Passing and returning numpy arrays to C++ methods via Cython
You've basically got it right. First, hopefully optimization shouldn't be a big deal. Ideally, most of the time is spent inside your C++ kernel, not in the cythnon wrapper code.
There are a few stylistic changes you can make that will simplify your code. (1) Reshaping between 1D and 2D arrays is not necessary. When you know the memory layout of your data (C-order vs. fortran order, striding, etc), you can see the array as just a chunk of memory that you're going to index yourself in C++, so numpy's ndim doesn't matter on the C++ side -- it's just seeing that pointer. (2) Using cython's address-of operator &
, you can get the pointer to the start of the array in a little cleaner way -- no explicit cast necessary -- using &X[0,0]
.
So this is my edited version of your original snippet:
cimport numpy as np
import numpy as np
cdef extern from "myclass.h":
cdef cppclass MyClass:
MyClass() except +
void run(double* X, int N, int D, double* Y)
def run(np.ndarray[np.double_t, ndim=2] X):
X = np.ascontiguousarray(X)
cdef np.ndarray[np.double_t, ndim=2, mode="c"] Y = np.zeros_like(X)
cdef MyClass myclass
myclass = MyClass()
myclass.run(&X[0,0], X.shape[0], X.shape[1], &Y[0,0])
return Y
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Michael Schubert
Updated on June 18, 2022Comments
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Michael Schubert almost 2 years
There are lots of questions about using numpy in cython on this site, a particularly useful one being Simple wrapping of C code with cython.
However, the cython/numpy interface api seems to have changed a bit, in particular with ensuring the passing of memory-contiguous arrays.
What is the best way to write a wrapper function in cython that:
- takes a numpy array that is likely but not necessarily contiguous
- calls a C++ class method with the signature
double* data_in, double* data_out
- returns a numpy array of the
double*
that the method wrote to?
My try is below:
cimport numpy as np import numpy as np # as suggested by jorgeca cdef extern from "myclass.h": cdef cppclass MyClass: MyClass() except + void run(double* X, int N, int D, double* Y) def run(np.ndarray[np.double_t, ndim=2] X): cdef int N, D N = X.shape[0] D = X.shape[1] cdef np.ndarray[np.double_t, ndim=1, mode="c"] X_c X_c = np.ascontiguousarray(X, dtype=np.double) cdef np.ndarray[np.double_t, ndim=1, mode="c"] Y_c Y_c = np.ascontiguousarray(np.zeros((N*D,)), dtype=np.double) cdef MyClass myclass myclass = MyClass() myclass.run(<double*> X_c.data, N, D, <double*> Y_c.data) return Y_c.reshape(N, 2)
This code compiles but is not necessarily optimal. Do you have any suggestions on improving the snippet above?
and (2) throws and "np is not defined on lineX_c = ...
") when calling it at runtime. The exact testing code and error message are the following:import numpy as np import mywrapper mywrapper.run(np.array([[1,2],[3,4]], dtype=np.double)) # NameError: name 'np' is not defined [at mywrapper.pyx":X_c = ...] # fixed!
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jorgeca almost 11 yearsYou still have to
import numpy as np
in your.pyx
file to use numpy functions (cimport numpy as np
"is used to import special compile-time information about the numpy module"). -
Saullo G. P. Castro almost 11 years@jorgeca I guess your comment answers the OP question...
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jorgeca almost 11 years@SaulloCastro I posted it as a comment because I thought it was a minor hurdle, but I don't know what's the best way to write these interfaces.
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Michael Schubert almost 11 years@jorgeca Thank you, it was indeed the missing statements that caused the error messages. And you are right, I'm mainly looking for optimisations :-)
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krishnab about 7 yearsSay, can this be done with a typed memoryview in Cython instead of passing the array? I was not sure if that would save some memory overhead, etc?