urlencoder/decoder for python

EDIT (I so stand corrected.):

I could have just used:
urllib.quote
urllib.unquote

---------------------------------------------------

Every once and a while I run into the need for a urlencoder/decoder function that just accepts a string and returns a string.

The one that comes w/ urllib(2) takes a dictionary and returns a "name=val" formatted string.
My only problem is that it does 2 things at once:
1) url encodes.
2) constructs a query or post string.

So I wrote a quick utility to do just #1:

_keys = [
 "$", "&", "+", ",", "/", ":", ";", "=", "?", "@", " ", '"',
 "<", ">", "#", "%", "{", "}", "|", "\\", "^", "~", "[", "]", "`"]

_vals = [
 '%24', '%26', '%2B', '%2C', '%2F', '%3A', '%3B', '%3D', '%3F',
 '%40', '%20', '%22', '%3C', '%3E', '%23', '%25', '%7B', '%7D',
 '%7C', '%5C', '%5E', '%7E', '%5B', '%5D', '%60']

def encode(str=""):
 """ URL Encodes a string with out side effects
 """
 return "".join([_swap(x) for x in str])

def decode(str=""):
 """ Takes a URL encoded string and decodes it with out side effects
 """
 if not str: return None
 for v in _vals:
   if v in str: str = str.replace(v, _keys[_vals.index(v)])
 return str

def _swap(x):
 """ Helper function for encode.
 """
 if x in _keys: return _vals[_keys.index(x)]
 return x

### units ###
if __name__ == "__main__":
 assert("".join(_keys) == decode(encode("".join(_keys))))
 assert("".join(_vals) == encode(decode("".join(_vals))))
 print "passed all unit tests."