Reword proxies section and add warning
@@ -603,10 +603,19 @@ Alternatively you can configure it once for an entire
session.get('http://example.org')
-When the proxies configuration is not overridden in python as shown above,
-by default Requests relies on the proxy configuration defined by standard
+.. warning:: Setting ``session.proxies`` may behave differently than expected.
+ Values provided will be overwritten by environmental proxies
+ (those returned by `urllib.request.getproxies <https://docs.python.org/3/library/urllib.request.html#urllib.request.getproxies>`_).
+ To ensure the use of proxies in the presence of environmental proxies,
+ explicitly specify the ``proxies`` argument on all individual requests as
+ initially explained above.
+
+ See `#2018 <https://github.com/psf/requests/issues/2018>`_ for details.
+
+When the proxies configuration is not overridden per request as shown above,
+Requests relies on the proxy configuration defined by standard
environment variables ``http_proxy``, ``https_proxy``, ``no_proxy``,
-``curl_ca_bundle``, and ``all_proxy``. Uppercase variants of these variables are also supported.
+and ``all_proxy``. Uppercase variants of these variables are also supported.
You can therefore set them to configure Requests (only set the ones relevant
to your needs)::
