array object can take many concrete forms. It might be a one-dimensional (1D) array of Booleans, or a three-dimensional (3D) array of 8-bit unsigned integers. As the built-in function isinstance() will show, every array is an instance of np.ndarray, regardless of shape or the type of elements stored in the array, i.e., the dtype. Similarly, many type-annotated interfaces still only specify np.ndarray:
import numpy as np
def process(
x: np.ndarray,
y: np.ndarray,
) -> np.ndarray: ...Such type annotations are insufficient: most interfaces have strong expectations of the shape or dtype of passed arrays. Most code will fail if a 3D array is passed where a 1D array is expected, or an array of dates is passed where an array of floats is expected.
Taking full advantage of the generic np.ndarray, array shape and dtype characteristics can now be fully specified:
def process(
x: np.ndarray[tuple[int], np.dtype[np.bool_]],
y: np.ndarray[tuple[int, int, int], np.dtype[np.uint8]],
) -> np.ndarray[tuple[int], np.dtype[np.float64]]: ...With such detail, recent versions of static analysis tools like mypy and pyright can find issues before code is even run. Further, run-time validators specialized for NumPy, like StaticFrame‘s sf.CallGuard, can re-use the same annotations for run-time validation.
Generic Types in Python
Generic built-in containers such as list and dict can be made concrete by specifying, for each interface, the contained types. A function can declare it takes a list of str with list[str]; or a dict of str to bool can be specified with dict[str, bool].
The Generic np.ndarray
An np.ndarray is an N-dimensional array of a single element type (or dtype). The np.ndarray generic takes two type parameters: the first defines the shape with a tuple, the second defines the element type with the generic np.dtype. While np.ndarray has taken two type parameters for some time, the definition of the first parameter, shape, was not full specified until NumPy 2.1.
The Shape Type Parameter
When creating an array with interfaces like np.empty or np.full, a shape argument is given as a tuple. The length of the tuple defines the array’s dimensionality; the magnitude of each position defines the size of that dimension. Thus a shape (10,) is a 1D array of 10 elements; a shape (10, 100, 1000) is a three dimensional array of size 10 by 100 by 1000.
When using a tuple to define shape in the np.ndarray generic, at present only the number of dimensions can generally be used for type checking. Thus, a tuple[int] can specify a 1D array; a tuple[int, int, int] can specify a 3D array; a tuple[int, ...], specifying a tuple of zero or more integers, denotes an N-dimensional array. It might be possible in the future to type-check an np.ndarray with specific magnitudes per dimension (using Literal), but this is not yet broadly supported.
The dtype Type Parameter
The NumPy dtype object defines element types and, for some types, other characteristics such as size (for Unicode and string types) or unit (for np.datetime64 types). The dtype itself is generic, taking a NumPy “generic” type as a type parameter. The most narrow types specify specific element characteristics, for example np.uint8, np.float64, or np.bool_. Beyond these narrow types, NumPy provides more general types, such as np.integer, np.inexact, or np.number.
Making np.ndarray Concrete
The following examples illustrate concrete np.ndarray definitions:
A 1D array of Booleans:
np.ndarray[tuple[int], np.dtype[np.bool_]]A 3D array of unsigned 8-bit integers:
np.ndarray[tuple[int, int, int], np.dtype[np.uint8]]A two-dimensional (2D) array of Unicode strings:
np.ndarray[tuple[int, int], np.dtype[np.str_]]A 1D array of any numeric type:
np.ndarray[tuple[int], np.dtype[np.number]]Static Type Checking with Mypy
Once the generic np.ndarray is made concrete, mypy or similar type checkers can, for some code paths, identify values that are incompatible with an interface.
For example, the function below requires a 1D array of signed integers. As shown below, unsigned integers, or dimensionalities other than one, fail mypy checks.
def process1(x: np.ndarray[tuple[int], np.dtype[np.signedinteger]]): ...
a1 = np.empty(100, dtype=np.int16)
process1(a1) # mypy passes
a2 = np.empty(100, dtype=np.uint8)
process1(a2) # mypy fails
# error: Argument 1 to "process1" has incompatible type
# "ndarray[tuple[int], dtype[unsignedinteger[_8Bit]]]";
# expected "ndarray[tuple[int], dtype[signedinteger[Any]]]" [arg-type]
a3 = np.empty((100, 100, 100), dtype=np.int64)
process1(a3) # mypy fails
# error: Argument 1 to "process1" has incompatible type
# "ndarray[tuple[int, int, int], dtype[signedinteger[_64Bit]]]";
# expected "ndarray[tuple[int], dtype[signedinteger[Any]]]"Runtime Validation with sf.CallGuard
Not all array operations can statically define the shape or dtype of a resulting array. For this reason, static analysis will not catch all mismatched interfaces. Better than creating redundant validation code across many functions, type annotations can be re-used for run-time validation with tools specialized for NumPy types.
The StaticFrame CallGuard interface offers two decorators, check and warn, which raise exceptions or warnings, respectively, on validation errors. These decorators will validate type-annotations against the characteristics of run-time objects.
For example, by adding sf.CallGuard.check to the function below, the arrays fail validation with expressive CallGuard exceptions:
import static_frame as sf
@sf.CallGuard.check
def process2(x: np.ndarray[tuple[int], np.dtype[np.signedinteger]]): ...
b1 = np.empty(100, dtype=np.uint8)
process2(b1)
# static_frame.core.type_clinic.ClinicError:
# In args of (x: ndarray[tuple[int], dtype[signedinteger]]) -> Any
# └── In arg x
# └── ndarray[tuple[int], dtype[signedinteger]]
# └── dtype[signedinteger]
# └── Expected signedinteger, provided uint8 invalid
b2 = np.empty((10, 100), dtype=np.int8)
process2(b2)
# static_frame.core.type_clinic.ClinicError:
# In args of (x: ndarray[tuple[int], dtype[signedinteger]]) -> Any
# └── In arg x
# └── ndarray[tuple[int], dtype[signedinteger]]
# └── tuple[int]
# └── Expected tuple length of 1, provided tuple length of 2Conclusion
More can be done to improve NumPy typing. For example, the np.object_ type could be made generic such that Python types contained in an object array could be defined. For example, a 1D object array of pairs of integers could be annotated as:
np.ndarray[tuple[int], np.dtype[np.object_[tuple[int, int]]]]Further, units of np.datetime64 cannot yet be statically specified. For example, date units could be distinguished from nanosecond units with annotations like np.dtype[np.datetime64[Literal['D']]] or np.dtype[np.datetime64[Literal['ns']]].
Even with limitations, fully-specified NumPy type annotations catch errors and improve code quality. As shown, Static Analysis can identify mismatched shape or dtype, and validation with sf.CallGuard can provide strong run-time guarantees.