Author Dusty Phillips Reviewers

d = defaultdict(tuple_counter)
When we run this code, we can access empty keys and insert into the list all in
one statement:
>>> d = defaultdict(tuple_counter)
>>> d['a'][1].append("hello")
>>> d['b'][1].append('world')
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Python Data Structures
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 166 
]
>>> d
defaultdict(,
{'a': (1, ['hello']), 'b': (2, ['world'])})
When we print 
dict
at the end, we see that the counter really was working.
This example, while succinctly demonstrating how to create our own 
function for 
defaultdict
, is not actually very good code; using a 
global variable means that if we created four different 
defaultdict
segments that each used 
tuple_counter
, it would count the number 
of entries in all dictionaries, rather than having a different count for 
each one. It would be better to create a class and pass a method on that 
class to 
defaultdict
.
Counter
You'd think that you couldn't get much simpler than 
defaultdict(int)
, but the 
"I want to count specific instances in an iterable" use case is common enough that 
the Python developers created a specific class for it. The previous code that counts 
characters in a string can easily be calculated in a single line:
from collections import Counter
def letter_frequency(sentence):
return Counter(sentence)
The 
Counter
object behaves like a beefed up dictionary where the keys are the items 
being counted and the values are the number of such items. One of the most useful 
functions is the 
most_common()
method. It returns a list of (key, count) tuples ordered 
by the count. You can optionally pass an integer argument into 
most_common()
to 
request only the top most common elements. For example, you could write a simple 
polling application as follows:
from collections import Counter
responses = [
"vanilla",
"chocolate",
"vanilla",
"vanilla",
"caramel",
"strawberry",
"vanilla"
]
print(
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Chapter 6