14.14.1.7 Specifying the required argument types (function prototypes)

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14.14.1.7 Specifying the required argument types (function prototypes)

It is possible to specify the required argument types of functions exported from DLLs by setting the `argtypes` attribute.

`argtypes` must be a sequence of C data types (the `printf` function is probably not a good example here, because it takes a variable number and different types of parameters depending on the format string, on the other hand this is quite handy to experiment with this feature):

    >>> printf.argtypes = [c_char_p, c_char_p, c_int, c_double]
    >>> printf("String '%s', Int %d, Double %f\n", "Hi", 10, 2.2)
    String 'Hi', Int 10, Double 2.200000
    37
    >>>

Specifying a format protects against incompatible argument types (just as a prototype for a C function), and tries to convert the arguments to valid types:

    >>> printf("%d %d %d", 1, 2, 3)
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
    ArgumentError: argument 2: exceptions.TypeError: wrong type
    >>> printf("%s %d %f", "X", 2, 3)
    X 2 3.00000012
    12
    >>>

If you have defined your own classes which you pass to function calls, you have to implement a `from_param` class method for them to be able to use them in the `argtypes` sequence. The `from_param` class method receives the Python object passed to the function call, it should do a typecheck or whatever is needed to make sure this object is acceptable, and then return the object itself, it's `_as_parameter_` attribute, or whatever you want to pass as the C function argument in this case. Again, the result should be an integer, string, unicode, a `ctypes` instance, or something having the `_as_parameter_` attribute.