NEP 42 — New and extensible DTypes#


New and extensible DTypes


Sebastian Berg


Ben Nathanson


Marten van Kerkwijk









This NEP is third in a series:

  • NEP 40 explains the shortcomings of NumPy’s dtype implementation.

  • NEP 41 gives an overview of our proposed replacement.

  • NEP 42 (this document) describes the new design’s datatype-related APIs.

  • NEP 43 describes the new design’s API for universal functions.


NumPy’s dtype architecture is monolithic – each dtype is an instance of a single class. There’s no principled way to expand it for new dtypes, and the code is difficult to read and maintain.

As NEP 41 explains, we are proposing a new architecture that is modular and open to user additions. dtypes will derive from a new DType class serving as the extension point for new types. np.dtype("float64") will return an instance of a Float64 class, a subclass of root class np.dtype.

This NEP is one of two that lay out the design and API of this new architecture. This NEP addresses dtype implementation; NEP 43 addresses universal functions.


Details of the private and external APIs may change to reflect user comments and implementation constraints. The underlying principles and choices should not change significantly.

Motivation and scope#

Our goal is to allow user code to create fully featured dtypes for a broad variety of uses, from physical units (such as meters) to domain-specific representations of geometric objects. NEP 41 describes a number of these new dtypes and their benefits.

Any design supporting dtypes must consider:

  • How shape and dtype are determined when an array is created

  • How array elements are stored and accessed

  • The rules for casting dtypes to other dtypes

In addition:

  • We want dtypes to comprise a class hierarchy open to new types and to subhierarchies, as motivated in NEP 41.

And to provide this,

  • We need to define a user API.

All these are the subjects of this NEP.

  • The class hierarchy, its relation to the Python scalar types, and its important attributes are described in nep42_DType class.

  • The functionality that will support dtype casting is described in Casting.

  • The implementation of item access and storage, and the way shape and dtype are determined when creating an array, are described in Coercion to and from Python objects.

  • The functionality for users to define their own DTypes is described in Public C-API.

The API here and in NEP 43 is entirely on the C side. A Python-side version will be proposed in a future NEP. A future Python API is expected to be similar, but provide a more convenient API to reuse the functionality of existing DTypes. It could also provide shorthands to create structured DTypes similar to Python’s dataclasses.

Backward compatibility#

The disruption is expected to be no greater than that of a typical NumPy release.

  • The main issues are noted in NEP 41 and will mostly affect heavy users of the NumPy C-API.

  • Eventually we will want to deprecate the API currently used for creating user-defined dtypes.

  • Small, rarely noticed inconsistencies are likely to change. Examples:

    • np.array(np.nan, dtype=np.int64) behaves differently from np.array([np.nan], dtype=np.int64) with the latter raising an error. This may require identical results (either both error or both succeed).

    • np.array([array_like]) sometimes behaves differently from np.array([np.array(array_like)])

    • array operations may or may not preserve dtype metadata

  • Documentation that describes the internal structure of dtypes will need to be updated.

The new code must pass NumPy’s regular test suite, giving some assurance that the changes are compatible with existing code.

Usage and impact#

We believe the few structures in this section are sufficient to consolidate NumPy’s present functionality and also to support complex user-defined DTypes.

The rest of the NEP fills in details and provides support for the claim.

Again, though Python is used for illustration, the implementation is a C API only; a future NEP will tackle the Python API.

After implementing this NEP, creating a DType will be possible by implementing the following outlined DType base class, that is further described in nep42_DType class:

class DType(np.dtype):
    type : type        # Python scalar type
    parametric : bool  # (may be indicated by superclass)

    def canonical(self) -> bool:
        raise NotImplementedError

    def ensure_canonical(self : DType) -> DType:
        raise NotImplementedError

For casting, a large part of the functionality is provided by the “methods” stored in _castingimpl

    def common_dtype(cls : DTypeMeta, other : DTypeMeta) -> DTypeMeta:
        raise NotImplementedError

    def common_instance(self : DType, other : DType) -> DType:
        raise NotImplementedError

    # A mapping of "methods" each detailing how to cast to another DType
    # (further specified at the end of the section)
    _castingimpl = {}

For array-coercion, also part of casting:

    def __dtype_setitem__(self, item_pointer, value):
        raise NotImplementedError

    def __dtype_getitem__(self, item_pointer, base_obj) -> object:
        raise NotImplementedError

    def __discover_descr_from_pyobject__(cls, obj : object) -> DType:
        raise NotImplementedError

    # initially private:
    def _known_scalar_type(cls, obj : object) -> bool:
        raise NotImplementedError

Other elements of the casting implementation is the CastingImpl:

casting = Union["safe", "same_kind", "unsafe"]

class CastingImpl:
    # Object describing and performing the cast
    casting : casting

    def resolve_descriptors(self, Tuple[DTypeMeta], Tuple[DType|None] : input) -> (casting, Tuple[DType]):
        raise NotImplementedError

    # initially private:
    def _get_loop(...) -> lowlevel_C_loop:
        raise NotImplementedError

which describes the casting from one DType to another. In NEP 43 this CastingImpl object is used unchanged to support universal functions. Note that the name CastingImpl here will be generically called ArrayMethod to accommodate both casting and universal functions.



The dtype instance; this is the object attached to a numpy array.


Any subclass of the base type np.dtype.


Conversion of Python types to NumPy arrays and values stored in a NumPy array.


