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?

Update: Python 3, doesn’t allow "rw" mode. You would get the error:

ValueError: must have exactly one of create/read/write/append mode

Answer 1

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.