This section describes the dictionary methods that are most often used.

copy()

Just like lists, if you assign a variable that contains a dictionary to another variable, you are not creating a copy of the dictionary; you are actually creating an alias (if you do not remember what an alias is, see Chapter 13). You cannot use the trick which is used for lists to create a copy, as it uses a slice-syntax, and dictionaries do not support slices. Therefore, there is a method copy() that returns a copy of a dictionary.

fruitbasket = { "apple":3, "banana":5, "cherry":50 }
fruitbasketalias = fruitbasket
fruitbasketcopy = fruitbasket.copy()

print( id( fruitbasket ) )
print( id( fruitbasketalias ) )
print( id( fruitbasketcopy ) )

Note that this method makes a shallow copy of the dictionary (see Chapter 13 if you do not remember the difference between shallow and deep copies). If you want to make a deep copy, use the deepcopy() function from the copy module.

keys(), values(), and items()

The method keys() provides an iterator that lists all the keys of a dictionary. The method values() provides an iterator that lists all the values of a dictionary. The method items() provides an iterator that lists all the key-value pairs of a dictionary as tuples.

I specifically say that these methods returns an iterator and not a list. If you want to turn them into lists, you have to use list casting (see Chapter 13).

fruitbasket = { "apple":3, "banana":5, "cherry":50 }
print( list( fruitbasket.keys() ) )
print( list( fruitbasket.values() ) )
print( list( fruitbasket.items() ) )

At this point you might be wondering when you can use an iterator. You mainly use iterators for for loops (though you can also use them as arguments for the functions max(), min() and sum()).

fruitbasket = { "apple":3, "banana":5, "cherry":50, "durian":0, "mango":2 }
for key in fruitbasket.keys():
    print( "{}:{}".format( key, fruitbasket[key] ) )
print( sum( fruitbasket.values() ) )

Since this code provides an unpredictable order for the keys, you might want to sort them before looping over them. Since keys() does not provide a list, it cannot be sorted directly, but you can turn the result into a list using list casting. After doing that, you can sort.

fruitbasket = { "apple":3, "banana":5, "cherry":50, "durian":0, "mango":2 }
keylist = list( fruitbasket.keys() )
keylist.sort()
for key in keylist:
    print( "{}:{}".format( key, fruitbasket[key] ) )

keylist = list( fruitbasket.key() ).sort() does not work, as you cannot apply the sort() method directly to the list casting. You must first create the list, then sort it. Neither can you write for key in keylist.sort(), as the sort() method has no return value.

If you wonder why Python seems to prefer iterators instead of lists: the answer is that iterators are more general and use much less memory. They are “lazy” methods, as they only provide an item when it is requested.

get()

The get() method can be used to get a value from a dictionary even when you do not know if the key for which you seek the value exists. You call the get() method with the key you are looking for, and it will return the corresponding value when the key exists in the dictionary, or the special value None when the key does not exist in the dictionary. If you want to return a specific value instead of None if the key does not exist, you can add that value as a second argument to the method.

fruitbasket = { "apple":3, "banana":5, "cherry":50, "durian":0, "mango":2 }

apple = fruitbasket.get( "apple" )
if apple:
    print( "apple is in the basket" )
else:
    print( "no apples in the basket")

orange = fruitbasket.get( "orange" )
if orange:
    print( "orange is in the basket" )
else:
    print( "no oranges in the basket")

banana = fruitbasket.get( "banana", 0 )
print( "number of bananas in the basket:", banana )

strawberry = fruitbasket.get( "strawberry", 0 )
print( "number of strawberries in the basket:", strawberry )

Run and study the example above closely, as what it demonstrates about the get() method is very useful. Suppose that you store a collection of items with corresponding quantities, for instance, the contents of a fruit basket with the keys being the names of the fruits and the values being the quantities. When you query the fruitbasket using the get() method with a second parameter zero, you can look for any fruit in the basket without the need to check first if the fruit exists in the basket, because if you ask for a fruit that is not there, the get() method returns zero, which is exactly what you want to hear.

Practice

The code below contains a list of words. Build a dictionary that contains all these words as keys, and their quantities as values. Print the words with their quantities.

wordlist = ["apple","durian","banana","durian","apple","cherry",
    "cherry","mango","apple","apple","cherry","durian","banana",
    "apple","apple","apple","apple","banana","apple"]

The code block below contains a string that is a list of words, separated by commas. Build a dictionary that contains all these words as keys, and how often they occur as values. Then print the words with their quantities.

text = "apple,durian,banana,durian,apple,cherry,cherry,mango," + \
    "apple,apple,cherry,durian,banana,apple,apple,apple," + \
    "apple,banana,apple"

The code block below contains a very small dictionary that contains the translations of English words to Dutch. Write a program that uses this dictionary to create a word-for-word translation of the given sentence. A word for which you cannot find a translation, you can leave “as is.” The dictionary is supposed to be used case-insensitively, but your translation may consist of all lower case words. It is nice if you leave punctuation in the translation, but if you take it out, that is acceptable (as leaving punctuation in is quite a bit of work, and does not really have anything to do with dictionaries – besides, leaving punctuation in is much easier to do once you have learned about regular expressions).

english_dutch = { "last":"laatst", "week":"week", "the":"de",
    "royal":"koninklijk", "festival":"feest", "hall":"hal", "saw":
    "zaag", "first":"eerst", "performance":"optreden", "of":"van",
    "a":"een", "new":"nieuw", "symphony":"symphonie", "by":"bij",
    "one":"een", "world":"wereld", "leading":"leidend", "modern":
    "modern", "composer":"componist", "composers":"componisten",
    "two":"twee", "shed":"schuur", "sheds":"schuren" }

sentence = "Last week The Royal Festival Hall saw the first \
performance of a new symphony by one of the world's leading \
modern composers, Arthur \"Two-Sheds\" Jackson."