python os.fdopen(os.open()) can't be used for writing?

Question

This question has to do with the answer to Write file with specific permissions in Python for opening a file for writing (in python) with specific permissions.

The code in the answer looks like:

with os.fdopen(os.open('foo', os.O_APPEND | os.O_CREAT, 0o644)) as out:
  out.write("hello\n")

This code in 2.7.1 (my company does not have 2.7.3 installed) produces:

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
IOError: File not open for writing

os.fdopen has its own mode argument, but setting that doesn't help:

>>> with os.fdopen(os.open('foo', os.O_APPEND | os.O_CREAT, 0o644), 'a') as out:
...   out.write("hello\n")
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 22] Invalid argument

Long story short, I have not been able to figure out how to actually write to a file that has been opened via os.fdopen and os.open. Any ideas? Known bug in 2.7.1?

Thanks in advance!

Answer 1

You must choose one of O_RDONLY, O_WRONLY or O_RDWR as a "basic" mode argument to open().

You did not explicitly do so, so O_RDONLY (zero on many systems) is assumed. Python's os.fdopen sees that you have specified a O_RDONLY and O_APPEND, which is a bit silly. Python complains about this combination with the EINVAL ("Invalid argument") error you see.

(Indeed, if you strace(1) your script — I'm assuming Linux here — I suspect you'll see that no "natural" EINVAL is encountered. Instead, python performs your os.open()/open(2), and then checks flags (F_GETFL) on the file descriptor just before raising the exception.)