Pass boolean array without copying with `f2py`?

Question

How should Fortran variables be typed if I intend to pass them a boolean, NumPy array with f2py? I have tried both integer*1 and logical*1, but these both indicate that the array is copied.

For example, if I compile the file, foo.f95, containing:

subroutine foo(x, n)
    logical*1 x(n)
    !f2py intent(in) x
    !f2py intent(hide), depend(x) :: n=shape(x,0)
    ...
end subroutine

with f2py -c -m foo foo.f90 -DF2PY_REPORT_ON_ARRAY_COPY=1 and run something like:

import numpy as np
import foo
x = np.random.randn(100) < 0
foo.foo(x)

it prints

copied an array: size=100, elsize=1

I get the same result if I change logical*1 to integer*1. What is the proper type for the boolean array in the Fortran file, so that the array is not copied?

Note that this is not a problem of memory contiguity since the arrays are 1D -- foo.foo(np.asfortranarray(x)) prints the same copy message.

Answer 1

From some experiments(*), it seems that Python/f2py treats np.int8 as compatible with logical*1, while np.bool or np.bool8 not for some reason. After inserting print *, "(fort) x = ", x into foo.f90, we get:

>>> foo.foo( np.array( [ True, False, False ], dtype=np.bool ) )
copied an array: size=3, elsize=1
 (fort) x =  T F F
>>> foo.foo( np.array( [ False, True, False ], dtype=np.bool8 ) )
copied an array: size=3, elsize=1
 (fort) x =  F T F
>>> foo.foo( np.array( [ False, False, True ], dtype=np.int8 ) ) # no copy
 (fort) x =  F F T

Because True and False are simply mapped to 1 and 0, using an int8 array on the Python side may be convenient.

---

(*) Some experiments

Here, I changed the f2py intent comment to inout to see whether we can modify the array from the Fortran side.

foo.f90:

subroutine foo(x, n)
    use iso_c_binding
    implicit none
    integer n
    logical*1 x( n )
    ! logical x( n )
    ! logical(c_bool) x( n )

    !f2py intent(inout) x
    !f2py intent(hide), depend(x) :: n=shape(x,0)

    print *, "(fort) x = ", x
    print *, "(fort) sizeof(x(1)) = ", sizeof(x(1))
    print *, "(fort) resetting x(:) to true"
    x(:) = .true.
end subroutine

test.py:

import numpy as np
import foo

for T in [ np.bool, np.bool8,
           np.int,  np.int8,  np.int32,  np.int64,
           np.uint, np.uint8, np.uint32, np.uint64,
           np.dtype('b'), np.dtype('int8'), np.dtype('int32') ]:

    print( "-------------------------" )
    print( "dtype =", T )

    x = np.array( [ True, False, True ], dtype=T )
    print( "input x =", x )

    try:
        foo.foo( x )
        print( "output x =", x )
    except:
        print( "failed" )

Results with logical*1:

-------------------------
dtype = <class 'bool'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'numpy.bool_'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'int'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int8'>
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = <class 'numpy.int32'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint8'>
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = <class 'numpy.uint32'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = int8
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = int8
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = int32
input x = [1 0 1]
failed

Results with logical (default kind):

-------------------------
dtype = <class 'bool'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'numpy.bool_'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'int'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int8'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int32'>
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     4
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = <class 'numpy.int64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint8'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint32'>
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     4
 (fort) resetting x(:) to true
output x = [1 1 1]
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = int8
input x = [1 0 1]
failed
-------------------------
dtype = int8
input x = [1 0 1]
failed
-------------------------
dtype = int32
input x = [1 0 1]
 (fort) x =  T F T
 (fort) sizeof(x(1)) =                     4
 (fort) resetting x(:) to true
output x = [1 1 1]

Results with logical(c_bool) (via iso_c_binding):

-------------------------
dtype = <class 'bool'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'numpy.bool_'>
input x = [ True False  True]
failed
-------------------------
dtype = <class 'int'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int8'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.int32'>
input x = [1 0 1]
 (fort) x =  T F F
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [65793     0     1]
-------------------------
dtype = <class 'numpy.int64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint8'>
input x = [1 0 1]
failed
-------------------------
dtype = <class 'numpy.uint32'>
input x = [1 0 1]
 (fort) x =  T F F
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [65793     0     1]
-------------------------
dtype = <class 'numpy.uint64'>
input x = [1 0 1]
failed
-------------------------
dtype = int8
input x = [1 0 1]
failed
-------------------------
dtype = int8
input x = [1 0 1]
failed
-------------------------
dtype = int32
input x = [1 0 1]
 (fort) x =  T F F
 (fort) sizeof(x(1)) =                     1
 (fort) resetting x(:) to true
output x = [65793     0     1]

For some reason, this last logical(c_bool) does not work with the above usage... (f2py seems to consider logical(c_bool) as 4 bytes, while gfortran treats it as 1 byte, so something is inconsistent...)