Simple Python script that encrypts text file using monoalphabetic substitution cypher

caesar.py

def encrypt_mono(file, key="key"):
  '''This function encrypts text in a text file using a key and monoalphabetic substituion cypher'''
  
  import string

  def strip_punctuation(s):
    return ''.join(c for c in s if c not in string.punctuation)

  # remove repeating characters from key and add remaining letters of alphabet
  alphabet = 'abcdefghijklmnopqrstuvwxyz'
  
  key = key + alphabet
  n = ''
  
  for i in key:
    if i not in string.ascii_letters:
      n += i
    elif i not in n:
      n += i

  dictionary = dict(zip(list(alphabet),n))

  with open(file,'r+') as text:
  
    lines = '\n'.join(text.readlines())
    lines = lines.lower()
    lines = lines.replace(' ','')
    lines = strip_punctuation(lines)
  
    for key, value in dictionary.iteritems():
      lines =  lines.replace(key,value.upper())
  
    text.seek(0)
    text.write(lines)
    text.truncate()