Conversion of an array to a different dtype.

parametric type#

A dtype whose representation can change based on a parameter value, like a string dtype with a length parameter. All members of the current flexible dtype class are parametric. See NEP 40.


Finding a dtype that can perform an operation on a mix of dtypes without loss of information.

safe cast#

A cast is safe if no information is lost when changing type.

On the C level we use descriptor or descr to mean dtype instance. In the proposed C-API, these terms will distinguish dtype instances from DType classes.


NumPy has an existing class hierarchy for scalar types, as seen in the figure of NEP 40, and the new DType hierarchy will resemble it. The types are used as an attribute of the single dtype class in the current NumPy; they’re not dtype classes. They neither harm nor help this work.

The DType class#

This section reviews the structure underlying the proposed DType class, including the type hierarchy and the use of abstract DTypes.

Class getter#

To create a DType instance from a scalar type users now call np.dtype (for instance, np.dtype(np.int64)). Sometimes it is also necessary to access the underlying DType class; this comes up in particular with type hinting because the “type” of a DType instance is the DType class. Taking inspiration from type hinting, we propose the following getter syntax:


to get the DType class corresponding to a scalar type. The notation works equally well with built-in and user-defined DTypes.

This getter eliminates the need to create an explicit name for every DType, crowding the np namespace; the getter itself signifies the type. It also opens the possibility of making np.ndarray generic over DType class using annotations like:


The above is fairly verbose, so it is possible that we will include aliases like:

Float64 = np.dtype[np.float64]

in numpy.typing, thus keeping annotations concise but still avoiding crowding the np namespace as discussed above. For a user-defined DType:

class UserDtype(dtype): ...

one can do np.ndarray[UserDtype], keeping annotations concise in that case without introducing boilerplate in NumPy itself. For a user-defined scalar type:

class UserScalar(generic): ...

we would need to add a typing overload to dtype:

__new__(cls, dtype: Type[UserScalar], ...) -> UserDtype

to allow np.dtype[UserScalar].

The initial implementation probably will return only concrete (not abstract) DTypes.

This item is still under review.

Hierarchy and abstract classes#

We will use abstract classes as building blocks of our extensible DType class hierarchy.

  1. Abstract classes are inherited cleanly, in principle allowing checks like isinstance(np.dtype("float64"), np.inexact).

  2. Abstract classes allow a single piece of code to handle a multiplicity of input types. Code written to accept Complex objects can work with numbers of any precision; the precision of the results is determined by the precision of the arguments.

  3. There’s room for user-created families of DTypes. We can envision an abstract Unit class for physical units, with a concrete subclass like Float64Unit. Calling Unit(np.float64, "m") (m for meters) would be equivalent to Float64Unit("m").

  4. The implementation of universal functions in NEP 43 may require a class hierarchy.

Example: A NumPy Categorical class would be a match for pandas Categorical objects, which can contain integers or general Python objects. NumPy needs a DType that it can assign a Categorical to, but it also needs DTypes like CategoricalInt64 and CategoricalObject such that common_dtype(CategoricalInt64, String) raises an error, but common_dtype(CategoricalObject, String) returns an object DType. In our scheme, Categorical is an abstract type with CategoricalInt64 and CategoricalObject subclasses.

Rules for the class structure, illustrated below:

  1. Abstract DTypes cannot be instantiated. Instantiating an abstract DType raises an error, or perhaps returns an instance of a concrete subclass. Raising an error will be the default behavior and may be required initially.

  2. While abstract DTypes may be superclasses, they may also act like Python’s abstract base classes (ABC) allowing registration instead of subclassing. It may be possible to simply use or inherit from Python ABCs.

  3. Concrete DTypes may not be subclassed. In the future this might be relaxed to allow specialized implementations such as a GPU float64 subclassing a NumPy float64.

The Julia language has a similar prohibition against subclassing concrete types. For example methods such as the later __common_instance__ or __common_dtype__ cannot work for a subclass unless they were designed very carefully. It helps avoid unintended vulnerabilities to implementation changes that result from subclassing types that were not written to be subclassed. We believe that the DType API should rather be extended to simplify wrapping of existing functionality.

The DType class requires C-side storage of methods and additional information, to be implemented by a DTypeMeta class. Each DType class is an instance of DTypeMeta with a well-defined and extensible interface; end users ignore it.


Miscellaneous methods and attributes#

This section collects definitions in the DType class that are not used in casting and array coercion, which are described in detail below.

  • Existing dtype methods (numpy.dtype) and C-side fields are preserved.

  • DType.type replaces dtype.type. Unless a use case arises, dtype.type will be deprecated. This indicates a Python scalar type which represents the same values as the DType. This is the same type as used in the proposed Class getter and for DType discovery during array coercion. (This can may also be set for abstract DTypes, this is necessary for array coercion.)

  • A new self.canonical property generalizes the notion of byte order to indicate whether data has been stored in a default/canonical way. For existing code, “canonical” will just signify native byte order, but it can take on new meanings in new DTypes – for instance, to distinguish a complex-conjugated instance of Complex which stores real - imag instead of real + imag. The ISNBO (“is native byte order”) flag might be repurposed as the canonical flag.

  • Support is included for parametric DTypes. A DType will be deemed parametric if it inherits from ParametricDType.

