Check if a string is encoded in base64 using Python

import base64
import binascii

try:
    base64.decodestring("foo")
except binascii.Error:
    print "no correct base64"

I was looking for a solution to the same problem, then a very simple one just struck me in the head. All you need to do is decode, then re-encode. If the re-encoded string is equal to the encoded string, then it is base64 encoded.
Here is the code:

import base64

def isBase64(s):
    try:
        return base64.b64encode(base64.b64decode(s)) == s
    except Exception:
        return False

That's it!

Edit: Here's a version of the function that works with both the string and bytes objects in Python 3:

import base64

def isBase64(sb):
        try:
                if isinstance(sb, str):
                        # If there's any unicode here, an exception will be thrown and the function will return false
                        sb_bytes = bytes(sb, 'ascii')
                elif isinstance(sb, bytes):
                        sb_bytes = sb
                else:
                        raise ValueError("Argument must be string or bytes")
                return base64.b64encode(base64.b64decode(sb_bytes)) == sb_bytes
        except Exception:
                return False