As of Pyramid 1.4 spaces are no longer accepted in the path names when rendering mako templates (changed in this commit: f71ed59#L1L79). Since no one else has noticed yet I don't know if this needs a bug fix, but it should at least be documented that the file path cannot have spaces.
For example a renderer argument to view config such as the following would fail when looking up the template:
This is a regression, thanks for the report. If you have any interest in submitting a test case or a patch it'd be appreciated.
I can submit a patch.
Spaces and dots may now be in mako renderer template paths. Fixes #950.