Usually, I use print(sorted(os.environ)) since I prefer a sorted list and it prevents such issue :-)

David:
> I agree that the result is surprising, but there may not be a generic fix.

What about something like:

vstinner@apu$ ./python -c 'import os; print(os.environ.keys())'
<KeysView ['LS_COLORS', 'XDG_MENU_PREFIX', 'LANG', 'GDM_LANG', 'HISTIGNORE', 'HISTCONTROL', 'DISPLAY', 'HOSTNAME', 'EDITOR', 'COLORTERM', 'DESKTOP_AUTOSTART_ID', 'USERNAME', 'XDG_VTNR', 'SSH_AUTH_SOCK', 'XDG_SESSION_ID', 'USER', 'DESKTOP_SESSION', 'GTK2_RC_FILES', 'PWD', 'SSH_ASKPASS', 'HOME', 'JOURNAL_STREAM', 'XDG_SESSION_TYPE', 'XDG_DATA_DIRS', 'XDG_SESSION_DESKTOP', 'LOADEDMODULES', 'MAIL', 'WINDOWPATH', 'VTE_VERSION', 'TERM', 'SHELL', 'QT_IM_MODULE', 'XMODIFIERS', 'XDG_CURRENT_DESKTOP', 'XDG_SEAT', 'SHLVL', 'PYTHONPATH', 'WINDOWID', 'MODULEPATH', 'GDMSESSION', 'GNOME_DESKTOP_SESSION_ID', 'LOGNAME', 'DBUS_SESSION_BUS_ADDRESS', 'XDG_RUNTIME_DIR', 'XAUTHORITY', 'PATH', 'PS1', 'MODULESHOME', 'MAKEFLAGS', 'HISTSIZE', 'SESSION_MANAGER', 'LESSOPEN', 'BASH_FUNC_module%%', 'BASH_FUNC_scl%%', '_', 'OLDPWD']>

vstinner@apu$ git diff
diff --git a/Lib/_collections_abc.py b/Lib/_collections_abc.py
index a5c7bfcda1..5e524dffbf 100644
--- a/Lib/_collections_abc.py
+++ b/Lib/_collections_abc.py
@@ -719,6 +719,9 @@ class KeysView(MappingView, Set):
     def __iter__(self):
         yield from self._mapping
 
+    def __repr__(self):
+        return '<KeysView %r>' % list(self)
+
 KeysView.register(dict_keys)
 
 

list(key_view) is valid. I mimicked dict views implementation:

static PyObject *
dictview_repr(_PyDictViewObject *dv)
{
    PyObject *seq;
    PyObject *result;

    seq = PySequence_List((PyObject *)dv);
    if (seq == NULL)
        return NULL;

    result = PyUnicode_FromFormat("%s(%R)", Py_TYPE(dv)->tp_name, seq);
    Py_DECREF(seq);
    return result;
}