(@cython.cfunc + Type Annotation) Fragility

Going through some of pandas’ cython modules to try to make code valid-python where possible, I’m seeing cythonize-time errors. It seems like some combinations are invalid for surprising reasons, e.g:

cpdef object get_rule_month(object source, object default='DEC')
# existing, works as expected

cpdef object get_rule_month(source: object, default: object='DEC'):
# seems OK

cpdef get_rule_month(source: object, default: object='DEC') -> object:
# breaks --> Syntax error in C variable declaration

@cython.ccall
def get_rule_month(source: object, default: object='DEC') -> object:
# breaks --> Exception clause not allowed for function returning Python object

@cython.ccall
def object get_rule_month(source: object, default: object='DEC'):
# breaks --> Expected '(', found 'get_rule_month'

@cython.ccall
@cython.returns(object)
def get_rule_month(source: object, default: object='DEC'):
# appears OK

Another example with a slightly different set of errors:

cdef inline bint is_null_datetime64(v):
# existing, OK

cdef inline is_null_datetime64(v) -> bint:
# breaks --> Syntax error in C variable declaration

@cython.cfunc
def inline is_null_datetime64(v) -> bint:
# breaks --> Expected '(', found 'is_null_datetime64

@cython.cfunc
@cython.inline
def is_null_datetime64(v) -> bint:
# breaks --> Function signature does not match previous declaration

This last one here is the one that is noticeably different from the first example.

Is this behavior intentional?

[Yobmod]

I’m having the similar issues on cython V0.29 with return types in the .pyx file. Any python object return type breaks compilation in python compatible mode. Cython types work for returns (int, float, cython.int, cython.void, etc), np.ndarray and python builtins don’t.

@cython.ccall
def get_object() -> object:  ...
# -> Exception clause not allowed for function returning Python object

@cython.cfunc
def get_str() -> str:  ...
# -> Exception clause not allowed for function returning Python object

Also returning None gives a different error:

@cython.ccall
def do_something_without_return() -> None:  ...
# -> Not a type

which makes it not compatible with typed python even with just builtin types. If returning None is redundant, can cython just ignore it? or alias it to a cython void / NULL? (Should this be a separate issue?)

[black-square]

@Yobmod It looks like that there is a workaround for -> object case: If we add a decorator @cython.exceptval(check=False) it works fine. The generated code still checks the return value for exceptions.