Data type promotion in NumPy#

When mixing two different data types, NumPy has to determine the appropriate dtype for the result of the operation. This step is referred to as promotion or finding the common dtype.

In typical cases, the user does not need to worry about the details of promotion, since the promotion step usually ensures that the result will either match or exceed the precision of the input.

For example, when the inputs are of the same dtype, the dtype of the result matches the dtype of the inputs:

>>> np.int8(1) + np.int8(1)
np.int8(2)

Mixing two different dtypes normally produces a result with the dtype of the higher precision input:

>>> np.int8(4) + np.int64(8)  # 64 > 8
np.int64(12)
>>> np.float32(3) + np.float16(3)  # 32 > 16
np.float32(6.0)

In typical cases, this does not lead to surprises. However, if you work with non-default dtypes like unsigned integers and low-precision floats, or if you mix NumPy integers, NumPy floats, and Python scalars, some details of NumPy promotion rules may be relevant. Note that these detailed rules do not always match those of other languages [1].

Numerical dtypes come in four “kinds” with a natural hierarchy.

  1. unsigned integers (uint)

  2. signed integers (int)

  3. float (float)

  4. complex (complex)

In addition to kind, NumPy numerical dtypes also have an associated precision, specified in bits. Together, the kind and precision specify the dtype. For example, a uint8 is an unsigned integer stored using 8 bits.

The result of an operation will always be of an equal or higher kind of any of the inputs. Furthermore, the result will always have a precision greater than or equal to those of the inputs. Already, this can lead to some examples which may be unexpected:

  1. When mixing floating point numbers and integers, the precision of the integer may force the result to a higher precision floating point. For example, the result of an operation involving int64 and float16 is float64.

  2. When mixing unsigned and signed integers with the same precision, the result will have higher precision than either inputs. Additionally, if one of them has 64bit precision already, no higher precision integer is available and for example an operation involving int64 and uint64 gives float64.

Please see the Numerical promotion section and image below for details on both.

Detailed behavior of Python scalars#

Since NumPy 2.0 [2], an important point in our promotion rules is that although operations involving two NumPy dtypes never lose precision, operations involving a NumPy dtype and a Python scalar (int, float, or complex) can lose precision. For instance, it is probably intuitive that the result of an operation between a Python integer and a NumPy integer should be a NumPy integer. However, Python integers have arbitrary precision whereas all NumPy dtypes have fixed precision, so the arbitrary precision of Python integers cannot be preserved.

More generally, NumPy considers the “kind” of Python scalars, but ignores their precision when determining the result dtype. This is often convenient. For instance, when working with arrays of a low precision dtype, it is usually desirable for simple operations with Python scalars to preserve the dtype.

>>> arr_float32 = np.array([1, 2.5, 2.1], dtype="float32")
>>> arr_float32 + 10.0  # undesirable to promote to float64
array([11. , 12.5, 12.1], dtype=float32)
>>> arr_int16 = np.array([3, 5, 7], dtype="int16")
>>> arr_int16 + 10  # undesirable to promote to int64
array([13, 15, 17], dtype=int16)

In both cases, the result precision is dictated by the NumPy dtype. Because of this, arr_float32 + 3.0 behaves the same as arr_float32 + np.float32(3.0), and arr_int16 + 10 behaves as arr_int16 + np.int16(10.).

As another example, when mixing NumPy integers with a Python float or complex, the result always has type float64 or complex128:

>> np.int16(1) + 1.0 np.float64(2.0)

However, these rules can also lead to surprising behavior when working with low precision dtypes.

First, since the Python value is converted to a NumPy one before the operation can by performed, operations can fail with an error when the result seems obvious. For instance, np.int8(1) + 1000 cannot continue because 1000 exceeds the maximum value of an int8. When the Python scalar cannot be coerced to the NumPy dtype, an error is raised:

>>> np.int8(1) + 1000
Traceback (most recent call last):
  ...
OverflowError: Python integer 1000 out of bounds for int8
>>> np.int64(1) * 10**100
Traceback (most recent call last):
...
OverflowError: Python int too large to convert to C long
>>> np.float32(1) + 1e300
np.float32(inf)
... RuntimeWarning: overflow encountered in cast

Second, since the Python float or integer precision is always ignored, a low precision NumPy scalar will keep using its lower precision unless explicitly converted to a higher precision NumPy dtype or Python scalar (e.g. via int(), float(), or scalar.item()). This lower precision may be detrimental to some calculations or lead to incorrect results, especially in the case of integer overflows:

>>> np.int8(100) + 100  # the result exceeds the capacity of int8
np.int8(-56)
... RuntimeWarning: overflow encountered in scalar add

Note that NumPy warns when overflows occur for scalars, but not for arrays; e.g., np.array(100, dtype="uint8") + 100 will not warn.

