Checking if the key is in the dict

When adding and accessing elements to dictionaries, we might accidentally try to access a key that hasn’t been added to the dictionary yet. In that case, Python will tell us that a KeyError occurred and there isn’t such a key in the dictionary. To check if the key is in the dictionary, we can use the in keyword (exactly as we would do in lists, tuples, or sets).

costs = {
    'Living room': 200,
    'Bathroom': 400,
    'Kitchen': 500,
    'Balcony': 100,
}
print(costs['Bedroom'])   # KeyError: 'Bedroom'

if 'Bedroom' in costs:
    print(costs['Bedroom'])
else:
    costs['Bedroom'] = 150

print(costs)
# {'Living room': 200, 'Bathroom': 400, 'Kitchen': 500, 'Balcony': 100, 'Bedroom': 150}