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 ValueError
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 ValueError, which generally indicates that there is something wrong with the value of a parameter.
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') >>> k = reverse_lookup(h, 2) >>> print k r
And an unsuccessful one:
>>> k = reverse_lookup(h, 3)Traceback (most recent call last):File "<stdin>", line 1, in ?File "<stdin>", line 5, in reverse_lookupValueError
The result when you raise an exception is the same as when Python raises one: it prints a traceback and an error message.
The raise statement takes a detailed error message as an optional argument. For example:
>>> k = reverse_lookup(h, 3) Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 5, in reverse_lookup ValueError
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.
Exercise 11.4.Modify reverse_lookupso that it builds and returns a list of all keys that map to v, or an empty list if there are none.