Numerical promotion#

The following image shows the numerical promotion rules with the kinds on the vertical axis and the precision on the horizontal axis.

../_images/nep-0050-promotion-no-fonts.svg

The input dtype with the higher kind determines the kind of the result dtype. The result dtype has a precision as low as possible without appearing to the left of either input dtype in the diagram.

Note the following specific rules and observations:

  1. When a Python float or complex interacts with a NumPy integer the result will be float64 or complex128 (yellow border). NumPy booleans will also be cast to the default integer.[#default-int] This is not relevant when additionally NumPy floating point values are involved.

  2. The precision is drawn such that float16 < int16 < uint16 because large uint16 do not fit int16 and large int16 will lose precision when stored in a float16. This pattern however is broken since NumPy always considers float64 and complex128 to be acceptable promotion results for any integer value.

  3. A special case is that NumPy promotes many combinations of signed and unsigned integers to float64. A higher kind is used here because no signed integer dtype is sufficiently precise to hold a uint64.

Exceptions to the general promotion rules#

In NumPy promotion refers to what specific functions do with the result and in some cases, this means that NumPy may deviate from what the np.result_type would give.

Behavior of sum and prod#

``np.sum`` and ``np.prod``: Will alway return the default integer type when summing over integer values (or booleans). This is usually an int64. The reason for this is that integer summations are otherwise very likely to overflow and give confusing results. This rule also applies to the underlying np.add.reduce and np.multiply.reduce.

Notable behavior with NumPy or Python integer scalars#

NumPy promotion refers to the result dtype and operation precision, but the operation will sometimes dictate that result. Division always returns floating point values and comparison always booleans.

This leads to what may appear as “exceptions” to the rules:

  • NumPy comparisons with Python integers or mixed precision integers always return the correct result. The inputs will never be cast in a way which loses precision.

  • Equality comparisons between types which cannot be promoted will be considered all False (equality) or all True (not-equal).

  • Unary math functions like np.sin that always return floating point values, accept any Python integer input by converting it to float64.

  • Division always returns floating point values and thus also allows divisions between any NumPy integer with any Python integer value by casting both to float64.

In principle, some of these exceptions may make sense for other functions. Please raise an issue if you feel this is the case.

Promotion of non-numerical datatypes#

NumPy extends the promotion to non-numerical types, although in many cases promotion is not well defined and simply rejected.

The following rules apply:

  • NumPy byte strings (np.bytes_) can be promoted to unicode strings (np.str_). However, casting the bytes to unicode will fail for non-ascii characters.

  • For some purposes NumPy will promote almost any other datatype to strings. This applies to array creation or concatenation.

  • The array constructers like np.array() will use object dtype when there is no viable promotion.

  • Structured dtypes can promote when their field names and order matches. In that case all fields are promoted individually.

  • NumPy timedelta can in some cases promote with integers.

Note

Some of these rules are somewhat surprising, and are being considered for change in the future. However, any backward-incompatible changes have to be weighed against the risks of breaking existing code. Please raise an issue if you have particular ideas about how promotion should work.

Details of promoted dtype instances#

The above discussion has mainly dealt with the behavior when mixing different DType classes. A dtype instance attached to an array can carry additional information such as byte-order, metadata, string length, or exact structured dtype layout.

While the string length or field names of a structured dtype are important, NumPy considers byte-order, metadata, and the exact layout of a structured dtype as storage details. During promotion NumPy does not take these storage details into account: * Byte-order is converted to native byte-order. * Metadata attached to the dtype may or may not be preserved. * Resulting structured dtypes will be packed (but aligned if inputs were).

This behaviors is the best behavior for most programs where storage details are not relevant to the final results and where the use of incorrect byte-order could drastically slow down evaluation.

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