input: Write a function in Python to create a random string of a specified length. The function should take an integer parameter representing the length of the string and return a string composed of random characters from the set of lowercase and uppercase letters, as well as digits.
  
  def generate_string(string_length):
      random_characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
      random_string = ''.join(random.choices(random_characters, k=string_length))
      return random_string
  
  assert len(generate_string(5))==5
  assert len(generate_string(10))==10
  assert len(generate_string(0))==0
  assert len(generate_string(20))==20
output: import random
  
  def generate_string(string_length):
      random_characters = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789'
      random_string = ''.join(random.choices(random_characters, k=string_length))
      return random_string