Python file modes detail

Question

In Python, the following statements do not work:

f = open("ftmp", "rw")
print >> f, "python"

I get the error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor

But with the following code it works:

g = open("ftmp", "r+")
print >> g, "python"

It looks like I need to revise the file modes. What are the deep intricacies of the file opening modes?

Answer 1

Better yet, let the documentation do it for you: http://docs.python.org/library/functions.html#open. Your issue in the question is that there is no "rw" mode... you probably want 'r+' as you wrote (or 'a+' if the file does not yet exist).

Answer 2

As an addition to @Jarret Hardie's answer here's how Python check file mode in the function fileio_init():

s = mode;
while (*s) {
    switch (*s++) {
    case 'r':
        if (rwa) {
        bad_mode:
            PyErr_SetString(PyExc_ValueError,
                    "Must have exactly one of read/write/append mode");
            goto error;
        }
        rwa = 1;
        self->readable = 1;
        break;
    case 'w':
        if (rwa)
            goto bad_mode;
        rwa = 1;
        self->writable = 1;
        flags |= O_CREAT | O_TRUNC;
        break;
    case 'a':
        if (rwa)
            goto bad_mode;
        rwa = 1;
        self->writable = 1;
        flags |= O_CREAT;
        append = 1;
        break;
    case 'b':
        break;
    case '+':
        if (plus)
            goto bad_mode;
        self->readable = self->writable = 1;
        plus = 1;
        break;
    default:
        PyErr_Format(PyExc_ValueError,
                 "invalid mode: %.200s", mode);
        goto error;
    }
}

if (!rwa)
    goto bad_mode;

That is: only "rwab+" characters are allowed; there must be exactly one of "rwa", at most one '+' and 'b' is a noop.