Path: csiph.com!usenet.pasdenom.info!aioe.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed1.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.009 X-Spam-Evidence: '*H*': 0.98; '*S*': 0.00; 'argument': 0.05; 'none:': 0.07; '22,': 0.09; 'indeed,': 0.09; 'cc:addr:python-list': 0.11; 'def': 0.12; 'jan': 0.12; 'stored': 0.12; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'mutable': 0.16; 'other,': 0.16; 'sense,': 0.16; 'subject:default': 0.16; 'successive': 0.16; 'wrote:': 0.18; 'variable': 0.18; 'wed,': 0.18; 'cc:addr:python.org': 0.22; 'error': 0.23; 'initial': 0.24; 'cc:2**0': 0.24; 'this:': 0.26; 'header:In-Reply-To:1': 0.27; 'function': 0.29; 'am,': 0.29; 'thus': 0.29; 'message- id:@mail.gmail.com': 0.30; 'equivalent.': 0.31; 'cases': 0.33; 'subject:the': 0.34; 'common': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'list': 0.37; 'expected': 0.38; 'is.': 0.60; 'most': 0.60; 'new': 0.61; 'simply': 0.61; 'results': 0.69; 'default': 0.69; 'acts': 0.74; '6:11': 0.84; 'idiom': 0.84; 'contents.': 0.91; 'to:none': 0.92 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:cc :content-type:content-transfer-encoding; bh=XC/rvvTX3EAdzkuJFimhlQwwKeSIRXfgnpeeCdaKerg=; b=Srr4CPwo5NYUi8t6S4AEyRnqu41ZdtR4vRTsD5sMFMEvgx2m7WS4LWjPqZvbgtyVnM J8WDvXUwNTaT0KFAvMK8+dPDQ6YrUoTuZ8BYTx5wDfVBrbyZ+dDc4LCzxFORQFmTw74k nmmqwJNt5HvVud87d8mR5i6RmlCRY7tTFc4ss8ChGj7Sqn2jY5INDOPnkS6v81W8jJ/+ uGNANU1XPcBSyLJf8x7DdmgzfGicHYdbTafRNQFNF3OR6D8jh9JiCc3FhVcMPfK5e21c abguuWLUpSPXNZQq2CQWwHoC/0QW0qFhU4R4vGIcurBK8bcfwEPobgDg3+pXqokkPlre eOGw== MIME-Version: 1.0 X-Received: by 10.66.118.71 with SMTP id kk7mr26602626pab.14.1390331952892; Tue, 21 Jan 2014 11:19:12 -0800 (PST) In-Reply-To: <52dec646$0$2934$426a74cc@news.free.fr> References: <52dec646$0$2934$426a74cc@news.free.fr> Date: Wed, 22 Jan 2014 06:19:12 +1100 Subject: Re: Modifying the default argument of function From: Chris Angelico Cc: "python-list@python.org" Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 23 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1390331956 news.xs4all.nl 2955 [2001:888:2000:d::a6]:57508 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:64444 On Wed, Jan 22, 2014 at 6:11 AM, M=C3=BB wrote: > The function acts as if there were a global variable x, but the call of x > results in an error (undefined variable). I don't understand why the > successive calls of f() don't return the same value: indeed, I thought th= at > [2,3] was the default argument of the function f, thus I expected the thr= ee > calls of f() to be exactly equivalent. In a sense, there is. The default for the argument is simply an object like any other, and it's stored in one place. For cases where you want a mutable default that is "reset" every time, the most common idiom is this: def f(x=3DNone): if x is None: x=3D[2,3] x.append(1) return x That will create a new list every time, with the same initial contents. ChrisA