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2.4: Reverse Lookup

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  • 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

    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_lookup

    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:

    >>> raise ValueError('value does not appear in the dictionary')
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ValueError: 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.

    Exercise \(\PageIndex{1}\)

    Modify reverse_lookup so that it builds and returns a list of all keys that map to v, or an empty list if there are none.

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