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The ctypes ArgumentError exception indicates the location of the problem by argument number. It counts arguments starting from 1, not 0 as is typical in Python.
Observed
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An example (anonymised) traceback:
 Traceback (most recent call last):
 ...
 foreign_function(a, b, c, d)
 ctypes.ArgumentError: argument 2: <class 'TypeError'>: wrong type
The error here was with the argument "b".
Expected
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Standard, zero-indexed Python counting would suggest that argument 2 should mean "c", and "b" would be argument 1.
Rationale
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This may be as intended, but for me it violates the principle of least surprise.
I *think* this is the vicinity of the bug:
http://hg.python.org/cpython/file/696d3631a4a1/Modules/_ctypes/callproc.c#l1103
 _ctypes_extend_error(PyExc_ArgError, "argument %d: ", i+1);
If I'm right and the "i+1" (here and/or in subsequent lines) is the cause, that definitely makes it look intentional.