax is becoming increasingly popular among Python developers because it's both elegant and easy to read.

Using the setdefault() Method

The setdefault() method is like a careful librarian who checks if a book exists before adding it to the shelf. This method is special because it only adds a new item if the key doesn't already exist in the dictionary.

✳️ Syntax:

dictionary.setdefault(key, default_value)

📌 Example:

In this case, "Rome" stays as Italy's capital because setdefault() sees that "Italy" already exists and leaves it alone. This makes setdefault() particularly useful when you want to add new items safely without risking overwriting existing data.

The method returns the value associated with the key, so you can also use it this way:

capital = capitals.setdefault("Germany", "Berlin")
print(capital)  # Prints: Berlin

Think of setdefault() as a polite way of adding items - it first asks "Is this already here?" before making any changes. This makes it perfect for situations where you want to be extra careful about preserving existing data while adding new items.

Are Dictionaries Ordered or Unordered?

Before Python 3.7, dictionaries were completely unordered. You couldn't rely on the items staying in the same order when iterating through them or printing the dictionary. The same dictionary might show items in different orders each time you ran your program!

However, since Python 3.7, dictionaries are ordered by insertion. This means they remember the order in which you add items.

📌 Example:

This will print: Greece, Italy, Spain in this exact order.

Quick Tips to Remember

• Square brackets [] are perfect for adding one item
• update() is your friend when adding multiple items
• setdefault() adds a new item only if it doesn't exist
• You can't have duplicate keys
• Adding an existing key updates its value instead of creating a duplicate

Happy dictionary building!

FAQs on Adding Items to Python Dictionaries

What happens if I add a key that already exists in the dictionary?

If you add a key that already exists, the new value will overwrite the old value.

Is it possible to add items to a dictionary while iterating through it?

While it's technically possible, it's not recommended as it can lead to unexpected behavior and may raise a RuntimeError. It's better to create a new dictionary or collect changes in a separate data structure before updating.