The urllib package serves as a namespace only. There are other modules under urllib like request and parse.
For optimization importing urllib doesn't import other modules under it. Because doing so would consume processor cycles and memory, but people may not need those other modules.
Individual modules under urllib must be imported separately depending on the needs.

Try these, the first one fails but the second succeeds because when flask is imported flask itself imports urllib.parse.

python3 -c 'import urllib, sys;print(sys.modules["urllib.parse"])'
python3 -c 'import flask, sys;print(sys.modules["urllib.parse"])'

When you run import urllib, it creates the module object of the urllib module (which is actually a package) without importing its submodules (parse, request etc.).

You need the parent module object (urllib) to be in your namespace if you want to access its submodule using attribute access. In addition to that, that submodule must already be loaded (imported). From the documentation:

if package spam has a submodule foo, after importing spam.foo, spam will have an attribute foo which is bound to the submodule. [...] The invariant holding is that if you have sys.modules['spam'] and sys.modules['spam.foo'] (as you would after the above import), the latter must appear as the foo attribute of the former.

There is only one instance of each module, thus any changes made to the urllib module object (stored in sys.modules['urllib']) get reflected everywhere.

You don't import urllib.parse, but IPython does. To prove this I'm going to create a startup file:

import urllib
print('Running the startup file: ', end='')
try:
    # After importing  'urllib.parse' ANYWHERE,
    # 'urllib' will have the 'parse' attribute.
    # You could also do "import sys; sys.modules['urllib'].parse"
    urllib.parse
except AttributeError:
    print("urllib.parse hasn't been imported yet")
else:
    print('urllib.parse has already been imported')
print('Exiting the startup file.')

and launch ipython

vaultah@base:~$ ipython
Running urllib/parse.py
Running the startup file: urllib.parse has already been imported
Exiting the startup file.
Python 3.6.0a0 (default:089146b8ccc6, Sep 25 2015, 14:16:56) 
Type "copyright", "credits" or "license" for more information.

IPython 4.0.0 -- An enhanced Interactive Python.

It is the side effect of importing pydoc during the startup of IPython (which ipython is /usr/local/bin/ipython):

/usr/local/bin/ipython, line 7:
  from IPython import start_ipython
/usr/local/lib/python3.6/site-packages/IPython/__init__.py, line 47:
  from .core.application import Application
/usr/local/lib/python3.6/site-packages/IPython/core/application.py, line 24:
  from IPython.core import release, crashhandler
/usr/local/lib/python3.6/site-packages/IPython/core/crashhandler.py, line 28:
  from IPython.core import ultratb
/usr/local/lib/python3.6/site-packages/IPython/core/ultratb.py, line 90:
  import pydoc
/usr/local/lib/python3.6/pydoc.py, line 68:
  import urllib.parse

This explains why the below code fails - you only import urllib and nothing seems to import urllib.parse:

$ python -c 'import urllib; print(urllib.parse)'

On the other hand, the following command works because datetime.datetime is not a module. It's a class that gets imported during import datetime.

$ python -c 'import datetime; print(datetime.datetime)'

You're not showing the imports in your program, so I can't be sure, but I bet you did

import urllib

which will not import and re-export the separate module urllib.parse. Do

import urllib.parse

instead.

(import urllib is rather senseless in Python 3.x, since all the functionality is in the submodules and these are not imported by the toplevel module.)

If you need to write code which is Python2 and Python3 compatible you can use the following import

try:
    from urllib.parse import urlparse
except ImportError:
     from urlparse import urlparse