Get the length of a Python object. This is equivalent to calling
the Python builtin len() function on the object.
py_len(x, default = NULL)The length of the object, as a numeric value.
A Python object.
The default length value to return, in the case that
the associated Python object has no __len__ method. When NULL
(the default), an error is emitted instead.
Not all Python objects have a defined length. For objects without a defined
length, calling py_len() will throw an error. If you'd like to instead
infer a default length in such cases, you can set the default argument
to e.g. 1L, to treat Python objects without a __len__ method as having
length one.