Question

I have studied generators feature and i think i got it but i would like to understand where i could apply it in my code.

I have in mind the following example i read in “Python essential reference” book:

# tail -f
 def tail(f):
  f.seek(0,2) 
  while True:
   line = f.readline() 
   if not line: 
     time.sleep(0.1)
     continue
   yield line

Do you have any other effective example where generators are the best tool for the job like tail -f?

How often do you use generators feature and in which kind of functionality\part of program do you usually apply it?

Answer 1

I use them a lot when I implement scanners (tokenizers) or when I iterate over data containers.

Edit: here is a demo tokenizer I used for a C++ syntax highlight program:

whitespace = ' \t\r\n'
operators = '~!%^&*()-+=[]{};:\'"/?.,<>\\|'

def scan(s):
    "returns a token and a state/token id"
    words = {0:'', 1:'', 2:''} # normal, operator, whitespace
    state = 2 # I pick ws as first state
    for c in s:
        if c in operators:
            if state != 1:
                yield (words[state], state)
                words[state] = ''
            state = 1
            words[state] += c
        elif c in whitespace:
            if state != 2:
                yield (words[state], state)
                words[state] = ''
            state = 2
            words[state] += c
        else:
            if state != 0:
                yield (words[state], state)
                words[state] = ''
            state = 0
            words[state] += c
    yield (words[state], state)

Usage example:

>>> it = scan('foo(); i++')
>>> it.next()
('', 2)
>>> it.next()
('foo', 0)
>>> it.next()
('();', 1)
>>> it.next()
(' ', 2)
>>> it.next()
('i', 0)
>>> it.next()
('++', 1)
>>>