Fused Types (Templates)

Fused types allow you to have one type definition that can refer to multiple types. This allows you to write a single static-typed cython algorithm that can operate on values of multiple types. Thus fused types allow generic programming and are akin to templates in C++ or generics in languages like Java / C#.


Fused types are not currently supported as attributes of extension types. Only variables and function/method arguments can be declared with fused types.


from __future__ import print_function

ctypedef fused char_or_float:

cpdef char_or_float plus_one(char_or_float var):
    return var + 1

def show_me():
        char a = 127
        float b = 127
    print('char', plus_one(a))
    print('float', plus_one(b))

This gives:

>>> show_me()
char -128
float 128.0

plus_one(a) “specializes” the fused type char_or_float as a char, whereas plus_one(b) specializes char_or_float as a float.

Declaring Fused Types

Fused types may be declared as follows:

cimport cython

ctypedef fused my_fused_type:

This declares a new type called my_fused_type which can be either an int or a double. Alternatively, the declaration may be written as:

my_fused_type = cython.fused_type(cython.int, cython.float)

Only names may be used for the constituent types, but they may be any (non-fused) type, including a typedef. i.e. one may write:

ctypedef double my_double
my_fused_type = cython.fused_type(cython.int, my_double)

Using Fused Types

Fused types can be used to declare parameters of functions or methods:

cdef cfunc(my_fused_type arg):
    return arg + 1

If the you use the same fused type more than once in an argument list, then each specialization of the fused type must be the same:

cdef cfunc(my_fused_type arg1, my_fused_type arg2):
    return cython.typeof(arg1) == cython.typeof(arg2)

In this case, the type of both parameters is either an int, or a double (according to the previous examples). However, because these arguments use the same fused type my_fused_type, both arg1 and arg2 are specialized to the same type. Therefore this function returns True for every possible valid invocation. You are allowed to mix fused types however:

def func(A x, B y):

where A and B are different fused types. This will result in specialized code paths for all combinations of types contained in A and B.

Fused types and arrays

Note that specializations of only numeric types may not be very useful, as one can usually rely on promotion of types. This is not true for arrays, pointers and typed views of memory however. Indeed, one may write:

def myfunc(A[:, :] x):

# and

cdef otherfunc(A *x):

Note that in Cython 0.20.x and earlier, the compiler generated the full cross product of all type combinations when a fused type was used by more than one memory view in a type signature, e.g.

def myfunc(A[:] a, A[:] b):
    # a and b had independent item types in Cython 0.20.x and earlier.

This was unexpected for most users, unlikely to be desired, and also inconsistent with other structured type declarations like C arrays of fused types, which were considered the same type. It was thus changed in Cython 0.21 to use the same type for all memory views of a fused type. In order to get the original behaviour, it suffices to declare the same fused type under different names, and then use these in the declarations:

ctypedef fused A:

ctypedef fused B:

def myfunc(A[:] a, B[:] b):
    # a and b are independent types here and may have different item types

To get only identical types also in older Cython versions (pre-0.21), a ctypedef can be used:

ctypedef A[:] A_1d

def myfunc(A_1d a, A_1d b):
    # a and b have identical item types here, also in older Cython versions

Selecting Specializations

You can select a specialization (an instance of the function with specific or specialized (i.e., non-fused) argument types) in two ways: either by indexing or by calling.


You can index functions with types to get certain specializations, i.e.:

cfunc[cython.p_double](p1, p2)

# From Cython space
func[float, double](myfloat, mydouble)

# From Python space
func[cython.float, cython.double](myfloat, mydouble)

If a fused type is used as a base type, this will mean that the base type is the fused type, so the base type is what needs to be specialized:

cdef myfunc(A *x):

# Specialize using int, not int *


A fused function can also be called with arguments, where the dispatch is figured out automatically:

cfunc(p1, p2)
func(myfloat, mydouble)

For a cdef or cpdef function called from Cython this means that the specialization is figured out at compile time. For def functions the arguments are typechecked at runtime, and a best-effort approach is performed to figure out which specialization is needed. This means that this may result in a runtime TypeError if no specialization was found. A cpdef function is treated the same way as a def function if the type of the function is unknown (e.g. if it is external and there is no cimport for it).

The automatic dispatching rules are typically as follows, in order of preference:

  • try to find an exact match
  • choose the biggest corresponding numerical type (biggest float, biggest complex, biggest int)

Built-in Fused Types

There are some built-in fused types available for convenience, these are:

cython.integral # short, int, long
cython.floating # float, double
cython.numeric  # short, int, long, float, double, float complex, double complex

Casting Fused Functions

Fused cdef and cpdef functions may be cast or assigned to C function pointers as follows:

cdef myfunc(cython.floating, cython.integral):

# assign directly
cdef object (*funcp)(float, int)
funcp = myfunc
funcp(f, i)

# alternatively, cast it
(<object (*)(float, int)> myfunc)(f, i)

# This is also valid
funcp = myfunc[float, int]
funcp(f, i)

Type Checking Specializations

Decisions can be made based on the specializations of the fused parameters. False conditions are pruned to avoid invalid code. One may check with is, is not and == and != to see if a fused type is equal to a certain other non-fused type (to check the specialization), or use in and not in to figure out whether a specialization is part of another set of types (specified as a fused type). In example:

ctypedef fused bunch_of_types:

ctypedef fused string_t:

cdef cython.integral myfunc(cython.integral i, bunch_of_types s):
    cdef int *int_pointer
    cdef long *long_pointer

    # Only one of these branches will be compiled for each specialization!
    if cython.integral is int:
        int_pointer = &i
        long_pointer = &i

    if bunch_of_types in string_t:
        print("s is a string!")


Finally, function objects from def or cpdef functions have an attribute __signatures__, which maps the signature strings to the actual specialized functions. This may be useful for inspection. Listed signature strings may also be used as indices to the fused function, but the index format may change between Cython versions:

specialized_function = fused_function["MyExtensionClass|int|float"]

It would usually be preferred to index like this, however:

specialized_function = fused_function[MyExtensionClass, int, float]

Although the latter will select the biggest types for int and float from Python space, as they are not type identifiers but builtin types there. Passing cython.int and cython.float would resolve that, however.

For memoryview indexing from python space we can do the following:

ctypedef fused my_fused_type:
    int[:, ::1]
    float[:, ::1]

def func(my_fused_type array):

my_fused_type[cython.int[:, ::1]](myarray)

The same goes for when using e.g. cython.numeric[:, :].