Most pythonic way to convert a string to a octal number

Most pythonic way to convert a string to a octal number

I am looking to change permissions on a file with the file mask stored in
a configuration file. Since os.chmod() requires an octal number, I need to
convert a string to an octal number. For example:
'000' ==> 0000 (or 0o000 for you python 3 folks)
'644' ==> 0644 (or 0o644)
'777' ==> 0777 (or 0o777)
After an obvious first attempt of creating every octal number from 0000 to
0777 and putting it in a dictionary lining it up with the string version,
I came up with the following:
def new_oct(octal_string):
if re.match('^[0-7]+$', octal_string) is None:
raise SyntaxError(octal_string)
power = 0
base_ten_sum = 0
for digit_string in octal_string[::-1]:
base_ten_digit_value = int(digit_string) * (8 ** power)
base_ten_sum += base_ten_digit_value
power += 1
return oct(base_ten_sum)
Is there a simpler way to do this?