python可选参数验证的可选参数

Question

In python, how to validate optional-to-optional keyword arguments?

this question is an extension to this question of mine about this optional-to-optional-arguments thing, (does it have a better name by the way?)

we know we can define optional arguments in this style:

os.fdopen(fd[, mode[, bufsize]])

so that if I mistakenly call fdopen by fdopen(sth, bufsize=16) , python will point out I must specify mode to use bufsize argument.

How is this implemented? I can obviously write so much if-elseses to make this work, but that would result in some really messed-up code for those really complicated functions, for example:

cv2.dilate(src, kernel[, dst[, anchor[, iterations[, borderType[, borderValue]]]]]) → dst

Answer 1

There's no specific syntax for this at Python level. You have to define ordinary optional arguments and do the validation yourself.

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The specific case you're looking at is implemented in C. Depending on which platform you're on, the C implementation is different. Here's the version for POSIX, Windows, and OS/2:

static PyObject *
posix_fdopen(PyObject *self, PyObject *args)
{
    ...
    if (!PyArg_ParseTuple(args, "i|si", &fd, &orgmode, &bufsize))
        return NULL;

The use of PyArg_ParseTuple means that this function doesn't actually accept any arguments by name. If you do os.fdopen(sth, bufsize=16) , you'll get a TypeError:

>>> os.fdopen('', bufsize=16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: fdopen() takes no keyword arguments

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Note that there's no real reason the bufsize argument should depend on the mode argument. I think this function probably predates keyword arguments, though it's hard to be sure. Keyword arguments were added to Python all the way back in 1.3 , and the earliest Python documentation available on python.org is for 1.4 , by which time os.fdopen was definitely around .