If py_call/2 or one of the other predicates that access Python causes Python to raise an exception, this exception is translated into a Prolog exception of the shape below. The library defines a rule for print_message/2 to render these errors in a human readable way.
error(python_error(ErrorType, Value), _)
Here, ErrorType is the name of the error type, as an atom,
e.g.,
’TypeError'. Value is the exception object
represented by a Python object reference. The library(janus)
defines the message formatting, which makes us end up with a message
like below.
?- py_call(nomodule:noattr). ERROR: Python 'ModuleNotFoundError': ERROR: No module named 'nomodule' ERROR: In: ERROR: [10] janus:py_call(nomodule:noattr)
The Python stack trace is handed embedded into the second
argument of the error(Formal, ImplementationDefined). If an
exception is printed, printing the Python backtrace, is controlled by
the Prolog flags py_backtrace (default true)
and
py_backtrace_depth (default 4).