I'd like to add a C extension to speed up a small bit of code in a package (Tornado), but make it optional both for compatibility with non-cpython implementations and for ease of installation on systems without a C compiler available. Ideally any user who runs "pip install tornado" on a system capable of compiling extensions would get the extensions; if this capability cannot be detected automatically I'd prefer the opt-out case to be the one that requires non-default arguments. Are there any packages that provide a good example to follow for this? PEP 426 uses "c-accelerators" as an example of an "extra", but it's unclear how this would work (based on the equivalent setuptools feature). There doesn't appear to be a way to know what extras are requested at build time. If the extra required a package like cython then you could build the extension whenever that package is present, but what about hand-written extensions? Extras are also opt-in instead of opt-out, so I'd have to recommend that most people use "pip install tornado[fast]" instead of "pip install tornado" (with "tornado[slow]" available as an option for limited environments). Thanks, -Ben