Note
This page uses two different syntax variants:
Cython specific cdef syntax, which was designed to make type declarations concise and easily readable from a C/C++ perspective.
Pure Python syntax which allows static Cython type declarations in pure Python code, following PEP-484 type hints and PEP 526 variable annotations.
To make use of C data types in Python syntax, you need to import the special cython module in the Python module that you want to compile, e.g.
If you use the pure Python syntax we strongly recommend you use a recent Cython 3 release, since significant improvements have been made here compared to the 0.29.x releases.
Python has a builtin array module supporting dynamic 1-dimensional arrays of primitive types. It is possible to access the underlying C array of a Python array from within Cython. At the same time they are ordinary Python objects which can be stored in lists and serialized between processes when using multiprocessing.
Compared to the manual approach with malloc() and free(), this gives the safe and automatic memory management of Python, and compared to a Numpy array there is no need to install a dependency, as the array module is built into both Python and Cython.
from cython.cimports.cpython import array
import array
a = cython.declare(array.array, array.array('i', [1, 2, 3]))
ca = cython.declare(cython.int[:], a)
print(ca[0])
from cpython cimport array
import array
cdef array.array a = array.array('i', [1, 2, 3])
cdef int[:] ca = a
print(ca[0])
NB: the import brings the regular Python array object into the namespace while the cimport adds functions accessible from Cython.
A Python array is constructed with a type signature and sequence of initial values. For the possible type signatures, refer to the Python documentation for the array module.
Notice that when a Python array is assigned to a variable typed as memory view, there will be a slight overhead to construct the memory view. However, from that point on the variable can be passed to other functions without overhead, so long as it is typed:
from cython.cimports.cpython import array
import array
a = cython.declare(array.array, array.array('i', [1, 2, 3]))
ca = cython.declare(cython.int[:], a)
@cython.cfunc
def overhead(a: cython.object) -> cython.int:
ca: cython.int[:] = a
return ca[0]
@cython.cfunc
def no_overhead(ca: cython.int[:]) -> cython.int:
return ca[0]
print(overhead(a)) # new memory view will be constructed, overhead
print(no_overhead(ca)) # ca is already a memory view, so no overhead
from cpython cimport array
import array
cdef array.array a = array.array('i', [1, 2, 3])
cdef int[:] ca = a
cdef int overhead(object a):
cdef int[:] ca = a
return ca[0]
cdef int no_overhead(int[:] ca):
return ca[0]
print(overhead(a)) # new memory view will be constructed, overhead
print(no_overhead(ca)) # ca is already a memory view, so no overhead
Zero-overhead, unsafe access to raw C pointer¶
To avoid any overhead and to be able to pass a C pointer to other functions, it is possible to access the underlying contiguous array as a pointer. There is no type or bounds checking, so be careful to use the right type and signedness.
from cython.cimports.cpython import array
import array
a = cython.declare(array.array, array.array('i', [1, 2, 3]))
# access underlying pointer:
print(a.data.as_ints[0])
from cython.cimports.libc.string import memset
memset(a.data.as_voidptr, 0, len(a) * cython.sizeof(cython.int))
from cpython cimport array
import array
cdef array.array a = array.array('i', [1, 2, 3])
# access underlying pointer:
print(a.data.as_ints[0])
from libc.string cimport memset
memset(a.data.as_voidptr, 0, len(a) * sizeof(int))
Note that any length-changing operation on the array object may invalidate the pointer.
Cloning, extending arrays¶To avoid having to use the array constructor from the Python module, it is possible to create a new array with the same type as a template, and preallocate a given number of elements. The array is initialized to zero when requested.
from cython.cimports.cpython import array
import array
int_array_template = cython.declare(array.array, array.array('i', []))
cython.declare(newarray=array.array)
# create an array with 3 elements with same type as template
newarray = array.clone(int_array_template, 3, zero=False)
from cpython cimport array
import array
cdef array.array int_array_template = array.array('i', [])
cdef array.array newarray
# create an array with 3 elements with same type as template
newarray = array.clone(int_array_template, 3, zero=False)
An array can also be extended and resized; this avoids repeated memory reallocation which would occur if elements would be appended or removed one by one.
from cython.cimports.cpython import array
import array
a = cython.declare(array.array, array.array('i', [1, 2, 3]))
b = cython.declare(array.array, array.array('i', [4, 5, 6]))
# extend a with b, resize as needed
array.extend(a, b)
# resize a, leaving just original three elements
array.resize(a, len(a) - len(b))
from cpython cimport array
import array
cdef array.array a = array.array('i', [1, 2, 3])
cdef array.array b = array.array('i', [4, 5, 6])
# extend a with b, resize as needed
array.extend(a, b)
# resize a, leaving just original three elements
array.resize(a, len(a) - len(b))
API reference¶ Data fields¶
data.as_voidptr data.as_chars data.as_schars data.as_uchars data.as_shorts data.as_ushorts data.as_ints data.as_uints data.as_longs data.as_ulongs data.as_longlongs # requires Python >=3 data.as_ulonglongs # requires Python >=3 data.as_floats data.as_doubles data.as_pyunicodes
Direct access to the underlying contiguous C array, with given type; e.g., myarray.data.as_ints.
The following functions are available to Cython from the array module
@cython.cfunc @cython.exceptval(-1) def resize(self: array.array, n: cython.Py_ssize_t) -> cython.int
cdef int resize(array.array self, Py_ssize_t n) except -1
Fast resize / realloc. Not suitable for repeated, small increments; resizes underlying array to exactly the requested amount.
@cython.cfunc @cython.exceptval(-1) def resize_smart(self: array.array, n: cython.Py_ssize_t) -> cython.int
cdef int resize_smart(array.array self, Py_ssize_t n) except -1
Efficient for small increments; uses growth pattern that delivers amortized linear-time appends.
@cython.cfunc @cython.inline def clone(template: array.array, length: cython.Py_ssize_t, zero: cython.bint) -> array.array
cdef inline array.array clone(array.array template, Py_ssize_t length, bint zero)
Fast creation of a new array, given a template array. Type will be same as template. If zero is True, new array will be initialized with zeroes.
@cython.cfunc @cython.inline def copy(self: array.array) -> array.array
cdef inline array.array copy(array.array self)
Make a copy of an array.
@cython.cfunc @cython.inline @cython.exceptval(-1) def extend_buffer(self: array.array, stuff: cython.p_char, n: cython.Py_ssize_t) -> cython.int
cdef inline int extend_buffer(array.array self, char* stuff, Py_ssize_t n) except -1
Efficient appending of new data of same type (e.g. of same array type) n: number of elements (not number of bytes!)
@cython.cfunc @cython.inline @cython.exceptval(-1) def extend(self: array.array, other: array.array) -> cython.int
cdef inline int extend(array.array self, array.array other) except -1
Extend array with data from another array; types must match.
@cython.cfunc @cython.inline def zero(self: array.array) -> cython.void
cdef inline void zero(array.array self)
Set all elements of array to zero.
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