--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-config-env.py	Sun Mar 26 21:33:37 2017 -0700
@@ -0,0 +1,48 @@
+# Test the config layer generated by environment variables
+
+from __future__ import absolute_import, print_function
+
+import os
+
+from mercurial import (
+    encoding,
+    rcutil,
+    ui as uimod,
+)
+
+testtmp = encoding.environ['TESTTMP']
+
+# prepare hgrc files
+def join(name):
+    return os.path.join(testtmp, name)
+
+with open(join('sysrc'), 'w') as f:
+    f.write('[ui]\neditor=e0\n[pager]\npager=p0\n')
+
+with open(join('userrc'), 'w') as f:
+    f.write('[ui]\neditor=e1')
+
+# replace rcpath functions so they point to the files above
+def systemrcpath():
+    return [join('sysrc')]
+
+def userrcpath():
+    return [join('userrc')]
+
+rcutil.systemrcpath = systemrcpath
+rcutil.userrcpath = userrcpath
+os.path.isdir = lambda x: False # hack: do not load default.d/*.rc
+
+# utility to print configs
+def printconfigs(env):
+    encoding.environ = env
+    rcutil._rccomponents = None # reset cache
+    ui = uimod.ui.load()
+    for section, name, value in ui.walkconfig():
+        source = ui.configsource(section, name)
+        print('%s.%s=%s # %s' % (section, name, value, source))
+    print('')
+
+# environment variable overrides
+printconfigs({})
+printconfigs({'EDITOR': 'e2', 'PAGER': 'p2'})