Mercurial's demandload system is incompatible with sqlalchemy.

This was reported upstream as

    https://bz.mercurial-scm.org/show_bug.cgi?id=5085

and is available in the upstream repository in changesets c25e3fd38ff1 and

9ff7261cc0f5.

diff --git a/mercurial/demandimport.py b/mercurial/demandimport.py

--- a/mercurial/demandimport.py

+++ b/mercurial/demandimport.py

@@ -174,7 +174,12 @@ def _demandimport(name, globals=None, lo

             """

             symbol = getattr(mod, attr, nothing)

             if symbol is nothing:

-                symbol = _demandmod(attr, mod.__dict__, locals, level=1)

+                mn = '%s.%s' % (mod.__name__, attr)

+                if mn in ignore:

+                    importfunc = _origimport

+                else:

+                    importfunc = _demandmod

+                symbol = importfunc(attr, mod.__dict__, locals, level=1)

                 setattr(mod, attr, symbol)

             # Record the importing module references this symbol so we can

@@ -250,6 +255,7 @@ ignore = [

     '_sre', # issue4920

     'rfc822',

     'mimetools',

+    'sqlalchemy.events', # has import-time side effects (issue5085)

     # setuptools 8 expects this module to explode early when not on windows

     'distutils.msvc9compiler'

     ]