source/release/1.19.0-notes.rst

.. currentmodule:: numpy

==========================
NumPy 1.19.0 Release Notes
==========================
This NumPy release is marked by the removal of much technical debt: support for
Python 2 has been removed, many deprecations have been expired, and
documentation has been improved. The polishing of the random module continues
apace with bug fixes and better usability from Cython.

The Python versions supported for this release are 3.6-3.8. Downstream
developers should use Cython >= 0.29.16 for Python 3.8 support and
OpenBLAS >= 3.7 to avoid problems on the Skylake architecture.


Highlights
==========

* Code compatibility with Python versions < 3.6 (including Python 2) was
  dropped from both the python and C code. The shims in ``numpy.compat`` will
  remain to support third-party packages, but they may be deprecated in a
  future release. Note that 1.19.x will *not* compile with earlier versions of
  Python due to the use of f-strings.

  (`gh-15233 <https://github.com/numpy/numpy/pull/15233>`__)


Expired deprecations
====================

``numpy.insert`` and ``numpy.delete`` can no longer be passed an axis on 0d arrays
----------------------------------------------------------------------------------
This concludes a deprecation from 1.9, where when an ``axis`` argument was
passed to a call to ``~numpy.insert`` and ``~numpy.delete`` on a 0d array, the
``axis`` and ``obj`` argument and indices would be completely ignored.
In these cases, ``insert(arr, "nonsense", 42, axis=0)`` would actually overwrite the
entire array, while ``delete(arr, "nonsense", axis=0)`` would be ``arr.copy()``

Now passing ``axis`` on a 0d array raises ``~numpy.AxisError``.

(`gh-15802 <https://github.com/numpy/numpy/pull/15802>`__)

``numpy.delete`` no longer ignores out-of-bounds indices
--------------------------------------------------------
This concludes deprecations from 1.8 and 1.9, where ``np.delete`` would ignore
both negative and out-of-bounds items in a sequence of indices. This was at
odds with its behavior when passed a single index.

Now out-of-bounds items throw ``IndexError``, and negative items index from the
end.

(`gh-15804 <https://github.com/numpy/numpy/pull/15804>`__)

``numpy.insert`` and ``numpy.delete`` no longer accept non-integral indices
---------------------------------------------------------------------------
This concludes a deprecation from 1.9, where sequences of non-integers indices
were allowed and cast to integers. Now passing sequences of non-integral
indices raises ``IndexError``, just like it does when passing a single
non-integral scalar.

(`gh-15805 <https://github.com/numpy/numpy/pull/15805>`__)

``numpy.delete`` no longer casts boolean indices to integers
------------------------------------------------------------
This concludes a deprecation from 1.8, where ``np.delete`` would cast boolean
arrays and scalars passed as an index argument into integer indices. The
behavior now is to treat boolean arrays as a mask, and to raise an error
on boolean scalars.

(`gh-15815 <https://github.com/numpy/numpy/pull/15815>`__)


Compatibility notes
===================

Changed random variate stream from ``numpy.random.Generator.dirichlet``
-----------------------------------------------------------------------
A bug in the generation of random variates for the Dirichlet distribution
with small 'alpha' values was fixed by using a different algorithm when
``max(alpha) < 0.1``.  Because of the change, the stream of variates
generated by ``dirichlet`` in this case will be different from previous
releases.

(`gh-14924 <https://github.com/numpy/numpy/pull/14924>`__)

Scalar promotion in ``PyArray_ConvertToCommonType``
---------------------------------------------------
The promotion of mixed scalars and arrays in ``PyArray_ConvertToCommonType``
has been changed to adhere to those used by ``np.result_type``.
This means that input such as ``(1000, np.array([1], dtype=np.uint8)))``
will now return ``uint16`` dtypes. In most cases the behaviour is unchanged.
Note that the use of this C-API function is generally discouraged.
This also fixes ``np.choose`` to behave the same way as the rest of NumPy
in this respect.

