Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!feeds.phibee-telecom.net!newsfeed.xs4all.nl!newsfeed4a.news.xs4all.nl!xs4all!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 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'example:': 0.03; 'explicitly': 0.05; 'assign': 0.07; 'assignment': 0.07; 'compiler': 0.07; 'modify': 0.07; 'variables': 0.07; 'function,': 0.09; 'referenced': 0.09; 'variable,': 0.09; 'def': 0.12; "'b'": 0.16; '1:09': 0.16; 'assignment.': 0.16; 'dictionary,': 0.16; 'modified.': 0.16; 'url:faq': 0.16; 'modification': 0.16; 'wrote:': 0.18; 'variable': 0.18; 'all,': 0.19; 'thu,': 0.19; '>>>': 0.22; 'email addr:gmail.com>': 0.22; '>>>': 0.24; 'question': 0.24; '>': 0.26; 'second': 0.26; 'header:In- Reply-To:1': 0.27; 'function': 0.29; 'dec': 0.30; 'message- id:@mail.gmail.com': 0.30; 'default,': 0.31; 'subject:About': 0.31; 'url:python': 0.33; 'checking': 0.33; "can't": 0.35; 'knows': 0.35; 'case,': 0.35; 'received:google.com': 0.35; 'functions.': 0.36; 'url:org': 0.36; 'to:addr:python-list': 0.38; 'pm,': 0.38; 'to:addr:python.org': 0.39; 'skip:u 10': 0.60; 'skip:\xc2 10': 0.60; 'url:3': 0.61; 'first': 0.61; 'within': 0.65; 'toy': 0.84 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :content-type; bh=6QmMIWnPKA0IWm8QBXJGQk6ZxnLmsG8IPJYqN3b8aYk=; b=uLyH7qhPVsPcLSqsVCFRQ8w4ahiU858JVvMCz7MpQzV9yfQVs/wTXXzoPklvoFPAn/ 9wIRBYoaPmHU0rHnDNTRtPf/G2WrsHIJdniMnFyWzTLaky7I+ZGPRsmMigwIgmhngHFS XcQk6WktW2gZASmh6WOOGMHNlyZep3Rj4JNyj+l07rXgx/SZEINeS3CmHWv/v4kGIT5t lpuBNpUO5SJZPhPGKWCSF0+cAuvROUdttQU5qlAfT8UskxSinsgGGIjr4IDfNkS7E7By gSM9Jr9aD1fApbqjrDK+y8Wh9XbOO3PzD2YlL7B0f6jCHgrCmAJZAfeLq3r7ajDhohy9 32lQ== X-Received: by 10.68.107.162 with SMTP id hd2mr28983807pbb.145.1417724540292; Thu, 04 Dec 2014 12:22:20 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: From: Ian Kelly Date: Thu, 4 Dec 2014 13:21:39 -0700 Subject: Re: About Modifying Globals To: Python Content-Type: multipart/alternative; boundary=047d7b6da6d49d1311050969b7c5 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: 72 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1417724544 news.xs4all.nl 2904 [2001:888:2000:d::a6]:38491 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:81944 --047d7b6da6d49d1311050969b7c5 Content-Type: text/plain; charset=UTF-8 On Thu, Dec 4, 2014 at 1:09 PM, LJ wrote: > > Hi All, > > I have a quick question regarding the modification of global variables within functions. To illustrate, consider the following toy example: > > a={"1": set()} > b=9 > > def gt(l): > a["1"] = a["1"] | set([l]) > > When calling this last function and checking the a dictionary, I get: > > >>> gt(5) > >>> a > {"1": set([5])} > > > The set in the dictionary was modified. The question is, why isn't it necessary to declare a as global within the gt function, as apposed to a case like > > def gt2(l): > b=b+l > > where I need to declare b as global within the function to avoid: > > UnboundLocalError: local variable 'b' referenced before assignment. https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python In the first case, you never assign to a; you only modify it. Since it's never assigned locally the compiler knows it can't be a local variable, so it automatically makes it a global. In the second case, the assignment to b causes the compiler to treat the variable as local by default, so it needs to be explicitly marked as global. --047d7b6da6d49d1311050969b7c5 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: quoted-printable
On Thu, Dec 4, 2014 at 1:09 PM, LJ <luisjosenovoa@gmail.com> w= rote:
>
> Hi All,
>
> I have a quick question regar= ding the modification of global variables within functions. To illustrate, = consider the following toy example:
>
> a=3D{"1": set= ()}
> b=3D9
>
> def gt(l):
> =C2=A0 =C2=A0a["1= "] =3D a["1"] | set([l])
>
> When calling this l= ast function and checking the a dictionary, I get:
>
> >>= > gt(5)
> >>> a
> {"1": set([5])}
>=
>
> The set in the dictionary was modified. The question is, w= hy isn't it necessary to declare a as global within the gt function, as= apposed to a case like
>
> def gt2(l):
> =C2=A0 =C2=A0b= =3Db+l
>
> where I need to declare b as global within the funct= ion to avoid:
>
> UnboundLocalError: local variable 'b'= referenced before assignment.

https://docs.python.org/3/faq/programming.html#what-are-the-rules-fo= r-local-and-global-variables-in-python

In the first case, y= ou never assign to a; you only modify it. Since it's never assigned loc= ally the compiler knows it can't be a local variable, so it automatical= ly makes it a global.

In the second case, the assi= gnment to b causes the compiler to treat the variable as local by default, = so it needs to be explicitly marked as global.
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