how to find user id from session_data from django_session table?

I had trouble with Paulo's method (see my comment on his answer), so I ended up using this method from a scottbarnham.com blog post:

from django.contrib.sessions.models import Session
from django.contrib.auth.models import User

session_key = '8cae76c505f15432b48c8292a7dd0e54'

session = Session.objects.get(session_key=session_key)
uid = session.get_decoded().get('_auth_user_id')
user = User.objects.get(pk=uid)

print user.username, user.get_full_name(), user.email

If you want to learn more about it and know how does encode or decode work, there are some relevant code. By the way the version of Django that i use is 1.9.4.

django/contrib/sessions/backends/base.py

class SessionBase(object):
    def _hash(self, value):
        key_salt = "django.contrib.sessions" + self.__class__.__name__
        return salted_hmac(key_salt, value).hexdigest()
    def encode(self, session_dict):
        "Returns the given session dictionary serialized and encoded as a string."
        serialized = self.serializer().dumps(session_dict)
        hash = self._hash(serialized)
        return base64.b64encode(hash.encode() + b":" + serialized).decode('ascii')
    def decode(self, session_data):
        encoded_data = base64.b64decode(force_bytes(session_data))
        try:
            # could produce ValueError if there is no ':'
            hash, serialized = encoded_data.split(b':', 1)
            expected_hash = self._hash(serialized)
            if not constant_time_compare(hash.decode(), expected_hash):
                raise SuspiciousSession("Session data corrupted")
            else:
                return self.serializer().loads(serialized)
        except Exception as e:
            # ValueError, SuspiciousOperation, unpickling exceptions. If any of
            # these happen, just return an empty dictionary (an empty session).
            if isinstance(e, SuspiciousOperation):
                logger = logging.getLogger('django.security.%s' %
                        e.__class__.__name__)
                logger.warning(force_text(e))
            return {}

django/contrib/sessions/serializer.py

class JSONSerializer(object):
    """
    Simple wrapper around json to be used in signing.dumps and
    signing.loads.
    """
    def dumps(self, obj):
        return json.dumps(obj, separators=(',', ':')).encode('latin-1')
    def loads(self, data):
        return json.loads(data.decode('latin-1'))

Let's focus on SessionBase's encode function.

1. Serialize the session dictionary to a json
2. create a hash salt
3. add the salt to serialized session , base64 the concatenation

So, decode is inverse. We can simplify the decode function in the following code.

import json
import base64
session_data = 'YTUyYzY1MjUxNzE4MzMxZjNjODFiNjZmZmZmMzhhNmM2NWQzMTllMTp7ImNvdW50Ijo0fQ=='
encoded_data = base64.b64decode(session_data)
hash, serialized = encoded_data.split(b':', 1)
json.loads(serialized.decode('latin-1'))

And that what session.get_decoded() did.

NOTE: format changed since original answer, for 1.4 and above see the update below

import pickle

data = pickle.loads(base64.decode(session_data))

>>> print data
{'_auth_user_id': 2L, '_auth_user_backend': 'django.contrib.auth.backends.ModelBackend',
 '_session_expiry': 0}

[update]

My base64.decode requires filename arguments, so then I tried base64.b64decode, but this returned "IndexError: list assignment index out of range".

I really don't know why I used the base64 module, I guess because the question featured it.

You can just use the str.decode method:

>>> pickle.loads(session_data.decode('base64'))
{'_auth_user_id': 2L, '_auth_user_backend': 'django.contrib.auth.backends.ModelBackend',
 '_session_expiry': 0}

I found a work-around (see answer below), but I am curious why this doesn't work.

Loading pickled data from user sources (cookies) is a security risk, so the session_data format was changed since this question was answered (I should go after the specific issue in Django's bug tracker and link it here, but my pomodoro break is gone).

The format now (since Django 1.4) is "hash:json-object" where the first 40 byte hash is a crypto-signature and the rest is a JSON payload. For now you can ignore the hash (it allows checking if the data was not tampered by some cookie hacker).

>>> json.loads(session_data.decode('base64')[41:])
{u'_auth_user_backend': u'django.contrib.auth.backends.ModelBackend',
 u'_auth_user_id': 1}