对字典创建的变量感到困惑

Question

# -*- coding: utf-8 -*-
states = {
    'oregon': 'OR',
    'florida': 'FL',
    'california': 'CA',
    'new york': 'NY',
    'michigan': 'MI'
}

cities = {
    'CA': 'san francisco',
    'MI': 'detroit',
    'FL': 'jacksonville'
}

cities['NY'] = 'new york'
cities['OR'] = 'portland'


for state, abbrev in states.items(): # add two variables
    print "%s is abbreviated %s" % (state, abbrev)

print '-' * 10    
for abbrev, city in cities.items():
    print "%s has the city %s" % (abbrev, city)

print '-' * 10
for state, abbrev in states.items():  # this is what i'm confusing about
    print "%s state is abbreviated %s and has city %s" % (state, abbrev, cities[abbrev])

I just want to know on the questionable line, there are only two variables inputed(state & abbrev), why there could be three variables quoted(state & abbrev & cities[abbrev])?

My guess is that 'abbrev' is being used twice, once in states dict and once in cities dict. So cities[abbrev] means to return the second value of each paired things? Can someone please confirm if my guess is correct?

If that's the case, why do I get a keyerror when I change cities[abbrev] into cities[state]? The error code is: KeyError: 'california'. It should return the first value of each pair.

I am confused on how this is working, can you please help me find a way out?

Answer 1

In your case, states.items() iterates of the key-value pairs, eg ('oregon', 'OR'). state will be 'oregon' and abbrev will be 'OR' . What cities[abbrev] does is finding the value of 'OR' in the dictionary cities . In the case of 'OR' that is 'portland' .

If you tried a value that is not in the keys of the dictionary, eg banana , then Python would throw a KeyError because the value banana is not a key in that dictionary.

To ensure that a key is present in a dictionary, you can use the in operator.

for state, abbrev in states.items():
    # if the value of abbrev (e.g. 'OR') is a value in the cities dictionary
    # we know that there is a city in this state. Print it.
    if abbrev in cities:
        print "%s state is abbreviated %s and has city %s" % (state, abbrev, cities[abbrev])
    else:
        print "%s is abbreviated %s" % (state, abbrev)