  • DType methods may resemble or even reuse existing Python slots. Thus Python special slots are off-limits for user-defined DTypes (for instance, defining Unit("m") > Unit("cm")), since we may want to develop a meaning for these operators that is common to all DTypes.

  • Sorting functions are moved to the DType class. They may be implemented by defining a method dtype_get_sort_function(self, sortkind="stable") -> sortfunction that must return NotImplemented if the given sortkind is not known.

  • Functions that cannot be removed are implemented as special methods. Many of these were previously defined part of the PyArray_ArrFuncs slot of the dtype instance (PyArray_Descr *) and include functions such as nonzero, fill (used for np.arange), and fromstr (used to parse text files). These old methods will be deprecated and replacements following the new design principles added. The API is not defined here. Since these methods can be deprecated and renamed replacements added, it is acceptable if these new methods have to be modified.

  • Use of kind for non-built-in types is discouraged in favor of isinstance checks. kind will return the __qualname__ of the object to ensure uniqueness for all DTypes. On the C side, kind and char are set to \0 (NULL character). While kind will be discouraged, the current np.issubdtype may remain the preferred method for this type of check.

  • A method ensure_canonical(self) -> dtype returns a new dtype (or self) with the canonical flag set.

  • Since NumPy’s approach is to provide functionality through unfuncs, functions like sorting that will be implemented in DTypes might eventually be reimplemented as generalized ufuncs.


We review here the operations related to casting arrays:

We show how casting arrays with astype(new_dtype) will be implemented.

Common DType operations#

When input types are mixed, a first step is to find a DType that can hold the result without loss of information – a “common DType.”

Array coercion and concatenation both return a common dtype instance. Most universal functions use the common DType for dispatching, though they might not use it for a result (for instance, the result of a comparison is always bool).

We propose the following implementation:

  • For two DType classes:

    __common_dtype__(cls, other : DTypeMeta) -> DTypeMeta

    Returns a new DType, often one of the inputs, which can represent values of both input DTypes. This should usually be minimal: the common DType of Int16 and Uint16 is Int32 and not Int64. __common_dtype__ may return NotImplemented to defer to other and, like Python operators, subclasses take precedence (their __common_dtype__ method is tried first).

  • For two instances of the same DType:

    __common_instance__(self: SelfT, other : SelfT) -> SelfT

    For nonparametric built-in dtypes, this returns a canonicalized copy of self, preserving metadata. For nonparametric user types, this provides a default implementation.

  • For instances of different DTypes, for example >float64 and S8, the operation is done in three steps:

    1. Float64.__common_dtype__(type(>float64), type(S8)) returns String (or defers to String.__common_dtype__).

    2. The casting machinery (explained in detail below) provides the information that ">float64" casts to "S32"

    3. String.__common_instance__("S8", "S32") returns the final "S32".

The benefit of this handoff is to reduce duplicated code and keep concerns separate. DType implementations don’t need to know how to cast, and the results of casting can be extended to new types, such as a new string encoding.

This means the implementation will work like this:

def common_dtype(DType1, DType2):
    common_dtype = type(dtype1).__common_dtype__(type(dtype2))
    if common_dtype is NotImplemented:
        common_dtype = type(dtype2).__common_dtype__(type(dtype1))
        if common_dtype is NotImplemented:
            raise TypeError("no common dtype")
    return common_dtype

def promote_types(dtype1, dtype2):
    common = common_dtype(type(dtype1), type(dtype2))

    if type(dtype1) is not common:
        # Find what dtype1 is cast to when cast to the common DType
        # by using the CastingImpl as described below:
        castingimpl = get_castingimpl(type(dtype1), common)
        safety, (_, dtype1) = castingimpl.resolve_descriptors(
                (common, common), (dtype1, None))
        assert safety == "safe"  # promotion should normally be a safe cast

    if type(dtype2) is not common:
        # Same as above branch for dtype1.

    if dtype1 is not dtype2:
        return common.__common_instance__(dtype1, dtype2)

Some of these steps may be optimized for nonparametric DTypes.

Since the type returned by __common_dtype__ is not necessarily one of the two arguments, it’s not equivalent to NumPy’s “safe” casting. Safe casting works for np.promote_types(int16, int64), which returns int64, but fails for:

np.promote_types("int64", "float32") -> np.dtype("float64")

It is the responsibility of the DType author to ensure that the inputs can be safely cast to the __common_dtype__.

Exceptions may apply. For example, casting int32 to a (long enough) string is at least at this time considered “safe”. However np.promote_types(int32, String) will not be defined.


object always chooses object as the common DType. For datetime64 type promotion is defined with no other datatype, but if someone were to implement a new higher precision datetime, then:


would return HighPrecisionDatetime, and the casting implementation, as described below, may need to decide how to handle the datetime unit.


  • We’re pushing the decision on common DTypes to the DType classes. Suppose instead we could turn to a universal algorithm based on safe casting, imposing a total order on DTypes and returning the first type that both arguments could cast to safely.

    It would be difficult to devise a reasonable total order, and it would have to accept new entries. Beyond that, the approach is flawed because importing a type can change the behavior of a program. For example, a program requiring the common DType of int16 and uint16 would ordinarily get the built-in type int32 as the first match; if the program adds import int24, the first match becomes int24 and the smaller type might make the program overflow for the first time. [1]

  • A more flexible common DType could be implemented in the future where __common_dtype__ relies on information from the casting logic. Since __commond_dtype__ is a method a such a default implementation could be added at a later time.

