Given a dictionary
d and a key
k, it is easy to find the corresponding value
v = d[k]. This operation is called a lookup.
But what if you have
v and you want to find
k? You have two problems: first, there might be more than one key that maps to the value
v. Depending on the application, you might be able to pick one, or you might have to make a list that contains all of them. Second, there is no simple syntax to do a reverse lookup; you have to search.
Here is a function that takes a value and returns the first key that maps to that value:
def reverse_lookup(d, v): for k in d: if d[k] == v: return k raise LookupError()
This function is yet another example of the search pattern, but it uses a feature we haven’t seen before,
raise. The raise statement causes an exception; in this case it causes a
LookupError, which is a built-in exception used to indicate that a lookup operation failed.
If we get to the end of the loop, that means
v doesn’t appear in the dictionary as a value, so we raise an exception.
Here is an example of a successful reverse lookup:
>>> h = histogram('parrot') >>> key = reverse_lookup(h, 2) >>> key 'r'
And an unsuccessful one:
>>> key = reverse_lookup(h, 3) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 5, in reverse_lookup LookupError
The effect when you raise an exception is the same as when Python raises one: it prints a traceback and an error message.
When you raise an exception, you can provide a detailed error message as an optional argument. For example:
>>> raise LookupError('value does not appear in the dictionary') Traceback (most recent call last): File "<stdin>", line 1, in ? LookupError: value does not appear in the dictionary
A reverse lookup is much slower than a forward lookup; if you have to do it often, or if the dictionary gets big, the performance of your program will suffer.