Python script that creates plenty of files and can alter them

plenty.py

#!/usr/bin/python
import os
import loremipsum
import random
import bisect
import numpy
import textwrap

class RandomWeight:
    def __init__(self, weight):
        self.items = weight.keys()
        self.count = len(self.items)
        self.breakpoints = numpy.cumsum(weight.values())

    def next(self): 
        i = bisect.bisect(self.breakpoints, random.random() * self.breakpoints[-1])
        return self.items[i] 

def alter_file(filepath):
    newtext = []
    if random.random() < 0.05:
      return
    
    for line in open(filepath, 'r').readlines():
        words = line.split()
        if len(words) > w.count:
            for _ in random.sample(range(0, len(words)), w.next()):
                words[random.randint(0, len(words) - 1)] = random.choice(loremipsum.get_sentence()[2:-1].split())
            newtext.append(' '.join(words) + '\n') 
    open(filepath, 'w').write('\n'.join(newtext))

w = RandomWeight({0:10, 1:1, 2:0.5, 3:0.25, 4:0.05})

for m in ['a', 'b', 'c']:
    for t in ['1', '2', '3', '4']:
        dirname = os.path.join('data', ''.join([m,t]))
        if not os.path.exists(dirname):
            os.makedirs(dirname)
        for i in range(512):
            filepath = os.path.join(dirname, '.'.join([str(i), 'yml']))
            if os.path.isfile(filepath):
                alter_file(filepath)
            else:              
                open(filepath, 'w').write('\n\n'.join('\n'.join(textwrap.wrap(i, 80)) for i in loremipsum.get_paragraphs(4)))