  • The three-step handling of differing dtypes could, of course, be coalesced. It would lose the value of splitting in return for a possibly faster execution. But few cases would benefit. Most cases, such as array coercion, involve a single Python type (and thus dtype).

The cast operation#

Casting is perhaps the most complex and interesting DType operation. It is much like a typical universal function on arrays, converting one input to a new output, with two distinctions:

  • Casting always requires an explicit output datatype.

  • The NumPy iterator API requires access to functions that are lower-level than what universal functions currently need.

Casting can be complex and may not implement all details of each input datatype (such as non-native byte order or unaligned access). So a complex type conversion might entail 3 steps:

  1. The input datatype is normalized and prepared for the cast.

  2. The cast is performed.

  3. The result, which is in a normalized form, is cast to the requested form (non-native byte order).

Further, NumPy provides different casting kinds or safety specifiers:

  • equivalent, allowing only byte-order changes

  • safe, requiring a type large enough to preserve value

  • same_kind, requiring a safe cast or one within a kind, like float64 to float32

  • unsafe, allowing any data conversion

and in some cases a cast may be just a view.

We need to support the two current signatures of arr.astype:

  • For DTypes: arr.astype(np.String)

    • current spelling arr.astype("S")

    • np.String can be an abstract DType

  • For dtypes: arr.astype(np.dtype("S8"))

We also have two signatures of np.can_cast:

  • Instance to class: np.can_cast(dtype, DType, "safe")

  • Instance to instance: np.can_cast(dtype, other_dtype, "safe")

On the Python level dtype is overloaded to mean class or instance.

A third can_cast signature, np.can_cast(DType, OtherDType, "safe"),may be used internally but need not be exposed to Python.

During DType creation, DTypes will be able to pass a list of CastingImpl objects, which can define casting to and from the DType.

One of them should define the cast between instances of that DType. It can be omitted if the DType has only a single implementation and is nonparametric.

Each CastingImpl has a distinct DType signature:

CastingImpl[InputDtype, RequestedDtype]

and implements the following methods and attributes:

  • To report safeness,

    resolve_descriptors(self, Tuple[DTypeMeta], Tuple[DType|None] : input) -> casting, Tuple[DType].

    The casting output reports safeness (safe, unsafe, or same-kind), and the tuple is used for more multistep casting, as in the example below.

  • To get a casting function,

    get_loop(...) -> function_to_handle_cast (signature to be decided)

    returns a low-level implementation of a strided casting function (“transfer function”) capable of performing the cast.

    Initially the implementation will be private, and users will only be able to provide strided loops with the signature.

  • For performance, a casting attribute taking a value of equivalent, safe, unsafe, or same-kind.

Performing a cast


The above figure illustrates a multistep cast of an int24 with a value of 42 to a string of length 20 ("S20").

We’ve picked an example where the implementer has only provided limited functionality: a function to cast an int24 to an S8 string (which can hold all 24-bit integers). This means multiple conversions are needed.

The full process is:

  1. Call

    CastingImpl[Int24, String].resolve_descriptors((Int24, String), (int24, "S20")).

    This provides the information that CastingImpl[Int24, String] only implements the cast of int24 to "S8".

  2. Since "S8" does not match "S20", use

    CastingImpl[String, String].get_loop()

    to find the transfer (casting) function to convert an "S8" into an "S20"

  3. Fetch the transfer function to convert an int24 to an "S8" using

    CastingImpl[Int24, String].get_loop()

  4. Perform the actual cast using the two transfer functions:

    int24(42) -> S8("42") -> S20("42").

    resolve_descriptors allows the implementation for

    np.array(42, dtype=int24).astype(String)

    to call

    CastingImpl[Int24, String].resolve_descriptors((Int24, String), (int24, None)).

    In this case the result of (int24, "S8") defines the correct cast:

    np.array(42, dtype=int24).astype(String) == np.array("42", dtype="S8").

Casting safety

To compute np.can_cast(int24, "S20", casting="safe"), only the resolve_descriptors function is required and is called in the same way as in the figure describing a cast.

In this case, the calls to resolve_descriptors, will also provide the information that int24 -> "S8" as well as "S8" -> "S20" are safe casts, and thus also the int24 -> "S20" is a safe cast.

In some cases, no cast is necessary. For example, on most Linux systems np.dtype("long") and np.dtype("longlong") are different dtypes but are both 64-bit integers. In this case, the cast can be performed using long_arr.view("longlong"). The information that a cast is a view will be handled by an additional flag. Thus the casting can have the 8 values in total: the original 4 of equivalent, safe, unsafe, and same-kind, plus equivalent+view, safe+view, unsafe+view, and same-kind+view. NumPy currently defines dtype1 == dtype2 to be True only if byte order matches. This functionality can be replaced with the combination of “equivalent” casting and the “view” flag.

(For more information on the resolve_descriptors signature see the Public C-API section below and NEP 43.)

Casting between instances of the same DType

To keep down the number of casting steps, CastingImpl must be capable of any conversion between all instances of this DType.

In general the DType implementer must include CastingImpl[DType, DType] unless there is only a singleton instance.

General multistep casting

We could implement certain casts, such as int8 to int24, even if the user provides only an int16 -> int24 cast. This proposal does not provide that, but future work might find such casts dynamically, or at least allow resolve_descriptors to return arbitrary dtypes.

