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

[Eric Blake]

On 9/25/20 8:52 AM, Markus Armbruster wrote:

> > > 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?

As long as you do a single pass over the output (you issue a stream of 
f.write() after the truncate, but never a seek), then this will work.

> 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).

Yes, that means that random access is impossible on such a stream.  But 
not all file creation patterns require random access.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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