Python 3: An interactive deep dive/

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Generators

Generators

We'll cover the following...

Wouldn’t it be grand to have a generic plural() function that parses the rules file? Get rules, check for a match, apply appropriate transformation, go to next rule. That’s all the plural() function has to do, and that’s all the plural() function should do.

Python 3.5
def rules(rules_filename):
    with open(rules_filename, encoding='utf-8') as pattern_file:
        for line in pattern_file:
            pattern, search, replace = line.split(None, 3)
            yield build_match_and_apply_functions(pattern, search, replace)
def plural(noun, rules_filename='plural5-rules.txt'):
    for matches_rule, apply_rule in rules(rules_filename):
        if matches_rule(noun):
            return apply_rule(noun)
    raise ValueError('no matching rule for {0}'.format(noun))

How the heck does that work? Let’s look at an interactive example first.

Python 3.5
def make_counter(x):
     print('entering make_counter')
     while True:
         yield x                           #①
         print('incrementing x')
         x = x + 1
 
counter = make_counter(2)                  #②
print (counter)                            #③
#<generator object at 0x001C9C10>
print (next(counter))                      #④
#entering make_counter
#2
print (next(counter))                      #⑤
#incrementing x
#3
print (next(counter))                      #⑥
#incrementing x
#4

① The presence of the yield keyword in make_counter means that this is not a normal function. It is a special kind of function which generates values one at a time. You can think of it as a resumable function. Calling it will return a generator that can be used to generate successive values of x.

② To create an instance of the make_counter generator, just call it like any other function. Note that this does not actually execute the function code. You can tell this because the first line of the make_counter() function calls print(), but nothing has been printed yet.

③ The make_counter() function returns a generator object.

④ The next() function takes a generator object and returns its next value. The first time you call next() with the counter generator, it executes the code in make_counter() up to the first yield statement, then returns the value that was yielded. In this case, that will be 2, because you ...