On 13/06/2006 1:45 PM, Redefined Horizons wrote: > I'm trying to understand the argument flags that are used in the > method table of an extension module written in C. > > First let me ask this question about the method table. Is it an C > array named "PyMethodDef"?
Its *type* is PyMethodDef (there is a typedef for that in python.h). You can name it what you like, but in a module called "foo", calling this thing "foo_methods" might be a reasonable idea. Please read section 1.4 of the Extending and Embedding Manual. > > Now, onto my questions about the arguments: > > [1] I see that even when the Python function we are supplying takes no > arguments, (the argument flag is METH_NOARGS), that we still pass the > C function a reference to "self". Is this "self" actually a pointer to > the section of memory that represents the object in Python that is > calling the method? This is independent of whether it is NOARGS or VARARGS. Irrespective of whether the C function is intended to appear to the Python caller as a method of a type/class, or as module-level function, its first arg is self. In the latter case, it should be unused by the called function. If you were to print its contents (using the %p format in printf, if available), you would find that it is NULL. Please read section 1.1 of the manual. > > [2] What is the difference between "positional" arguments indicated by > the METH_VARARGS argument flag, and "keyword" arguments that are > indicated by the METH_KEYWORDS argument flag? Does one determine the > identity of an argument based on the order or position in which the > arguments are passed to the C function, while the other uses keys in a > dictionary to identify the arguments? The difference is the same as if the called function is a Python function. In one sense, there is no difference -- even if you declare keywords, a call can still use positional arguments. Note that METH_KEYWORDS is not a substitute for METH_VARARGS; it's an add-on -- or a bitwise-or-on :-) def afunction(foo, bar, zot): # positional args, caller's only option is afunction(42, 1, 666) def bfunction(foo=0, bar="plugh", zot=None): # keyword args, caller can do: bfunction(zot=10**100) # using a keyword: args foo & bar get default values bfunction(10, "xyzzy") # positional: foo = 10; bar = "xyzzy"; zot = 10**100 In a Python function, nothing extra needs to be done. In a C-extension function, a char * [] is required to supply the names of the arguments. The default value caper is handled by the caller initialising the C variables that will receive the argument values. Manual: please read section 1.4 and section 1.8. > That should be the start of my customization questions... :] > > If it is any consolation I hope to take what I learn and put together > a tutorial on expanding Python for those that only have a little > experience with C programming, like myself. I plan to explain > everything like I was teaching a 6-year old. :] > > I was just going to put this on a wiki, but is there a way to include > with other Python documentation on an "official" site? The manual is in fact written as a tutorial. You may prefer to propose changes to the manual, rather than to publish yet another document. A good idea is to download the Python source and go looking in the Modules directory. If module foo is implemented in C, its source will be there as foomodule.c. For a simple module that provides only functions, look at e.g. binascii. For something that defines types, try datetime. They and others should provide you with good examples. HTH, John -- http://mail.python.org/mailman/listinfo/python-list
