Frequently Encountered Bugs

There are a number of bugs that can be accidentally written into code which will give obscure or difficult error messages. This guide aims to provide a useful list of the ones most frequently encountered. Some are obvious but it is worth checking just in case, it can save a lot of time debugging. They are grouped by which language's "fault" they are, not which language they occur in.

Python based

• When calling C from python: function takes X args, (X given) probably means you gave 1 too many arguments (I think self counts as an argument in the first number it gives but not the second)
• Make sure to use PyObject * instead of PyObject
• When parsing arguments from Python in C: make sure the format string is correct.
• When returning anything from C to Python you must use Py_BuildValue()

Java based

• Java method not found error is often because the wrong types were given when declaring JNI_METH_CLASS() (Note: pyObject is not the type for pyString)
• When using (*env)->Call<type>Method(), the <type> must be the correct return type of the method(or field)
• When calling (*env)->Call<type>Method(), the arguments passed must be of type j<type>, this may involve converting a PyObject to a JythonPyObject or converting char* to jstring or converting PyString to char* to jstring

C based

• Conflicting definitions for a function can mean it is used before being declared.
• Generally be careful with types, they are hard to check and: Jython String in C(jobject) != Python String in C(PyString) != C String(char*) != Java String(jstring)