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Mon, 28 Sep 2020 14:09:12 +0200 (CEST) From: Markus Armbruster To: Eric Blake Subject: Re: [PATCH v2 28/38] qapi/gen.py: update write() to be more idiomatic References: <20200922210101.4081073-1-jsnow@redhat.com> <20200922210101.4081073-29-jsnow@redhat.com> <20200923152624.GO3312949@habkost.net> <20200924155959.GC191229@localhost.localdomain> <87r1qqro8i.fsf@dusky.pond.sub.org> <20200925132458.GJ2178035@redhat.com> <874knmq7yg.fsf@dusky.pond.sub.org> Date: Mon, 28 Sep 2020 14:09:12 +0200 In-Reply-To: (Eric Blake's message of "Fri, 25 Sep 2020 10:47:18 -0500") Message-ID: <87y2kugl1z.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/28 03:29:35 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.687, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Maydell , "Daniel P. =?utf-8?Q?Berrang=C3=A9?=" , Eduardo Habkost , John Snow , qemu-devel@nongnu.org, Michael Roth , Cleber Rosa , Alex =?utf-8?Q?Benn=C3=A9e?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Eric Blake writes: > 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. Well, I do seek(), right before the truncate. >> 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. To be honest, I still prefer the code I wrote, because there the reader only wonders why I didn't just open(), while here we get to argue about subtleties of mode "a+".