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; 'represents': 0.05; 'subject:Python': 0.06; 'absent': 0.09; 'assuming': 0.09; 'happen.': 0.09; 'imported': 0.09; 'subject:script': 0.09; 'suppress': 0.09; 'try:': 0.09; 'python': 0.11; 'be:': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'importerror': 0.16; 'module?': 0.16; 'nameerror:': 0.16; 'raised,': 0.16; 'semantics': 0.16; 'subject:top': 0.16; 'sys.exit(1)': 0.16; '(not': 0.18; 'successful,': 0.19; 'work,': 0.20; 'import': 0.22; 'to:name:python-list@python.org': 0.22; 'idea': 0.28; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; 'that.': 0.31; 'implicit': 0.31; 'intellectual': 0.31; 'raised': 0.31; 'open': 0.33; 'subject:the': 0.34; 'except': 0.35; 'something': 0.35; 'done.': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'there': 0.35; 'done': 0.36; 'possible': 0.36; 'should': 0.36; 'implement': 0.38; 'to:addr :python-list': 0.38; 'bad': 0.39; 'extremely': 0.39; 'itself': 0.39; 'to:addr:python.org': 0.39; 'skip:p 20': 0.39; 'skip:u 10': 0.60; 'exercise,': 0.84 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:date:message-id:subject:from:to:content-type; bh=6NDaiZLzrJ0dzvZUIGxEMLhdWfUn6BgnLl4zvyLFkKY=; b=lPk/ho1rShyilSOx16R1Sl3eBBrfyjhWBOMN7tLU9+oOkUyn2BnV3L5Dr35M76eGby MpByAzQTi3OsdqcZ68hbYsGcTjUlZZ6F/+fKgPxHTqTfurSBSSIeprOHPhGHEJr4/Tba o922oBEljT86AlFYvIOL4PlZAIjct/AGnVj1rOQi0NHacXk/h3WmspwVOlvulKf6L4mE hGXyiu7HIRoOK2QNVQEjzaOilqd9s24Bz9zPTfu42ckOaB9P+y6twYgyjK0jUyfy3Bz5 pQd3txKKkFf/3LYE1eiNNSDgVE8TLV6F1gP+GAOzx5pk+PWweFNRpf1kkNrJFPAZSrGy yMEw== MIME-Version: 1.0 X-Received: by 10.107.128.106 with SMTP id b103mr14023408iod.42.1415575881498; Sun, 09 Nov 2014 15:31:21 -0800 (PST) Date: Mon, 10 Nov 2014 10:31:21 +1100 Subject: __missing__ for the top-level Python script From: Chris Angelico To: "python-list@python.org" Content-Type: text/plain; charset=UTF-8 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: 31 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1415575883 news.xs4all.nl 2930 [2001:888:2000:d::a6]:34275 X-Complaints-To: abuse@xs4all.nl Path: csiph.com!usenet.pasdenom.info!bete-des-vosges.org!feed.ac-versailles.fr!nerim.net!novso.com!newsfeed.xs4all.nl!newsfeed1.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Xref: csiph.com comp.lang.python:80447 Let's have some fun nutting out possible implementations for a bad idea :) If you want a dictionary that prepopulates itself on demand, you implement __missing__. Is there a way to implement the same thing for the __main__ module? Since it isn't imported (as such), I don't think "switch out what's in sys.modules" will work, though I'm open to correction on that. Desired result: Implicit imports. ## test.py print("Path separator is",os.sep) try: print("Version", psycopg2.__version__, "of psycopg2 is installed.") except NameError: print("You don't have psycopg2 installed.") sys.exit(1) So the semantics should be: If NameError would be raised (not including UnboundLocalError, which still represents an error), attempt to import the absent name. If successful, continue as if it had already been done. If ImportError is raised, suppress it and let the original NameError happen. Yes, this is extremely unPythonic. But just as an intellectual exercise, can this be done? I'm assuming Python 3 here, so if there's something neat that can be done with import hooks, go for it. ChrisA