Keys and values

What you will be able to do

After this lesson you can

• make a dictionary of keys and values,
• look up a value by its key,
• ask how many pairs there are and whether a key is there, and
• read a KeyError and a TypeError about keys.

Find things by a label

A list finds things by position: item 0, item 1, and so on. But what if you want the phone number of Ravi? You would like to ask for "Ravi", and not for position 7. You would have to remember where Ravi is in the list.

A dictionary works like a real dictionary. You look up a word, and you get its meaning. Python's dictionary stores pairs:

• the key: the label that you look up by,
• the value: the thing that is stored.

Think of a cupboard with labelled drawers. The label on the drawer is the key, and what is inside is the value. You never count the drawers. You read the label.

Making a dictionary

A dictionary is written in curly brackets. Each pair is key: value, and the pairs are separated by commas.

Predict first. Before you run it, write down what you think each line shows. Then run it and compare.

• ages["Ravi"] looks up the key "Ravi" and gives 13.
• len(ages) counts the pairs: 3.
• "Meera" in ages asks: is "Meera" a key? Yes, so True. And "Kabir" in ages is False.
• 12 in ages is False, although the value 12 is there. The word in asks about the keys, never the values. This surprises many learners. To ask about the values, use 12 in ages.values().
• type(ages) is dict, the sixth type of value that you know.
• {} is an empty dictionary. Do not confuse it with an empty list [].

Longer dictionaries

For a longer dictionary, write one pair on each line. A comma after the last pair is allowed, and makes it easy to add one more.

The values can be anything: numbers, text, lists, even other dictionaries (a later lesson). The keys must follow two rules.

1. A key is unique. If you write the same key twice, the second value replaces the first. A dictionary never holds two pairs with the same key.
2. A key is fixed. It must be a value that cannot change: text, a whole number, a decimal number, or a tuple. A list cannot be a key.

When the key is not there

And what if the key is a list?

The words under the bars are the keys. Asking for a key that is not there is the usual mistake, so check with in or use get().

Lists or dictionaries?

In a dictionary, finding a key is fast, even for a million pairs. Python does not look through them one by one. In a list, the word in looks at the items one by one.

Common mistakes

• Using [ ] instead of { } to make the dictionary.
• Forgetting the colon between a key and its value, or the comma between pairs.
• Looking up a key that is not there. A KeyError.
• Asking value in d and expecting it to look at the values.
• A list as a key. A TypeError.
• Writing the same key twice, and losing the first value.

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 Read a dictionary. You look up values, and the checker confirms each answer.
2. Build. Open the problem Read a dictionary and the problem Which container?
3. Stretch. Open the problem A tiny translator and make a dictionary that translates five words.

Remember these

• A dictionary stores {key: value} pairs. Look up with d[key].
• len(d) counts the pairs. key in d asks about the keys.
• A key is unique and fixed: text, a number or a tuple. Not a list.
• A missing key gives a KeyError.

Check yourself

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

1. What is {"a": 1, "b": 2}["b"]?

2. What does {"a": 1}["z"] do?

3. What is 1 in {"a": 1}?

4. Why cannot a list be a key?