URLFetch service and it may not be
the best choice for your application. There are three options for using
urllib3 on Google App Engine:

1. You can use :class:`AppEngineManager` with URLFetch. URLFetch is
   cost-effective in many circumstances as long as your usage is within the
   limitations.
2. You can use a normal :class:`~urllib3.PoolManager` by enabling sockets.
   Sockets also have `limitations and restrictions
   <https://cloud.google.com/appengine/docs/python/sockets/   #limitations-and-restrictions>`_ and have a lower free quota than URLFetch.
   To use sockets, be sure to specify the following in your ``app.yaml``::

        env_variables:
            GAE_USE_SOCKETS_HTTPLIB : 'true'

3. If you are using `App Engine Flexible
<https://cloud.google.com/appengine/docs/flexible/>`_, you can use the standard
:class:`PoolManager` without any configuration or special environment variables.
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