If CastingImpl[Int8, Int24].resolve_descriptors((Int8, Int24), (int8, int24)) returns (int16, int24), the actual casting process could be extended to include the int8 -> int16 cast. This adds a step.


The implementation for casting integers to datetime would generally say that this cast is unsafe (because it is always an unsafe cast). Its resolve_descriptors function may look like:

def resolve_descriptors(self, DTypes, given_dtypes):
   from_dtype, to_dtype = given_dtypes
   from_dtype = from_dtype.ensure_canonical()  # ensure not byte-swapped
   if to_dtype is None:
       raise TypeError("Cannot convert to a NumPy datetime without a unit")
   to_dtype = to_dtype.ensure_canonical()  # ensure not byte-swapped

   # This is always an "unsafe" cast, but for int64, we can represent
   # it by a simple view (if the dtypes are both canonical).
   # (represented as C-side flags here).
   safety_and_view = NPY_UNSAFE_CASTING | _NPY_CAST_IS_VIEW
   return safety_and_view, (from_dtype, to_dtype)


While NumPy currently defines integer-to-datetime casts, with the possible exception of the unit-less timedelta64 it may be better to not define these casts at all. In general we expect that user defined DTypes will be using custom methods such as unit.drop_unit(arr) or arr * unit.seconds.


  • Our design objectives are: - Minimize the number of DType methods and avoid code duplication. - Mirror the implementation of universal functions.

  • The decision to use only the DType classes in the first step of finding the correct CastingImpl in addition to defining CastingImpl.casting, allows to retain the current default implementation of __common_dtype__ for existing user defined dtypes, which could be expanded in the future.

  • The split into multiple steps may seem to add complexity rather than reduce it, but it consolidates the signatures of np.can_cast(dtype, DTypeClass) and np.can_cast(dtype, other_dtype).

    Further, the API guarantees separation of concerns for user DTypes. The user Int24 dtype does not have to handle all string lengths if it does not wish to do so. Further, an encoding added to the String DType would not affect the overall cast. The resolve_descriptors function can keep returning the default encoding and the CastingImpl[String, String] can take care of any necessary encoding changes.

  • The main alternative is moving most of the information that is here pushed into the CastingImpl directly into methods on the DTypes. But this obscures the similarity between casting and universal functions. It does reduce indirection, as noted below.

  • An earlier proposal defined two methods __can_cast_to__(self, other) to dynamically return CastingImpl. This removes the requirement to define all possible casts at DType creation (of one of the involved DTypes).

    Such an API could be added later. It resembles Python’s __getattr__ in providing additional control over attribute lookup.


CastingImpl is used as a name in this NEP to clarify that it implements all functionality related to a cast. It is meant to be identical to the ArrayMethod proposed in NEP 43 as part of restructuring ufuncs to handle new DTypes. All type definitions are expected to be named ArrayMethod.

The way dispatching works for CastingImpl is planned to be limited initially and fully opaque. In the future, it may or may not be moved into a special UFunc, or behave more like a universal function.

Coercion to and from Python objects#

When storing a single value in an array or taking it out, it is necessary to coerce it – that is, convert it – to and from the low-level representation inside the array.

Coercion is slightly more complex than typical casts. One reason is that a Python object could itself be a 0-dimensional array or scalar with an associated DType.

Coercing to and from Python scalars requires two to three methods that largely correspond to the current definitions:

  1. __dtype_setitem__(self, item_pointer, value)

  2. __dtype_getitem__(self, item_pointer, base_obj) -> object; base_obj is for memory management and usually ignored; it points to an object owning the data. Its only role is to support structured datatypes with subarrays within NumPy, which currently return views into the array. The function returns an equivalent Python scalar (i.e. typically a NumPy scalar).

  3. __dtype_get_pyitem__(self, item_pointer, base_obj) -> object (initially hidden for new-style user-defined datatypes, may be exposed on user request). This corresponds to the arr.item() method also used by arr.tolist() and returns Python floats, for example, instead of NumPy floats.

(The above is meant for C-API. A Python-side API would have to use byte buffers or similar to implement this, which may be useful for prototyping.)

When a certain scalar has a known (different) dtype, NumPy may in the future use casting instead of __dtype_setitem__.

A user datatype is (initially) expected to implement __dtype_setitem__ for its own DType.type and all basic Python scalars it wishes to support (e.g. int and float). In the future a function known_scalar_type may be made public to allow a user dtype to signal which Python scalars it can store directly.

Implementation: The pseudocode implementation for setting a single item in an array from an arbitrary Python object value is (some functions here are defined later):

def PyArray_Pack(dtype, item_pointer, value):
    DType = type(dtype)
    if DType.type is type(value) or DType.known_scalartype(type(value)):
        return dtype.__dtype_setitem__(item_pointer, value)

    # The dtype cannot handle the value, so try casting:
    arr = np.array(value)
    if arr.dtype is object or arr.ndim != 0:
        # not a numpy or user scalar; try using the dtype after all:
        return dtype.__dtype_setitem__(item_pointer, value)


where the call to np.array() represents the dtype discovery and is not actually performed.

Example: Current datetime64 returns np.datetime64 scalars and can be assigned from np.datetime64. However, the datetime __dtype_setitem__ also allows assignment from date strings (“2016-05-01”) or Python integers. Additionally the datetime __dtype_get_pyitem__ function actually returns a Python datetime.datetime object (most of the time).

