The improved version of "caesar.py" using a list of 153,484 words. "dictionary.pkl" is available from http://ushitora.net/archives/456 .

caesar_dictionary.py

import pickle

ALPHA = ["A", "B", "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M", "N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"]
alpha = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]

word = input("Please Enter Words: ")

def plus_i(i, w):
    if w in alpha:
        n = alpha.index(w) + i
        if n > 25:
            n -= 26
        return alpha[n]
    elif w in ALPHA:
        n = ALPHA.index(w) + i
        if n > 25:
            n -= 26
        return ALPHA[n]
    else:
        return w

candidates = ["".join([plus_i(i, w) for w in word]) for i in range(26)]

with open("dictionary.pkl", "rb") as f:  # "dictionary.pkl" is available from 
    obj = pickle.load(f)                 # http://ushitora.net/archives/456
f.close()                                # The list of 153,484 words in about 100 books.
 
c_scores = [sum([1 if c.lower() in obj else 0 for c in candidate.split(" ")]) for candidate in candidates]

print(candidates[c_scores.index(max(c_scores))])