It turns out almost anything is technically an array-like. “Array-like” is more of a statement of how the input will be interpreted than a restriction on what the input can be; if a parameter is documented as array-like, NumPy will try to interpret it as an array.
There is no formal definition of array-like beyond the nearly tautological one — an array-like is any Python object that np.array can convert to an ndarray. To go beyond this, you’d need to study the source code.
NPY_NO_EXPORT PyObject *
PyArray_FromAny(PyObject *op, PyArray_Descr *newtype, int min_depth,
int max_depth, int flags, PyObject *context)
{
/*
* This is the main code to make a NumPy array from a Python
* Object. It is called from many different places.
*/
PyArrayObject *arr = NULL, *ret;
PyArray_Descr *dtype = NULL;
int ndim = 0;
npy_intp dims[NPY_MAXDIMS];
/* Get either the array or its parameters if it isn't an array */
if (PyArray_GetArrayParamsFromObject(op, newtype,
0, &dtype,
&ndim, dims, &arr, context) < 0) {
Py_XDECREF(newtype);
return NULL;
}
...
Particularly interesting is PyArray_GetArrayParamsFromObject, whose comments enumerate the types of objects np.array expects:
NPY_NO_EXPORT int
PyArray_GetArrayParamsFromObject(PyObject *op,
PyArray_Descr *requested_dtype,
npy_bool writeable,
PyArray_Descr **out_dtype,
int *out_ndim, npy_intp *out_dims,
PyArrayObject **out_arr, PyObject *context)
{
PyObject *tmp;
/* If op is an array */
/* If op is a NumPy scalar */
/* If op is a Python scalar */
/* If op supports the PEP 3118 buffer interface */
/* If op supports the __array_struct__ or __array_interface__ interface */
/*
* If op supplies the __array__ function.
* The documentation says this should produce a copy, so
* we skip this method if writeable is true, because the intent
* of writeable is to modify the operand.
* XXX: If the implementation is wrong, and/or if actual
* usage requires this behave differently,
* this should be changed!
*/
/* Try to treat op as a list of lists */
/* Anything can be viewed as an object, unless it needs to be writeable */
}
So by studying the source code we can conclude an array-like is
- a NumPy array, or
- a NumPy scalar, or
- a Python scalar, or
- any object which supports the PEP 3118 buffer interface, or
- any object that supports the
__array_struct__or__array_interface__interface, or - any object that supplies the
__array__function, or - any object that can be treated as a list of lists, or
- anything! If it doesn’t fall under one of the other cases, it’ll be treated as a 0-dimensional array of
objectdtype.