(`gh-14933 <https://github.com/numpy/numpy/pull/14933>`__)

Fasttake and fastputmask slots are deprecated and NULL'ed
---------------------------------------------------------
The fasttake and fastputmask slots are now never used and
must always be set to NULL. This will result in no change in behaviour.
However, if a user dtype should set one of these a DeprecationWarning
will be given.

(`gh-14942 <https://github.com/numpy/numpy/pull/14942>`__)

``np.ediff1d`` casting behaviour with ``to_end`` and ``to_begin``
-----------------------------------------------------------------
``np.ediff1d`` now uses the ``"same_kind"`` casting rule for
its additional ``to_end`` and ``to_begin`` arguments. This
ensures type safety except when the input array has a smaller
integer type than ``to_begin`` or ``to_end``.
In rare cases, the behaviour will be more strict than it was
previously in 1.16 and 1.17. This is necessary to solve issues
with floating point NaN.

(`gh-14981 <https://github.com/numpy/numpy/pull/14981>`__)

Converting of empty array-like objects to NumPy arrays
------------------------------------------------------
Objects with ``len(obj) == 0`` which implement an "array-like" interface,
meaning an object implementing ``obj.__array__()``,
``obj.__array_interface__``, ``obj.__array_struct__``, or the python
buffer interface and which are also sequences (i.e. Pandas objects)
will now always retain there shape correctly when converted to an array.
If such an object has a shape of ``(0, 1)`` previously, it could
be converted into an array of shape ``(0,)`` (losing all dimensions
after the first 0).

(`gh-14995 <https://github.com/numpy/numpy/pull/14995>`__)

Removed ``multiarray.int_asbuffer``
-----------------------------------
As part of the continued removal of Python 2 compatibility,
``multiarray.int_asbuffer`` was removed. On Python 3, it threw a
``NotImplementedError`` and was unused internally. It is expected that there
are no downstream use cases for this method with Python 3.

(`gh-15229 <https://github.com/numpy/numpy/pull/15229>`__)

``numpy.distutils.compat`` has been removed
-------------------------------------------
This module contained only the function ``get_exception()``, which was used as::

    try:
        ...
    except Exception:
        e = get_exception()

Its purpose was to handle the change in syntax introduced in Python 2.6, from
``except Exception, e:`` to ``except Exception as e:``, meaning it was only
necessary for codebases supporting Python 2.5 and older.

(`gh-15255 <https://github.com/numpy/numpy/pull/15255>`__)

``issubdtype`` no longer interprets ``float`` as ``np.floating``
----------------------------------------------------------------
``numpy.issubdtype`` had a FutureWarning since NumPy 1.14 which
has expired now. This means that certain input where the second
argument was neither a datatype nor a NumPy scalar type
(such as a string or a python type like ``int`` or ``float``)
will now be consistent with passing in ``np.dtype(arg2).type``.
This makes the result consistent with expectations and leads to
a false result in some cases which previously returned true.

(`gh-15773 <https://github.com/numpy/numpy/pull/15773>`__)

Change output of ``round`` on scalars to be consistent with Python
------------------------------------------------------------------

Output of the ``__round__`` dunder method and consequently the Python
built-in ``round`` has been changed to be a Python ``int`` to be consistent
with calling it on Python ``float`` objects when called with no arguments.
Previously, it would return a scalar of the ``np.dtype`` that was passed in.

(`gh-15840 <https://github.com/numpy/numpy/pull/15840>`__)

The ``numpy.ndarray`` constructor no longer interprets ``strides=()`` as ``strides=None``
-----------------------------------------------------------------------------------------
The former has changed to have the expected meaning of setting
``numpy.ndarray.strides`` to ``()``, while the latter continues to result in
strides being chosen automatically.

