gh-135788: Support NETRC environment variable in netrc

Doc/library/netrc.rst

@@ -18,9 +18,10 @@ the Unix :program:`ftp` program and other FTP clients.
 .. class:: netrc([file])
 
    A :class:`~netrc.netrc` instance or subclass instance encapsulates data from  a netrc
-   file.  The initialization argument, if present, specifies the file to parse.  If
-   no argument is given, the file :file:`.netrc` in the user's home directory --
-   as determined by :func:`os.path.expanduser` -- will be read.  Otherwise,
+   file. The initialization argument, if present, specifies the file to parse. If no
+   argument is given, it will look for the file path in the :envvar:`!NETRC` environment variable.
+   If that is not set, it defaults to reading the file :file:`.netrc` in the user's home
+   directory -- as determined by :func:`os.path.expanduser`. If the file cannot be found,
    a :exc:`FileNotFoundError` exception will be raised.
    Parse errors will raise :exc:`NetrcParseError` with diagnostic
    information including the file name, line number, and terminating token.

Lib/netrc.py

@@ -65,9 +65,10 @@ def push_token(self, token):
 
 class netrc:
     def __init__(self, file=None):
-        default_netrc = file is None
+        netrc_envvar = os.environ.get("NETRC", "")
+        default_netrc = file is None and not bool(netrc_envvar)
         if file is None:
-            file = os.path.join(os.path.expanduser("~"), ".netrc")
+            file = netrc_envvar or os.path.join(os.path.expanduser("~"), ".netrc")
         self.hosts = {}
         self.macros = {}
         try: