convert octal permission number to a text string

perm2txt.py

#this function correctly deals with four digit octals , unlike most other scripts
#When the octal is 4 digits long, the first digit is a setuid, setguid or sticky flag. 
#See here for more info: https://linuxize.com/post/chmod-command-in-linux/ 
#and here: https://en.wikipedia.org/wiki/File_system_permissions#Notation_of_traditional_Unix_permissions

#examples:
#print(perm_to_text('3777'))
#rwxrwsrwt
#print(perm_to_text('2775'))
#rwxrwsr-x

def perm_to_text(octal):
   result =  ''
   first = 0
   octal = str(octal)
   #if there are more than 4 digits, just take the last 4
   if len(octal)>4:
      #separate initial digit
      octal = octal [-4:]
   #if there are 4 digits, deal with first (setuid, setgid, and sticky flags) separately
   if len(octal)==4:
      if octal[0]!='0': 
         first = int(octal [:1])
      octal = octal [-3:]
   value_letters = [(4, 'r'), (2, 'w'), (1, 'x')]
   # Iterate over each of the digits in octal
   for permission in [int(n) for n in octal]:
      # Check for each of the permissions values
      for value, letter in value_letters:
         if permission >= value:
            result += letter
            permission -= value
         else:
            result += '-'
   if first!=0:
      for value in [4,2,1]:
         if first >= value:
            if value==4:
               if result[2] == 'x':
                  result = result[:2]+'s'+result[3:]
               elif result[2] == '-':
                  result = result[:2]+'S'+result[3:]
            if value==2:
               if result[5] == 'x':
                  result = result[:5]+'s'+result[6:]
               elif result[5] == '-':
                  result = result[:5]+'S'+result[6:]            
            if value==1:
               if result[8] == 'x':
                  result = result[:8]+'t'+result[9:]
               elif result[8] == '-':
                  result = result[:8]+'T'+result[9:]             
            first -= value   
   
   return result

print(perm_to_text('3777'))
#rwxrwsrwt
print(perm_to_text('2775'))
#rwxrwsr-x