(`gh-15882 <https://github.com/numpy/numpy/pull/15882>`__)

C-Level string to datetime casts changed
----------------------------------------
The C-level casts from strings were simplified. This changed
also fixes string to datetime and timedelta casts to behave
correctly (i.e. like Python casts using ``string_arr.astype("M8")``
while previously the cast would behave like
``string_arr.astype(np.int_).astype("M8")``.
This only affects code using low-level C-API to do manual casts
(not full array casts) of single scalar values or using e.g.
``PyArray_GetCastFunc``, and should thus not affect the vast majority
of users.

(`gh-16068 <https://github.com/numpy/numpy/pull/16068>`__)

``SeedSequence`` with small seeds no longer conflicts with spawning
-------------------------------------------------------------------
Small seeds (less than ``2**96``) were previously implicitly 0-padded out to
128 bits, the size of the internal entropy pool. When spawned, the spawn key
was concatenated before the 0-padding. Since the first spawn key is ``(0,)``,
small seeds before the spawn created the same states as the first spawned
``SeedSequence``.  Now, the seed is explicitly 0-padded out to the internal
pool size before concatenating the spawn key. Spawned ``SeedSequences`` will
produce different results than in the previous release. Unspawned
``SeedSequences`` will still produce the same results.

(`gh-16551 <https://github.com/numpy/numpy/pull/16551>`__)


Deprecations
============

Deprecate automatic ``dtype=object`` for ragged input
-----------------------------------------------------
Calling ``np.array([[1, [1, 2, 3]])`` will issue a ``DeprecationWarning`` as
per `NEP 34`_. Users should explicitly use ``dtype=object`` to avoid the
warning.

.. _`NEP 34`: https://numpy.org/neps/nep-0034.html

(`gh-15119 <https://github.com/numpy/numpy/pull/15119>`__)

Passing ``shape=0`` to factory functions in ``numpy.rec`` is deprecated
-----------------------------------------------------------------------
``0`` is treated as a special case and is aliased to ``None`` in the functions:

* ``numpy.core.records.fromarrays``
* ``numpy.core.records.fromrecords``
* ``numpy.core.records.fromstring``
* ``numpy.core.records.fromfile``

In future, ``0`` will not be special cased, and will be treated as an array
length like any other integer.

(`gh-15217 <https://github.com/numpy/numpy/pull/15217>`__)

Deprecation of probably unused C-API functions
----------------------------------------------
The following C-API functions are probably unused and have been
deprecated:

* ``PyArray_GetArrayParamsFromObject``
* ``PyUFunc_GenericFunction``
* ``PyUFunc_SetUsesArraysAsData``

In most cases ``PyArray_GetArrayParamsFromObject`` should be replaced
by converting to an array, while ``PyUFunc_GenericFunction`` can be
replaced with ``PyObject_Call`` (see documentation for details).

(`gh-15427 <https://github.com/numpy/numpy/pull/15427>`__)

Converting certain types to dtypes is Deprecated
------------------------------------------------
The super classes of scalar types, such as ``np.integer``, ``np.generic``,
or ``np.inexact`` will now give a deprecation warning when converted
to a dtype (or used in a dtype keyword argument).
The reason for this is that ``np.integer`` is converted to ``np.int_``,
while it would be expected to represent *any* integer (e.g. also
``int8``, ``int16``, etc.
For example, ``dtype=np.floating`` is currently identical to
``dtype=np.float64``, even though also ``np.float32`` is a subclass of
``np.floating``.

(`gh-15534 <https://github.com/numpy/numpy/pull/15534>`__)

Deprecation of ``round`` for ``np.complexfloating`` scalars
-----------------------------------------------------------
Output of the ``__round__`` dunder method and consequently the Python built-in
``round`` has been deprecated on complex scalars. This does not affect
``np.round``.

(`gh-15840 <https://github.com/numpy/numpy/pull/15840>`__)

``numpy.ndarray.tostring()`` is deprecated in favor of ``tobytes()``
--------------------------------------------------------------------
``~numpy.ndarray.tobytes`` has existed since the 1.9 release, but until this
release ``~numpy.ndarray.tostring`` emitted no warning. The change to emit a
warning brings NumPy in line with the builtin ``array.array`` methods of the
same name.

