Changing a dictionary

What you will be able to do

After this lesson you can

• add, change and remove pairs,
• look up a value safely, without a KeyError,
• look at all the keys, values and pairs, and
• use sum, max and sorted on a dictionary.

Adding, changing and removing

Unlike a string, a dictionary can change, as a list can. You change it by writing to a key.

• d["Meera"] = 64 adds a new pair if "Meera" is not a key yet.
• d["Asha"] = 78 replaces the value if "Asha" is already a key.
• del d["Ravi"] removes a pair.
• d.pop("Meera") also removes a pair, and gives the value back.

Predict first, and pay attention to the last lines. Then run it.

• After the first two changes, the dictionary has three pairs: Asha with the new mark 78, Ravi, and the new Meera.
• After del, Ravi is gone.
• marks.pop("Meera") removes Meera and gives back her mark, 64, which we store in gone.

The same line, d[key] = value, both adds and changes. Python decides by asking: is the key already there? This is useful: you do not need to check.

The safe way to look up: get()

You saw that marks["Kabir"] stops with a KeyError if Kabir is not a key. The method get is safe.

Which one to use? get is shorter when you have a sensible default, such as 0 for a count. The if ... in form is better when the two cases need different actions, as in a message.

Looking at the whole dictionary

The same line, stock[key] = value, does both jobs: it changes the value of a key that exists, and adds a key that does not.

The order of a dictionary

A dictionary remembers the order in which the keys were added. So the output of print(marks) shows the pairs in the order of adding, and a new pair goes at the end. But do not rely on the order to find things. You find things by key. If you need a particular order, use sorted.

Common mistakes

• d[key] when the key may be missing. A KeyError. Use get or in.
• Forgetting that d[key] = value replaces. An old value is lost without a message.
• del d[key] for a missing key. A KeyError. Use d.pop(key, None) if it may be missing.
• Changing a dictionary while looping over it. Python stops with a RuntimeError. Make a list of the keys first: for key in list(d):.
• Using d.keys() where a list is needed. Wrap it in list().

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Change the pairs. You add, change and remove, and the checker confirms the final dictionary.
2. Build. Open the problem Add, change and remove and the problem A safe lookup.
3. Stretch. Open the problem The total and the average of the values.

Remember these

• d[key] = value adds a new pair or replaces the old value.
• del d[key] removes. d.pop(key) removes and gives the value back.
• d.get(key, default) never gives a KeyError.
• keys(), values() and items() show the whole dictionary. sorted(d) gives the keys in order.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. After d = {"a": 1} and d["b"] = 2, what is len(d)?

2. What does {"a": 1}.get("z", 5) give?

3. What does d["a"] = 9 do when "a" is already a key?

4. How do you add up all the values of d?