python - for statement and replace keyword in the code -

i new python, wk2 trying follow book, learn python hard way.

for code in ex41.py, have 2 questions.

1. i don't quite understand why has following statement:

   for in range(0, snippet.count("@@@")):

   since snippet.count("@@@") 1, have 1 value 0. why loop through anyway? why can't go straight set variables param_count , param_names?

2. i confused by:

   for word in param_names:     result = result.replace("@@@", word, 1) 

   i know param_names list of words 3 words, since @@@ show once in both snippet , phrase, how can replaced more 1 times?

thanks,

import random urllib import urlopen import sys  word_url = "http://learncodethehardway.org/words.txt" words = []  phrases = { "class %%%(%%%):":  "make class named %%% is-a %%%.", "class %%%(object):\n\tdef __init__(self, ***)" :  "class %%% has-a __init__ takes self , *** parameters.", "class %%%(object):\n\tdef ***(self, @@@)":  "class %%% has-a function named *** takes self , @@@ parameters.", "*** = %%%()":  "set *** instance of class %%%.", "***.***(@@@)":  "from *** *** function, , call parameters self, @@@.", "***.*** = '***'":  "from *** *** attribute , set '***'." }   phrase_first = false if len(sys.argv) == 2 , sys.argv[1] == "english":     phrase_first = true   word in urlopen(word_url).readlines():     words.append(word.strip())  def convert(snippet, phrase):     class_names = [w.capitalize() w in                     random.sample(words, snippet.count("%%%"))]     other_names = random.sample(words, snippet.count("***"))     results = []     param_names = []      in range(0, snippet.count("@@@")):         param_count = random.randint(1,3)         param_names.append(', '.join(random.sample(words, param_count)))      sentence in snippet, phrase:         result = sentence[:]            word in class_names:             result = result.replace("%%%", word, 1)          word in other_names:             result = result.replace("***", word, 1)          word in param_names:             result = result.replace("@@@", word, 1)          results.append(result)      return results   try:     while true:         snippets = phrases.keys()         random.shuffle(snippets)          snippet in snippets:             phrase = phrases[snippet]             question, answer = convert(snippet, phrase)              if phrase_first:                 question, answer = answer, question              print question              raw_input("> ")             print "answer: %s\n\n" % answer except eoferror:     print "\nbye" 

param_names contains subsequent names of parameters, if have ['param1', 'param2'] , string '(@@@, @@@)' want first replace result.replace("@@@", 'param1', 1) replace first occurrence of @@@ resulting in '(param1, @@@)'. then, want replace second @@@ have '(param1, param2)'. without constraint end '(param1, param1)', i.e. occurrences of @@@ replaced name of first parameter. same applies other place holders (classes , "others").

the fact seems redundant particular input data not imply it's redundant in general. needed other possible inputs example in script, without algorithm incorrect.