(`gh-15867 <https://github.com/numpy/numpy/pull/15867>`__)


C API changes
=============

Better support for ``const`` dimensions in API functions
--------------------------------------------------------
The following functions now accept a constant array of ``npy_intp``:

* ``PyArray_BroadcastToShape``
* ``PyArray_IntTupleFromIntp``
* ``PyArray_OverflowMultiplyList``

Previously the caller would have to cast away the const-ness to call these
functions.

(`gh-15251 <https://github.com/numpy/numpy/pull/15251>`__)

Const qualify UFunc inner loops
-------------------------------
``UFuncGenericFunction`` now expects pointers to const ``dimension`` and
``strides`` as arguments. This means inner loops may no longer modify
either ``dimension`` or ``strides``. This change leads to an
``incompatible-pointer-types`` warning forcing users to either ignore
the compiler warnings or to const qualify their own loop signatures.

(`gh-15355 <https://github.com/numpy/numpy/pull/15355>`__)


New Features
============

``numpy.frompyfunc`` now accepts an identity argument
-----------------------------------------------------
This allows the :attr:``numpy.ufunc.identity`` attribute to be set on the
resulting ufunc, meaning it can be used for empty and multi-dimensional
calls to :meth:``numpy.ufunc.reduce``.

(`gh-8255 <https://github.com/numpy/numpy/pull/8255>`__)

``np.str_`` scalars now support the buffer protocol
---------------------------------------------------
``np.str_`` arrays are always stored as UCS4, so the corresponding scalars
now expose this through the buffer interface, meaning
``memoryview(np.str_('test'))`` now works.

(`gh-15385 <https://github.com/numpy/numpy/pull/15385>`__)

``subok`` option for ``numpy.copy``
-----------------------------------
A new kwarg, ``subok``, was added to ``numpy.copy`` to allow users to toggle
the behavior of ``numpy.copy`` with respect to array subclasses. The default
value is ``False`` which is consistent with the behavior of ``numpy.copy`` for
previous numpy versions. To create a copy that preserves an array subclass with
``numpy.copy``, call ``np.copy(arr, subok=True)``. This addition better
documents that the default behavior of ``numpy.copy`` differs from the
``numpy.ndarray.copy`` method which respects array subclasses by default.

(`gh-15685 <https://github.com/numpy/numpy/pull/15685>`__)

``numpy.linalg.multi_dot`` now accepts an ``out`` argument
----------------------------------------------------------

``out`` can be used to avoid creating unnecessary copies of the final product
computed by ``numpy.linalg.multidot``.

(`gh-15715 <https://github.com/numpy/numpy/pull/15715>`__)

``keepdims`` parameter for ``numpy.count_nonzero``
--------------------------------------------------
The parameter ``keepdims`` was added to ``numpy.count_nonzero``. The
parameter has the same meaning as it does in reduction functions such
as ``numpy.sum`` or ``numpy.mean``.

(`gh-15870 <https://github.com/numpy/numpy/pull/15870>`__)

``equal_nan`` parameter for ``numpy.array_equal``
-------------------------------------------------
The keyword argument ``equal_nan`` was added to ``numpy.array_equal``.
``equal_nan`` is a boolean value that toggles whether or not ``nan`` values are
considered equal in comparison (default is ``False``). This matches API used in
related functions such as ``numpy.isclose`` and ``numpy.allclose``.

(`gh-16128 <https://github.com/numpy/numpy/pull/16128>`__)


Improvements
============

Improve detection of CPU features
=================================
Replace ``npy_cpu_supports`` which was a gcc specific mechanism to test support
of AVX with more general functions ``npy_cpu_init`` and ``npy_cpu_have``, and
expose the results via a ``NPY_CPU_HAVE`` c-macro as well as a python-level
``__cpu_features__`` dictionary.

