shuffling word's character order

shuffle_word.py

import random
import numpy as np

class shuffle():
    
    def __init__(self):
        np.random.seed(1234)
        
    def shuffle_string(self, string):
        chars = list(string)
        random.shuffle(chars)
        return ''.join(chars)
    
    def garble_word(self, word):
        shuffle_rate = (len(word) // 4) + 1 
        # No operation needed on sufficiently small words
        # (Also, main algorithm requires word length >= 2)
        if len(word) <= 3:
            return word
    
        # Split word into first & last letter, and middle letters
        first, mid, last = word[0:shuffle_rate], word[shuffle_rate:-shuffle_rate], word[-shuffle_rate:]
    
        return first + self.shuffle_string(mid) + last

    def shuffle_corpus(self, corpus, vocabulary, shuffle_rate=2):
        np.random.seed(1234)
        for sent in corpus:           
            index = [i for i in range(len(sent)) if sent[i] in vocabulary]
            r_list = np.random.choice(index, len(index)//shuffle_rate , replace = False)
            
            for j in r_list:
                sent[j] = self.garble_word(sent[j])