input: Write a function named `generate_password` that generates a random password of length 8 characters, containing at least one uppercase letter, one lowercase letter, one digit, and one special character. The function should return the generated password as a string.
  
  assert generate_password()==<some_password_with_required_characters>
  assert len(generate_password())==8
  assert any(char.isupper() for char in generate_password())==True
  assert any(char.islower() for char in generate_password())==True
  assert any(char.isdigit() for char in generate_password())==True
  assert any(char in string.punctuation for char in generate_password())==True
  
  Note: The first assert statement cannot be fully asserted without knowing the actual generated password, hence it should be tested manually.
output: import random
  import string
  
  def generate_password():
      while True:
          password = ''.join(random.choices(string.ascii_letters + string.digits + string.punctuation, k=8))
          if (any(c.isupper() for c in password) and
              any(c.islower() for c in password) and
              any(c.isdigit() for c in password) and
              any(c in string.punctuation for c in password)):
              return password