Question

Iam trying to do the exercises in the book “Learn Python the hard way” page: 106. The example is below:

cities = {'CA': 'San Francisco', 'MI': 'Detroit', 'FL': 'Jacksonville'}

cities['NY'] = 'New York'
cities['OR'] = 'Portland'

def find_city(themap, state):
    if state in themap:
        return themap[state]
    else:
        return "Not found."

# ok pay attention!
cities['_find'] = find_city

while True:
    print "State? (ENTER to quit)",
    state = raw_input("> ")
    if not state: break

    # this line is the most important ever! study!
    city_found = cities['_find'](cities, state)
    print city_found

I do not understand what cities['_find'] = find_city does? What is _find? In particular, why the underscore? Similarly, I am not sure what city_found = cities['_find'](cities, state) does. I have seen a similar post on the same question:
learn python the hard way exercise 40 help

which basically says that cities['_find'] = find_city adds the function find_city to the dictionary, but I still dont understand what city_found = cities['_find'](cities, state) does(?)

I’d really appreciate if someone could explain me the above two lines. Thanks for your time.

Answer 1

This code:

cities['_find'] = find_city

simply inserts the function find_city into the cities dictionary, using the key _find. The underscore has no particular meaning, it’s just part of the key string. Probably chosen to not collide with actual city names.

This code:

city_found = cities['_find'](cities, state)

Calls the find_city function, by first looking it up in the dictionary using the _find key.

It could be rewritten as:

city_found = find_city(cities, state)

There doesn’t seem to be any real point in doing it like this, there’s no benefit in having the dictionary (which is called a “map” in the code) contain the find function, that I can see.