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Dtype kind vs char #2864

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Merged
YannickJadoul merged 8 commits into from
Feb 23, 2021
Merged

Dtype kind vs char #2864

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df1be99
[dtype]: add type() method to access type attribute of PyArray_Descr …
Feb 15, 2021
b59299d
[dtype] change type() name method to char_() to be compliant with Pyt…
Feb 15, 2021
1400520
[dtype] fix by pre-commit
Feb 17, 2021
f455222
[dtype] Change comments and solutions format for test
Feb 17, 2021
400ffea
Clarify documentation and move note about dtype.char vs PyArray_Descr…
YannickJadoul Feb 17, 2021
f3ba900
Fix and extend tests
YannickJadoul Feb 17, 2021
14c9f7d
Fix the supposedly fixed tests
YannickJadoul Feb 17, 2021
46701ec
Fix the fixed tests again
YannickJadoul Feb 17, 2021
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12 changes: 11 additions & 1 deletion include/pybind11/numpy.h
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Original file line number Diff line number Diff line change
Expand Up @@ -507,11 +507,21 @@ class dtype : public object {
return detail::array_descriptor_proxy(m_ptr)->names != nullptr;
}

/// Single-character type code.
/// Single-character code for dtype's kind.
/// For example, floating point types are 'f' and integral types are 'i'.
char kind() const {
return detail::array_descriptor_proxy(m_ptr)->kind;
}

/// Single-character for dtype's type.
/// For example, ``float`` is 'f', ``double`` 'd', ``int`` 'i', and ``long`` 'd'.
char char_() const {
// Note: The signature, `dtype::char_` follows the naming of NumPy's
// public Python API (i.e., ``dtype.char``), rather than its internal
// C API (``PyArray_Descr::type``).
return detail::array_descriptor_proxy(m_ptr)->type;
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@YannickJadoul YannickJadoul Feb 17, 2021

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Is it worth adding a comment that we're following the Python API of dtype (i.e., dtype.char), rather than NumPy's internal C API (PyArray_Descr::type), for the name?

See also https://numpy.org/devdocs/reference/c-api/types-and-structures.html#c.PyArray_Descr for the root cause of the type vs. char confusion.

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@bemichel bemichel Feb 17, 2021

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okay !

}

private:
static object _dtype_from_pep3118() {
static PyObject *obj = module_::import("numpy.core._internal")
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34 changes: 34 additions & 0 deletions tests/test_numpy_dtypes.cpp
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Expand Up @@ -251,6 +251,38 @@ py::list test_dtype_ctors() {
return list;
}

py::list test_dtype_kind() {
py::list list;
for (auto& dt : {
py::dtype("bool8"), // bool
py::dtype("int16"), // short
py::dtype("int32"), // int
py::dtype("int64"), // long int
py::dtype("float32"), // float
py::dtype("float64"), // double
py::dtype("float128") // long double
}) {
list.append(dt.kind());
}
return list;
}

py::list test_dtype_char_() {
py::list list;
for (auto& dt : {
py::dtype("bool8"), // bool
py::dtype("int16"), // short
py::dtype("int32"), // int
py::dtype("int64"), // long int
py::dtype("float32"), // float
py::dtype("float64"), // double
py::dtype("float128") // long double
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@bstaletic bstaletic Feb 17, 2021

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On Winblows sizeof(long double) == sizeof(double), which might explain the error.

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@YannickJadoul YannickJadoul Feb 17, 2021

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I was about to say: MSVC doesn't really like this, @bemichel.

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@YannickJadoul YannickJadoul Feb 17, 2021

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Also, those comments are not completely right: sometimes np.int32 == np.int, but sometimes np.int64 == np.int. It really depends on the platform, I think?

But then, as far as I remember (I once contributed to this part of NumPy's docstrings in a sprint at a conference), the letters are actually linked to np.bool, np.int, np.float`, etc, and not to the sized variants!

Scratch that. Seems that either they changed, or I misremember.

No, I was right. But np.int is an alias for a Python int, not for a C int, that's np.intc.

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@YannickJadoul YannickJadoul Feb 17, 2021

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At any rate, here are the docs:
https://numpy.org/doc/stable/reference/arrays.scalars.html#built-in-scalar-types

intN or floatN are aliases, so I'd suggest using the non-alias versions.

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@YannickJadoul YannickJadoul Feb 17, 2021

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Actually. It's easier if I just try fixing this. Let's see.

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@YannickJadoul YannickJadoul Feb 17, 2021

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Done. Let's see what this says and does.

}) {
list.append(dt.char_());
}
return list;
}

struct A {};
struct B {};

Expand Down Expand Up @@ -376,6 +408,8 @@ TEST_SUBMODULE(numpy_dtypes, m) {
return l;
});
m.def("test_dtype_ctors", &test_dtype_ctors);
m.def("test_dtype_kind", &test_dtype_kind);
m.def("test_dtype_char_", &test_dtype_char_);
m.def("test_dtype_methods", []() {
py::list list;
auto dt1 = py::dtype::of<int32_t>();
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3 changes: 3 additions & 0 deletions tests/test_numpy_dtypes.py
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Expand Up @@ -169,6 +169,9 @@ def test_dtype(simple_dtype):
np.zeros(1, m.trailing_padding_dtype())
)

assert m.test_dtype_kind() == list("biiifff")
assert m.test_dtype_char_() == list("?hilfdg")


def test_recarray(simple_dtype, packed_dtype):
elements = [(False, 0, 0.0, -0.0), (True, 1, 1.5, -2.5), (False, 2, 3.0, -5.0)]
Expand Down