Alternatives: This functionality could also be implemented as a cast to and from the object dtype. However, coercion is slightly more complex than typical casts. One reason is that in general a Python object could itself be a zero-dimensional array or scalar with an associated DType. Such an object has a DType, and the correct cast to another DType is already defined:

np.array(np.float32(4), dtype=object).astype(np.float64)

is identical to:

np.array(4, dtype=np.float32).astype(np.float64)

Implementing the first object to np.float64 cast explicitly, would require the user to take to duplicate or fall back to existing casting functionality.

It is certainly possible to describe the coercion to and from Python objects using the general casting machinery, but the object dtype is special and important enough to be handled by NumPy using the presented methods.

Further issues and discussion:

  • The __dtype_setitem__ function duplicates some code, such as coercion from a string.

    datetime64 allows assignment from string, but the same conversion also occurs for casting from the string dtype to datetime64.

    We may in the future expose the known_scalartype function to allow the user to implement such duplication.

    For example, NumPy would normally use


    but datetime64 could choose to use its __dtype_setitem__ instead for performance reasons.

  • There is an issue about how subclasses of scalars should be handled. We anticipate to stop automatically detecting the dtype for np.array(float64_subclass) to be float64. The user can still provide dtype=np.float64. However, the above automatic casting using np.array(scalar_subclass).astype(requested_dtype) will fail. In many cases, this is not an issue, since the Python __float__ protocol can be used instead. But in some cases, this will mean that subclasses of Python scalars will behave differently.


Example: np.complex256 should not use __float__ in its __dtype_setitem__ method in the future unless it is a known floating point type. If the scalar is a subclass of a different high precision floating point type (e.g. np.float128) then this currently loses precision without notifying the user. In that case np.array(float128_subclass(3), dtype=np.complex256) may fail unless the float128_subclass is first converted to the np.float128 base class.

DType discovery during array coercion#

An important step in the use of NumPy arrays is creation of the array from collections of generic Python objects.

Motivation: Although the distinction is not clear currently, there are two main needs:

np.array([1, 2, 3, 4.])

needs to guess the correct dtype based on the Python objects inside. Such an array may include a mix of datatypes, as long as they can be promoted. A second use case is when users provide the output DType class, but not the specific DType instance:

np.array([object(), None], dtype=np.dtype[np.string_])  # (or `dtype="S"`)

In this case the user indicates that object() and None should be interpreted as strings. The need to consider the user provided DType also arises for a future Categorical:

np.array([1, 2, 1, 1, 2], dtype=Categorical)

which must interpret the numbers as unique categorical values rather than integers.

There are three further issues to consider:

  1. It may be desirable to create datatypes associated with normal Python scalars (such as datetime.datetime) that do not have a dtype attribute already.

  2. In general, a datatype could represent a sequence, however, NumPy currently assumes that sequences are always collections of elements (the sequence cannot be an element itself). An example would be a vector DType.

  3. An array may itself contain arrays with a specific dtype (even general Python objects). For example: np.array([np.array(None, dtype=object)], dtype=np.String) poses the issue of how to handle the included array.

Some of these difficulties arise because finding the correct shape of the output array and finding the correct datatype are closely related.

Implementation: There are two distinct cases above:

  1. The user has provided no dtype information.

  2. The user provided a DType class – as represented, for example, by "S" representing a string of any length.

In the first case, it is necessary to establish a mapping from the Python type(s) of the constituent elements to the DType class. Once the DType class is known, the correct dtype instance needs to be found. In the case of strings, this requires to find the string length.

These two cases shall be implemented by leveraging two pieces of information:

  1. DType.type: The current type attribute to indicate which Python scalar type is associated with the DType class (this is a class attribute that always exists for any datatype and is not limited to array coercion).

  2. __discover_descr_from_pyobject__(cls, obj) -> dtype: A classmethod that returns the correct descriptor given the input object. Note that only parametric DTypes have to implement this. For nonparametric DTypes using the default instance will always be acceptable.

The Python scalar type which is already associated with a DType through the DType.type attribute maps from the DType to the Python scalar type. At registration time, a DType may choose to allow automatically discover for this Python scalar type. This requires a lookup in the opposite direction, which will be implemented using global a mapping (dictionary-like) of:

known_python_types[type] = DType

Correct garbage collection requires additional care. If both the Python scalar type (pytype) and DType are created dynamically, they will potentially be deleted again. To allow this, it must be possible to make the above mapping weak. This requires that the pytype holds a reference of DType explicitly. Thus, in addition to building the global mapping, NumPy will store the DType as pytype.__associated_array_dtype__ in the Python type. This does not define the mapping and should not be accessed directly. In particular potential inheritance of the attribute does not mean that NumPy will use the superclasses DType automatically. A new DType must be created for the subclass.


Python integers do not have a clear/concrete NumPy type associated right now. This is because during array coercion NumPy currently finds the first type capable of representing their value in the list of long, unsigned long, int64, unsigned int64, and object (on many machines long is 64 bit).

Instead they will need to be implemented using an AbstractPyInt. This DType class can then provide __discover_descr_from_pyobject__ and return the actual dtype which is e.g. np.dtype("int64"). For dispatching/promotion in ufuncs, it will also be necessary to dynamically create AbstractPyInt[value] classes (creation can be cached), so that they can provide the current value based promotion functionality provided by np.result_type(python_integer, array) [2] .

