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LESSON 127 OF 673 · DICTIONARIES

Checking for a Key in a Python Dictionary

Lesson Read the full explanation

TL ; DR:

Write key in d to ask whether a dictionary has a key. The answer is True or False, so it goes in an if.


capitals = {"France": "Paris", "Japan": "Tokyo"}

if "Japan" in capitals:
    print(capitals["Japan"])    # Tokyo

if "Spain" not in capitals:
    print("No Spain")           # No Spain

Full lesson:

Reading a key that isn't there is an error. capitals["Spain"] stops the program with a KeyError.

So check first. "Spain" in capitals is True when "Spain" is a key. It is False when it isn't.

not in asks the opposite. "Spain" not in capitals is True here, so No Spain prints.

A value is not a key

in looks only at the keys. "Paris" in capitals is False, because "Paris" is a value.

To look in the values, write "Paris" in capitals.values().

A key must match exactly, upper and lower case too. "japan" in capitals is False.

Your task

In pets, each key is a pet's name, and its value is the pet's kind.

First, check whether "Tom" is a key of pets. If it is, print Tom is a followed by Tom's kind.

Then check that "cat" is not a key, with not in. If so, print cat is not a key.

"cat" is in pets, but only as a value. It is Tom's kind.

Expected output:

Tom is a cat
cat is not a key
STEP 1 OF 2
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    PYTHON · DICTIONARIES

    Dictionary Key Check in Python, explained

    Why check first

    Reading a key that isn't in a dictionary is an error. With capitals = {"France": "Paris", "Japan": "Tokyo"}, the expression capitals["Spain"] stops the program with KeyError: 'Spain'. So when a key might be missing, ask first.

    in and not in

    key in d is True when key is one of the dictionary's keys and False when it isn't. key not in d asks the opposite. The answer is a bool, so it goes straight into an if.

    capitals = {"France": "Paris", "Japan": "Tokyo"}
if "Japan" in capitals:
    print(capitals["Japan"])
if "Spain" not in capitals:
    print("No Spain")

    Tokyo
No Spain

    The check is quick, even for a large dictionary: Python looks the key up directly instead of scanning every entry.

    A value is not a key

    The operator in looks only at the keys. The value "Paris" is not a key, so "Paris" in capitals is False. To look among the values, ask the values: "Paris" in capitals.values() is True. A key must also match exactly, upper and lower case included, so "japan" in capitals is False.

    print("Paris" in capitals)
print("Paris" in capitals.values())
print("japan" in capitals)

    False
True
False

    The lesson's task

    In pets, each key is a pet's name and each value is its kind: {"Rex": "dog", "Tom": "cat", "Polly": "parrot"}. The task has two checks.

    First, check whether "Tom" is a key. It is, so the code prints Tom is a followed by Tom's kind, read with pets["Tom"]. The read is safe because the if already proved the key exists. Second, check that "cat" is not a key, with not in. The word "cat" appears in pets only as a value, Tom's kind, so it is not a key and the second line prints.

    pets = {"Rex": "dog", "Tom": "cat", "Polly": "parrot"}
if "Tom" in pets:
    print("Tom is a " + pets["Tom"])
if "cat" not in pets:
    print("cat is not a key")

    Tom is a cat
cat is not a key

    Two meanings of the word in

    The word in is also part of the for loop: for name in pets: walks through the keys, one per pass. That is a loop, not a question, and it does not give True or False. In an if condition, in is the question. Both use the keys of the dictionary, which is why checking a key and looping over keys feel alike.

    Check, then choose

    The usual shape is an if with an else: do one thing when the key exists and another when it doesn't. The check is about the key's presence, and the two branches say what to do about each case:

    ages = {"Ana": 31}
name = "Ben"
if name in ages:
    print(ages[name])
else:
    print(name, "is not listed")

    Ben is not listed

    Keys are not only text. Any key type works with in, and the type matters: with d = {1: "one"}, the check 1 in d is True but "1" in d is False, because the number 1 and the text "1" are different keys. The same is true for tuples used as keys, such as grid positions.

    Common mistakes

    • Mixing up keys and values. "cat" in pets is False while "cat" in pets.values() is True. The task is about keys, so the plain form is the right one.
    • Reading before checking. pets["Max"] raises a KeyError. Put the read inside the if, or use get, which the next lesson covers.
    • Forgetting the space in the text. The output is Tom is a cat, so the string being joined ends with a space: "Tom is a ". Without it you get Tom is acat.
    • Adding .keys() for no reason. "Tom" in pets.keys() works and gives True, but "Tom" in pets says the same thing more simply.

    Key takeaways

    • key in d and key not in d test the keys of a dictionary and give a bool.
    • Values are not keys; use d.values() to search them.
    • Keys match exactly, including case.
    • Check before reading when a key may be missing.
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