Python file open function modes

Question

I have noticed that, in addition to the documented mode characters, Python 2.7.5.1 in Windows XP and 8.1 also accepts modes U and D at least when reading files. Mode U is used in numpy's genfromtxt. Mode D has the effect that the file is deleted, as per the following code fragment:

 f = open('text.txt','rD')
 print(f.next())
 f.close()  # file text.txt is deleted when closed

Does anybody know more about these modes, especially whether they are a permanent feature of the language applicable also on Linux systems?

Answer 1

The D flag seems to be Windows specific. Windows seems to add several flags to the fopen function in its CRT, as described here.

While Python does filter the mode string to make sure no errors arise from it, it does allow some of the special flags, as can be seen in the Python sources here. Specifically, it seems that the N flag is filtered out, while the T and D flags are allowed:

while (*++mode) {
    if (*mode == ' ' || *mode == 'N') /* ignore spaces and N */
        continue;
    s = "+TD"; /* each of this can appear only once */
    ...

I would suggest sticking to the documented options to keep the code cross-platform.