Documentation for manual proxy configuration, by Andy Gimblett.

This closes SF patch #523415.

Doc/lib/liburllib.tex

@@ -77,9 +77,17 @@ the proxy server before starting the Python interpreter.  For example
 ...
 \end{verbatim}
 
+In a Windows environment, if no proxy envvironment variables are set,
+proxy settings are obtained from the registry's Internet Settings
+section.
+
 In a Macintosh environment, \function{urlopen()} will retrieve proxy
 information from Internet\index{Internet Config} Config.
 
+The \function{urlopen()} function does not support explicit proxy
+specification.  If you need to override environmental proxy settings,
+use \class{URLopener}, or a subclass such as \class{FancyURLopener}.
+
 Proxies which require authentication for use are not currently
 supported; this is considered an implementation limitation.
 \end{funcdesc}
@@ -195,6 +203,12 @@ define their own \mailheader{User-Agent} header by subclassing
 attribute \member{version} to an appropriate string value before the
 \method{open()} method is called.
 
+The optional \var{proxies} parameter should be a dictionary mapping
+scheme names to proxy URLs, where an empty dictionary turns proxies
+off completely.  Its default value is None, in which case
+environmental proxy settings will be used if present, as discussed in
+the definition of \function{urlopen()}, above.
+
 Additional keyword parameters, collected in \var{x509}, are used for
 authentication with the \file{https:} scheme.  The keywords
 \var{key_file} and \var{cert_file} are supported; both are needed to
@@ -360,3 +374,24 @@ The following example uses the \samp{POST} method instead:
 >>> f = urllib.urlopen("http://www.musi-cal.com/cgi-bin/query", params)
 >>> print f.read()
 \end{verbatim}
+
+The following example uses an explicitly specified HTTP proxy,
+overriding environment settings:
+
+\begin{verbatim}
+>>> import urllib
+>>> proxies = {'http': 'http://proxy.example.com:8080/'}
+>>> opener = urllib.FancyURLopener(proxies)
+>>> f = opener.open("http://www.python.org")
+>>> f.read()
+\end{verbatim}
+
+The following example uses no proxies at all, overriding environment
+settings:
+
+\begin{verbatim}
+>>> import urllib
+>>> opener = urllib.FancyURLopener({})
+>>> f = opener.open("http://www.python.org/")
+>>> f.read()
+\end{verbatim}