Query string safe Base64

Encoding and decoding arbitrary strings into strings that are safe to put into a URL query param.

The problem

urlsafe_b64encode and urlsafe_b64decode from base64 are not enough because they leave = chars unquoted:

>>> import base64

>>> base64.urlsafe_b64encode('a')
>>> 'YQ=='

And there are 2 problems with that

I. = sign gets quoted:

>>> import urllib

>>> urllib.quote('=')
'%3D'

II. Some libraries tolerate the = in query string values:

>>> from urlparse import urlsplit, parse_qs

>>> parse_qs(urlsplit('http://aaa.com/asa?q=AAAA=BBBB=CCCC').query)
{'q': ['AAAA=BBBB=CCCC']}

but the RFC 3986 underspecifies the query string so we cannot rely on = chars being handled by all web applications as it is done by urlparse.

Therefore we consider chars: ['+', '/', '='] unsafe and we replace them with ['-', '_', '.']. Characters + and / are already handled by urlsafe_* functions from base64 so only = is left for us. The . character has been chosen because it often appears in real world query strings and it is not used by base64.

The solution

>>> import querystringsafe_base64

>>> querystringsafe_base64.encode('foo-bar')
>>> 'Zm9vLWJhcg..'

>>> querystringsafe_base64.decode('Zm9vLWJhcg..')
>>> 'foo-bar'