Brett Cannon wrote:
> 
> Modules/getpath.c seems to be where the C code does it when getting paths for sys.path. So it would be possible to use that same algorithm to set some sys attribute (e.g. in_checkout or something) much like sys.gettotalrefcount is optional and only shown when built with --with-pydebug. Otherwise some directory structure check could be done (e.g. find importlib/_bootstrap.py off of sys.path, and then see if ../Modules/Setup or something also exists that would never show up in an installed CPython).

Why not simply use a flag that get's set based on an environment
variable, say PYTHONDEVMODE ?

Adding more cruft to getpath.c or similar routines is just going to
slow down startup time even more...

Python 2.7 has a startup time of 70ms on my machine; compare that to
Python 2.1 with 10ms and
Perl 5 with just 4ms.