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Wed, 16 Dec 2020 09:23:13 +0100 (CET) From: Markus Armbruster To: John Snow Subject: Re: [PATCH 03/12] qapi/main: handle theoretical None-return from re.match() References: <20201214235327.1007124-1-jsnow@redhat.com> <20201214235327.1007124-4-jsnow@redhat.com> Date: Wed, 16 Dec 2020 09:23:13 +0100 In-Reply-To: <20201214235327.1007124-4-jsnow@redhat.com> (John Snow's message of "Mon, 14 Dec 2020 18:53:18 -0500") Message-ID: <87eejqtb0u.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 10.5.11.12 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=63.128.21.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_H4=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: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , qemu-devel@nongnu.org, Eduardo Habkost , Cleber Rosa Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" John Snow writes: > Mypy cannot understand that this match can never be None, so help it > along. > > Signed-off-by: John Snow > --- > scripts/qapi/main.py | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/scripts/qapi/main.py b/scripts/qapi/main.py > index 42517210b805..271d9e84da94 100644 > --- a/scripts/qapi/main.py > +++ b/scripts/qapi/main.py > @@ -23,7 +23,8 @@ > > def invalid_prefix_char(prefix: str) -> Optional[str]: > match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', prefix) @match can't be None because the regexp always matches a prefix, possibly the empty prefix. > - if match.end() != len(prefix): > + # match cannot be None, but mypy cannot infer that. > + if match and match.end() != len(prefix): > return prefix[match.end()] > return None High-level logic: if there is an invalid prefix character: return it return None The actual code takes the return None when the match fails. If this could happen, it would be wrong. I can't, so it doesn't matter, but I dislike it anyway. Alternative 1: turn "match cannot be None" from comment to code match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', prefix) + assert match if match.end() != len(prefix): return prefix[match.end()] return None Alternative 2: turn empty prefix into a special case - match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', prefix) + match = re.match(r'[A-Za-z_.-][A-Za-z0-9_.-]*', prefix) + if not match: + return prefix[0] if match.end() != len(prefix): return prefix[match.end()] return None I'd prefer alternative 1.