In fcntl module, the documentation states the following regarding the mutate_flag in ioctl method:

***
If it is false, the buffer’s mutability is ignored and behaviour is as for a read-only buffer, except that the 1024 byte limit mentioned above is avoided – so long as the buffer you pass is as least as long as what the operating system wants to put there, things should work.
***

However, looking at the code (fcntlmodule.c) it seems that the 1024 bytes limitation is avoided when the mutate_flag is set to True (the opposite of what is stated in the doc).

@Yuval sorry about the delay in replying.  Can a *nix person comment on this please as I stick with Windows.

Looking at the C code, if a writable buffer is passed, mutate_arg is false, and the buffer is longer than 1024 bytes, then a ValueError is raised:

>>> ioctl(0, 0, bytearray(2048), False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: ioctl string arg too long

So it seems the implementation does not match the documentation’s claim that “the 1024 byte limit . . . is avoided”. IMO this is an implementation bug.

But instead of (or in addition to) fixing it, maybe we could consider removing “mutate_flag” parameter altogether. It was originally added as a backwards compatibility helper for Issue 555817 in Python 2.3, and changed to True by default in Python 2.5.