Type Promotion Rules

Array API specification for type promotion rules.

A conforming implementation of the array API standard must implement the following type promotion rules governing the common result type for two array operands during an arithmetic operation.

A conforming implementation of the array API standard may support additional type promotion rules beyond those described in this specification.


Type codes are used here to keep tables readable; they are not part of the standard. In code, use the data type objects specified in Data Types (e.g., int16 rather than 'i2' ).


The following type promotion tables specify the casting behaviour for operations involving two arrays. In situations where more than two arrays participate, the table can be used repeatedy on pairs of input arrays (the result does not depend on the order in which the arrays are given).

Signed integer type promotion table

i1 i2 i4 i8
i1 i1 i2 i4 i8
i2 i2 i2 i4 i8
i4 i4 i4 i4 i8
i8 i8 i8 i8 i8


  • i1 : 8-bit signed integer (i.e., int8 )

  • i2 : 16-bit signed integer (i.e., int16 )

  • i4 : 32-bit signed integer (i.e., int32 )

  • i8 : 64-bit signed integer (i.e., int64 )

Unsigned integer type promotion table

u1 u2 u4 u8
u1 u1 u2 u4 u8
u2 u2 u2 u4 u8
u4 u4 u4 u4 u8
u8 u8 u8 u8 u8


  • u1 : 8-bit unsigned integer (i.e., uint8 )

  • u2 : 16-bit unsigned integer (i.e., uint16 )

  • u4 : 32-bit unsigned integer (i.e., uint32 )

  • u8 : 64-bit unsigned integer (i.e., uint64 )

Mixed unsigned and signed integer type promotion table

u1 u2 u4
i1 i2 i4 i8
i2 i2 i4 i8
i4 i4 i4 i8

Floating-point type promotion table

f4 f8
f4 f4 f8
f8 f8 f8


  • f4 : single-precision (32-bit) floating-point number (i.e., float32 )

  • f8 : double-precision (64-bit) floating-point number (i.e., float64 )


  • Type promotion rules strictly apply when determining the common result type for two array operands during an arithmetic operation, regardless of array dimension. Accordingly, zero-dimensional arrays are subject to the same type promotion rules as dimensional arrays.

  • Type promotion of non-numerical data types to numerical data types is unspecified (e.g., bool to intxx or floatxx ).

  • Non-array (“scalar”) operands must not participate in type promotion.


Mixed integer and floating-point type promotion rules are not specified because behavior varies between implementations.

Mixing arrays with Python scalars

Using Python scalars (i.e. instances of bool , int , float ) together with arrays must be supported for:

  • array <op> scalar ,

  • scalar <op> array ,

where <op> is a built-in operator (see Operators for operators supported by the array object), and scalar is of the same kind as the array dtype (e.g. a float scalar if the array’s dtype is float32 or float64 ). The expected behaviour is then equivalent to:

  1. Convert the scalar to a 0-D array with the same dtype as that of the array used in the expression.

  2. Execute the operation for array <op> 0-D array (or 0-D array <op> array if scalar was the left-hand argument).


Note again that mixed integer and floating-point behaviour is not specified. Mixing an integer array with a Python float may give float32 , float64 , or raise an exception - behaviour of implementations will differ.