Re: [PATCH v2 28/38] qapi/gen.py: update write() to be more idiomatic

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Fri, Sep 25, 2020 at 03:15:57PM +0200, Markus Armbruster wrote:
>> Cleber Rosa <crosa@redhat.com> writes:
>> 
>> > On Wed, Sep 23, 2020 at 02:37:27PM -0400, John Snow wrote:
>> >> On 9/23/20 11:26 AM, Eduardo Habkost wrote:
>> >> > On Tue, Sep 22, 2020 at 05:00:51PM -0400, John Snow wrote:
>> >> > > Make the file handling here just a tiny bit more idiomatic.
>> >> > > (I realize this is heavily subjective.)
>> >> > > 
>> >> > > Use exist_ok=True for os.makedirs and remove the exception,
>> >> > > use fdopen() to wrap the file descriptor in a File-like object,
>> >> > > and use a context manager for managing the file pointer.
>> >> > > 
>> >> > > Signed-off-by: John Snow <jsnow@redhat.com>
>> >> > 
>> >> > Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>> >> > 
>> >> > I really miss a comment below explaining why we use
>> >> > open(os.open(pathname, ...), ...) instead of open(pathname, ...).
>> 
>> This code:
>> 
>>         fd = os.open(pathname, os.O_RDWR | os.O_CREAT, 0o666)
>>         f = open(fd, 'r+', encoding='utf-8')
>> 
>> >> Not known to me. It was introduced in 907b846653 as part of an effort to
>> >> reduce rebuild times. Maybe this avoids a modification time change if the
>> >> file already exists?
>> >> 
>> >> Markus?
>> >
>> > AFACIT the change on 907b846653 is effective because of the "is new
>> > text different from old text?" conditional.  I can not see how the
>> > separate/duplicate open/fdopen would contribute to that.
>> >
>> > But, let's hear from Markus.
>> 
>> This was my best attempt to open the file read/write, creating it if it
>> doesn't exist.
>> 
>> Plain
>> 
>>         f = open(pathname, "r+", encoding='utf-8')
>> 
>> fails instead of creates, and
>> 
>>         f = open(pathname, "w+", encoding='utf-8')
>> 
>> truncates.
>> 
>> If you know a better way, tell me!
>
> IIUC, you need  "a+" as the mode, rather than "w+"

Sure this lets me do

            f.seek(0)
            f.truncate(0)
            f.write(text)

to overwrite the old contents on all systems?

Documentation cautions:

    [...] 'a' for appending (which on some Unix systems, means that all
    writes append to the end of the file regardless of the current seek
    position).