(`gh-13421 <https://github.com/numpy/numpy/pull/13421>`__)

Use 64-bit integer size on 64-bit platforms in fallback lapack_lite
-------------------------------------------------------------------
Use 64-bit integer size on 64-bit platforms in the fallback LAPACK library,
which is used when the system has no LAPACK installed, allowing it to deal with
linear algebra for large arrays.

(`gh-15218 <https://github.com/numpy/numpy/pull/15218>`__)

Use AVX512 intrinsic to implement ``np.exp`` when input is ``np.float64``
-------------------------------------------------------------------------
Use AVX512 intrinsic to implement ``np.exp`` when input is ``np.float64``,
which can improve the performance of ``np.exp`` with ``np.float64`` input 5-7x
faster than before. The ``_multiarray_umath.so`` module has grown about 63 KB
on linux64.

(`gh-15648 <https://github.com/numpy/numpy/pull/15648>`__)

Ability to disable madvise hugepages
------------------------------------
On Linux NumPy has previously added support for madavise hugepages which can
improve performance for very large arrays.  Unfortunately, on older Kernel
versions this led to peformance regressions, thus by default the support has
been disabled on kernels before version 4.6. To override the default, you can
use the environment variable::

    NUMPY_MADVISE_HUGEPAGE=0

or set it to 1 to force enabling support. Note that this only makes
a difference if the operating system is set up to use madvise
transparent hugepage.

(`gh-15769 <https://github.com/numpy/numpy/pull/15769>`__)

``numpy.einsum`` accepts NumPy ``int64`` type in subscript list
---------------------------------------------------------------
There is no longer a type error thrown when ``numpy.einsum`` is passed
a NumPy ``int64`` array as its subscript list.

(`gh-16080 <https://github.com/numpy/numpy/pull/16080>`__)

``np.logaddexp2.identity`` changed to ``-inf``
----------------------------------------------
The ufunc ``~numpy.logaddexp2`` now has an identity of ``-inf``, allowing it to
be called on empty sequences.  This matches the identity of ``~numpy.logaddexp``.

(`gh-16102 <https://github.com/numpy/numpy/pull/16102>`__)


Changes
=======

Remove handling of extra argument to ``__array__``
--------------------------------------------------
A code path and test have been in the code since NumPy 0.4 for a two-argument
variant of ``__array__(dtype=None, context=None)``. It was activated when
calling ``ufunc(op)`` or ``ufunc.reduce(op)`` if ``op.__array__`` existed.
However that variant is not documented, and it is not clear what the intention
was for its use. It has been removed.

(`gh-15118 <https://github.com/numpy/numpy/pull/15118>`__)

``numpy.random._bit_generator`` moved to ``numpy.random.bit_generator``
-----------------------------------------------------------------------
In order to expose ``numpy.random.BitGenerator`` and
``numpy.random.SeedSequence`` to Cython, the ``_bitgenerator`` module is now
public as ``numpy.random.bit_generator``

Cython access to the random distributions is provided via a ``pxd`` file
------------------------------------------------------------------------
``c_distributions.pxd`` provides access to the c functions behind many of the
random distributions from Cython, making it convenient to use and extend them.

(`gh-15463 <https://github.com/numpy/numpy/pull/15463>`__)

Fixed ``eigh`` and ``cholesky`` methods in ``numpy.random.multivariate_normal``
-------------------------------------------------------------------------------
Previously, when passing ``method='eigh'`` or ``method='cholesky'``,
``numpy.random.multivariate_normal`` produced samples from the wrong
distribution. This is now fixed.

(`gh-15872 <https://github.com/numpy/numpy/pull/15872>`__)

Fixed the jumping implementation in ``MT19937.jumped``
------------------------------------------------------
This fix changes the stream produced from jumped MT19937 generators. It does
not affect the stream produced using ``RandomState`` or ``MT19937`` that
are directly seeded.

The translation of the jumping code for the MT19937 contained a reversed loop
ordering. ``MT19937.jumped`` matches the Makoto Matsumoto's original
implementation of the Horner and Sliding Window jump methods.

(`gh-16153 <https://github.com/numpy/numpy/pull/16153>`__)