To allow for a DType to accept inputs as scalars that are not basic Python types or instances of DType.type, we use known_scalar_type method. This can allow discovery of a vector as a scalar (element) instead of a sequence (for the command np.array(vector, dtype=VectorDType)) even when vector is itself a sequence or even an array subclass. This will not be public API initially, but may be made public at a later time.

Example: The current datetime DType requires a __discover_descr_from_pyobject__ which returns the correct unit for string inputs. This allows it to support:

np.array(["2020-01-02", "2020-01-02 11:24"], dtype="M8")

By inspecting the date strings. Together with the common dtype operation, this allows it to automatically find that the datetime64 unit should be “minutes”.

NumPy internal implementation: The implementation to find the correct dtype will work similar to the following pseudocode:

def find_dtype(array_like):
    common_dtype = None
    for element in array_like:
        # default to object dtype, if unknown
        DType = known_python_types.get(type(element), np.dtype[object])
        dtype = DType.__discover_descr_from_pyobject__(element)

        if common_dtype is None:
            common_dtype = dtype
            common_dtype = np.promote_types(common_dtype, dtype)

In practice, the input to np.array() is a mix of sequences and array-like objects, so that deciding what is an element requires to check whether it is a sequence. The full algorithm (without user provided dtypes) thus looks more like:

def find_dtype_recursive(array_like, dtype=None):
    Recursively find the dtype for a nested sequences (arrays are not
    supported here).
    DType = known_python_types.get(type(element), None)

    if DType is None and is_array_like(array_like):
        # Code for a sequence, an array_like may have a DType we
        # can use directly:
        for element in array_like:
            dtype = find_dtype_recursive(element, dtype=dtype)
        return dtype

    elif DType is None:
        DType = np.dtype[object]

    # dtype discovery and promotion as in `find_dtype` above

If the user provides DType, then this DType will be tried first, and the dtype may need to be cast before the promotion is performed.

Limitations: The motivational point 3. of a nested array np.array([np.array(None, dtype=object)], dtype=np.String) is currently (sometimes) supported by inspecting all elements of the nested array. User DTypes will implicitly handle these correctly if the nested array is of object dtype. In some other cases NumPy will retain backward compatibility for existing functionality only. NumPy uses such functionality to allow code such as:

>>> np.array([np.array(["2020-05-05"], dtype="S")], dtype=np.datetime64)
array([['2020-05-05']], dtype='datetime64[D]')

which discovers the datetime unit D (days). This possibility will not be accessible to user DTypes without an intermediate cast to object or a custom function.

The use of a global type map means that an error or warning has to be given if two DTypes wish to map to the same Python type. In most cases user DTypes should only be implemented for types defined within the same library to avoid the potential for conflicts. It will be the DType implementor’s responsibility to be careful about this and use avoid registration when in doubt.


  • Instead of a global mapping, we could rely on the scalar attribute scalar.__associated_array_dtype__. This only creates a difference in behavior for subclasses, and the exact implementation can be undefined initially. Scalars will be expected to derive from a NumPy scalar. In principle NumPy could, for a time, still choose to rely on the attribute.

  • An earlier proposal for the dtype discovery algorithm used a two-pass approach, first finding the correct DType class and only then discovering the parametric dtype instance. It was rejected as needlessly complex. But it would have enabled value-based promotion in universal functions, allowing:

    np.add(np.array([8], dtype="uint8"), [4])

    to return a uint8 result (instead of int16), which currently happens for:

    np.add(np.array([8], dtype="uint8"), 4)

    (note the list [4] instead of scalar 4). This is not a feature NumPy currently has or desires to support.

Further issues and discussion: It is possible to create a DType such as Categorical, array, or vector which can only be used if dtype=DType is provided. Such DTypes cannot roundtrip correctly. For example:

np.array(np.array(1, dtype=Categorical)[()])

will result in an integer array. To get the original Categorical array dtype=Categorical will need to be passed explicitly. This is a general limitation, but round-tripping is always possible if dtype=original_arr.dtype is passed.

Public C-API#

DType creation#

To create a new DType the user will need to define the methods and attributes outlined in the Usage and impact section and detailed throughout this proposal.

In addition, some methods similar to those in PyArray_ArrFuncs will be needed for the slots struct below.

As mentioned in NEP 41, the interface to define this DType class in C is modeled after PEP 384: Slots and some additional information will be passed in a slots struct and identified by ssize_t integers:

static struct PyArrayMethodDef slots[] = {
    {NPY_dt_method, method_implementation},
    {0, NULL}

typedef struct{
  PyTypeObject *typeobj;    /* type of python scalar or NULL */
  int flags                 /* flags, including parametric and abstract */
  /* NULL terminated CastingImpl; is copied and references are stolen */
  CastingImpl *castingimpls[];
  PyType_Slot *slots;
  PyTypeObject *baseclass;  /* Baseclass or NULL */
} PyArrayDTypeMeta_Spec;

PyObject* PyArray_InitDTypeMetaFromSpec(PyArrayDTypeMeta_Spec *dtype_spec);

All of this is passed by copying.

TODO: The DType author should be able to define new methods for the DType, up to defining a full object, and, in the future, possibly even extending the PyArrayDTypeMeta_Type struct. We have to decide what to make available initially. A solution may be to allow inheriting only from an existing class: class MyDType(np.dtype, MyBaseclass). If np.dtype is first in the method resolution order, this also prevents an undesirable override of slots like ==.

