input: Write a python function to generate a password of a given length using random letters and digits.
  assert len(generate_password(8)) == 8
  assert len(generate_password(12)) == 12
  assert isinstance(generate_password(10), str)
output: import random
  import string
  
  def generate_password(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))