Limitations

This page used to list bugs in Cython that made the semantics of compiled code differ from that in Python. Most of the missing features have been fixed in Cython 0.15. The bug tracker has an up-to-date list of remaining compatibility issues. Note that a future version 1.0 of Cython is planned to provide full Python language compatibility.

Below is a list of differences that we will probably not be addressing. Most of these things that fall more into the implementation details rather than semantics, and we may decide not to fix (or require a –pedantic flag to get).

Nested tuple argument unpacking

def f((a,b), c):
    pass

This was removed in Python 3.

Inspect support

While it is quite possible to emulate the interface of functions in Cython’s own function type, and recent Cython releases have seen several improvements here, the “inspect” module does not consider a Cython implemented function a “function”, because it tests the object type explicitly instead of comparing an abstract interface or an abstract base class. This has a negative impact on code that uses inspect to inspect function objects, but would require a change to Python itself.

Stack frames

Currently we generate fake tracebacks as part of exception propagation, but don’t fill in locals and can’t fill in co_code. To be fully compatible, we would have to generate these stack frame objects at function call time (with a potential performance penalty). We may have an option to enable this for debugging.

Identity vs. equality for inferred literals

a = 1.0          # a inferred to be C type 'double'
b = c = None     # b and c inferred to be type 'object'
if some_runtime_expression:
    b = a        # creates a new Python float object
    c = a        # creates a new Python float object
print b is c     # most likely not the same object