The slots will be identified by names which are prefixed with NPY_dt_ and are:

  • is_canonical(self) -> {0, 1}

  • ensure_canonical(self) -> dtype

  • default_descr(self) -> dtype (return must be native and should normally be a singleton)

  • setitem(self, char *item_ptr, PyObject *value) -> {-1, 0}

  • getitem(self, char *item_ptr, PyObject (base_obj) -> object or NULL

  • discover_descr_from_pyobject(cls, PyObject) -> dtype or NULL

  • common_dtype(cls, other) -> DType, NotImplemented, or NULL

  • common_instance(self, other) -> dtype or NULL

Where possible, a default implementation will be provided if the slot is omitted or set to NULL. Nonparametric dtypes do not have to implement:

  • discover_descr_from_pyobject (uses default_descr instead)

  • common_instance (uses default_descr instead)

  • ensure_canonical (uses default_descr instead).

Sorting is expected to be implemented using:

  • get_sort_function(self, NPY_SORTKIND sort_kind) -> {out_sortfunction, NotImplemented, NULL}.

For convenience, it will be sufficient if the user implements only:

  • compare(self, char *item_ptr1, char *item_ptr2, int *res) -> {-1, 0, 1}

Limitations: The PyArrayDTypeMeta_Spec struct is clumsy to extend (for instance, by adding a version tag to the slots to indicate a new, longer version). We could use a function to provide the struct; it would require memory management but would allow ABI-compatible extension (the struct is freed again when the DType is created).


The external API for CastingImpl will be limited initially to defining:

  • casting attribute, which can be one of the supported casting kinds. This is the safest cast possible. For example, casting between two NumPy strings is of course “safe” in general, but may be “same kind” in a specific instance if the second string is shorter. If neither type is parametric the resolve_descriptors must use it.

  • resolve_descriptors(PyArrayMethodObject *self, PyArray_DTypeMeta *DTypes[2], PyArray_Descr *dtypes_in[2], PyArray_Descr *dtypes_out[2], NPY_CASTING *casting_out) -> int {0, -1} The out dtypes must be set correctly to dtypes which the strided loop (transfer function) can handle. Initially the result must have instances of the same DType class as the CastingImpl is defined for. The casting will be set to NPY_EQUIV_CASTING, NPY_SAFE_CASTING, NPY_UNSAFE_CASTING, or NPY_SAME_KIND_CASTING. A new, additional flag, _NPY_CAST_IS_VIEW, can be set to indicate that no cast is necessary and a view is sufficient to perform the cast. The cast should return -1 when an error occurred. If a cast is not possible (but no error occurred), a -1 result should be returned without an error set. This point is under consideration, we may use ``-1`` to indicate a general error, and use a different return value for an impossible cast. This means that it is not possible to inform the user about why a cast is impossible.

  • strided_loop(char **args, npy_intp *dimensions, npy_intp *strides, ...) -> int {0, -1} (signature will be fully defined in NEP 43)

This is identical to the proposed API for ufuncs. The additional ... part of the signature will include information such as the two dtypes. More optimized loops are in use internally, and will be made available to users in the future (see notes).

Although verbose, the API will mimic the one for creating a new DType:

typedef struct{
  int flags;                  /* e.g. whether the cast requires the API */
  int nin, nout;              /* Number of Input and outputs (always 1) */
  NPY_CASTING casting;        /* The "minimal casting level" */
  PyArray_DTypeMeta *dtypes;  /* input and output DType class */
  /* NULL terminated slots defining the methods */
  PyType_Slot *slots;
} PyArrayMethod_Spec;

The focus differs between casting and general ufuncs. For example, for casts nin == nout == 1 is always correct, while for ufuncs casting is expected to be usually “no”.

Notes: We may initially allow users to define only a single loop. Internally NumPy optimizes far more, and this should be made public incrementally in one of two ways:

  • Allow multiple versions, such as:

    • contiguous inner loop

    • strided inner loop

    • scalar inner loop

  • Or, more likely, expose the get_loop function which is passed additional information, such as the fixed strides (similar to our internal API).

  • The casting level denotes the minimal guaranteed casting level and can be -1 if the cast may be impossible. For most non-parametric casts, this value will be the casting level. NumPy may skip the resolve_descriptors call for np.can_cast() when the result is True based on this level.

The example does not yet include setup and error handling. Since these are similar to the UFunc machinery, they will be defined in NEP 43 and then incorporated identically into casting.

The slots/methods used will be prefixed with NPY_meth_.


  • Aside from name changes and signature tweaks, there seem to be few alternatives to the above structure. The proposed API using *_FromSpec function is a good way to achieve a stable and extensible API. The slots design is extensible and can be changed without breaking binary compatibility. Convenience functions can still be provided to allow creation with less code.

  • One downside is that compilers cannot warn about function-pointer incompatibilities.


Steps for implementation are outlined in the Implementation section of NEP 41. In brief, we first will rewrite the internals of casting and array coercion. After that, the new public API will be added incrementally. We plan to expose it in a preliminary state initially to gain experience. All functionality currently implemented on the dtypes will be replaced systematically as new features are added.


The space of possible implementations is large, so there have been many discussions, conceptions, and design documents. These are listed in NEP 40. Alternatives were also been discussed in the relevant sections above.