Path: csiph.com!news.swapon.de!fu-berlin.de!uni-berlin.de!not-for-mail From: Jason Swails Newsgroups: comp.lang.python Subject: Re: What is the meaning of Py_INCREF a static PyTypeObject? Date: Thu, 12 Nov 2015 13:52:10 -0500 Lines: 47 Message-ID: References: <56444835.5020803@126.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Trace: news.uni-berlin.de dgEuUhqhVjVPwBucm1ry9wUaofFVTsuUVteo3OPJUXug== Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'static': 0.03; 'api.': 0.04; 'cc:addr:python-list': 0.09; 'destroyed.': 0.09; 'garbage': 0.09; 'imported': 0.09; 'macros': 0.09; 'namespace': 0.09; 'python': 0.10; 'python.': 0.11; 'thu,': 0.15; '*always*': 0.16; 'attribute,': 0.16; 'cc:name:python list': 0.16; 'definition)': 0.16; 'module).': 0.16; 'received:io': 0.16; 'received:psf.io': 0.16; 'segfault': 0.16; 'unnecessary.': 0.16; 'wrote:': 0.16; 'module,': 0.18; '2015': 0.20; 'cc:2**0': 0.20; 'cc:addr:python.org': 0.20; '(the': 0.22; 'struct': 0.22; 'trying': 0.22; 'am,': 0.23; 'seems': 0.23; 'import': 0.24; 'header:In-Reply-To:1': 0.24; 'module': 0.25; 'message- id:@mail.gmail.com': 0.27; 'defining': 0.27; 'function': 0.28; 'objects': 0.29; 'tutorial': 0.29; 'normally': 0.30; 'point': 0.33; 'url:python': 0.33; 'received:google.com': 0.35; 'could': 0.35; 'done': 0.35; 'fail': 0.35; 'jason': 0.35; 'nov': 0.35; 'something': 0.35; 'skip:p 30': 0.35; 'sometimes': 0.35; 'but': 0.36; 'url:org': 0.36; 'received:209.85': 0.36; 'subject:?': 0.36; 'subject:: ': 0.37; 'really': 0.37; '12,': 0.37; 'doing': 0.38; 'received:209': 0.38; 'skip:p 20': 0.38; 'mean': 0.38; 'why': 0.39; 'does': 0.39; 'subject:the': 0.39; 'some': 0.40; 'url:3': 0.60; '&': 0.61; 'skip:n 10': 0.62; 'you.': 0.64; 'zhang': 0.84 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=u/9OSh1lJfPtCXvuDrKx10VW91w6xVWrKzVBQyF6PWM=; b=ZgtJPIYVPTcGTU/ZH9oiu4SmIOOjB5BSRAAlImX/6tQMtoyOFX/S8HdxncHg6vbE61 PptALuBLme7MZEbStAyPlfAeqRQG2pJod3PCi+f8lOb7SY6/7F3mBeju9JR2IpNeblMe 6O0DxZ0LlyuG5nZFWqQ7C2TvBj8n3rHXI5lzE6YaQqPI48Wa5KNiCZxLgPchW//qRV72 n+n4m4l3JTiOAu58zyH13fQctXMrbrizLh8naFEO8O6dNeXKGQmZOOfddPImFf1xp13V MsxzBlvpHcNZLaS4JqZfl2TwmESrrFEmwKGoBpBwnGGHdy1Ky1aQ+Z248iou32xs/7vT +ZUg== X-Received: by 10.13.229.3 with SMTP id o3mr16200632ywe.341.1447354330545; Thu, 12 Nov 2015 10:52:10 -0800 (PST) In-Reply-To: <56444835.5020803@126.com> X-Content-Filtered-By: Mailman/MimeDel 2.1.20+ X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.20+ Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Xref: csiph.com comp.lang.python:98710 On Thu, Nov 12, 2015 at 3:05 AM, Xiang Zhang <18518281186@126.com> wrote: > Recently I am learning Python C API. > > When I read the tutorial < > https://docs.python.org/3/extending/newtypes.html#the-basics>, defining > new types, I feel confused. After PyType_Ready(&noddy_NoddyType) comes > Py_INCREF(&noddy_NoddyType). Actually noddy_NoddyType is a static struct = so > I don't understand why I need to Py_INCREF it. Since it's Py_INCREFed, do= es > it mean sometimes we also need to Py_DECREF it? But then it seems that > type_dealloc will be invoked and it will fail assert(type->tp_flags & > Py_TPFLAGS_HEAPTYPE); > =E2=80=8BIt is a module attribute, so when the module is imported it has to= have a single reference (the reference *in* the module). If you don't INCREF it, then it will have a refcount of 0, and immediately be ready for garbage collection. So if you try to use the type from the module, you could get a segfault because it's trying to use an object (type definition) that was already destroyed. Note that you don't *always* have to INCREF objects after you create them in C. Some macros and function do that for you. And in some cases, all you want or need is a borrowed reference. In those cases, Py_INCREF is unnecessary. The DECREF will be done when it's normally done in Python. If you do something like import noddy del noddy.NoddyType =E2=80=8BAll that's really doing is removing NoddyType from the noddy names= pace and Py_DECREFing it. Alternatively, doing import noddy noddy.NoddyType =3D 10 # rebind the name Then the original object NoddyType was pointing to will be DECREFed and NoddyType will point to an object taking the value of 10. HTH, Jason