Re: Style question (Poll)
On Wednesday, 14 March 2012 21:16:05 UTC, Terry Reedy wrote: On 3/14/2012 4:49 PM, Arnaud Delobelle wrote: On 14 March 2012 20:37, Croephacroe...@gmail.com wrote: Which is preferred: for value in list: if not value is another_value: value.do_something() break Do you really mean 'is' or '=='? If you mean x is not y, write it that way. 'not x is y' can be misread and misunderstood, depending on whether the 'is' is true or not. not 1 is 1 False not (1 is 1) False (not 1) is 1 False Does not matter how read. not (1 is 0) True (not 1) is 0 False not 1 is 0 True Does matter how read. if list and not list[0] is another_value: list[0].do_something() Or try: value = mylist[0] if value is not another_value: value.dosomething except IndexError: pass I would not do this in this case of index 0, but if the index were a complicated expression or expensive function call, making 'if list' an inadequate test, I might. Hard to say, since they don't do the same thing :) I suspect you meant: for value in list: if not value is another_value: value.do_something() break I always feel uncomfortable with this because it's misleading: a loop that never loops. I agree. Please do not do this in public ;-). -- Terry Jan Reedy I'm not sure it's efficient or even if I like it, but it avoids try/except and the use of a for loop. if next( iter(mylist), object() ) is not another_value: # ... Just my 2p, Jon. -- http://mail.python.org/mailman/listinfo/python-list
Style question (Poll)
Which is preferred: for value in list: if not value is another_value: value.do_something() break --or-- if list and not list[0] is another_value: list[0].do_something() Comments are welcome, Thanks -- http://mail.python.org/mailman/listinfo/python-list
Re: Style question (Poll)
On 14 March 2012 20:37, Croepha croe...@gmail.com wrote: Which is preferred: for value in list: if not value is another_value: value.do_something() break --or-- if list and not list[0] is another_value: list[0].do_something() Hard to say, since they don't do the same thing :) I suspect you meant: for value in list: if not value is another_value: value.do_something() break I always feel uncomfortable with this because it's misleading: a loop that never loops. -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
Re: Style question (Poll)
On 3/14/2012 4:49 PM, Arnaud Delobelle wrote: On 14 March 2012 20:37, Croephacroe...@gmail.com wrote: Which is preferred: for value in list: if not value is another_value: value.do_something() break Do you really mean 'is' or '=='? If you mean x is not y, write it that way. 'not x is y' can be misread and misunderstood, depending on whether the 'is' is true or not. not 1 is 1 False not (1 is 1) False (not 1) is 1 False Does not matter how read. not (1 is 0) True (not 1) is 0 False not 1 is 0 True Does matter how read. if list and not list[0] is another_value: list[0].do_something() Or try: value = mylist[0] if value is not another_value: value.dosomething except IndexError: pass I would not do this in this case of index 0, but if the index were a complicated expression or expensive function call, making 'if list' an inadequate test, I might. Hard to say, since they don't do the same thing :) I suspect you meant: for value in list: if not value is another_value: value.do_something() break I always feel uncomfortable with this because it's misleading: a loop that never loops. I agree. Please do not do this in public ;-). -- Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
RE: Style question (Poll)
Which is preferred: for value in list: if not value is another_value: value.do_something() break Do you really mean 'is' or '=='? Let me expound on how 'is' and '==' are very different. It may work for some comparisons but often not for others. Certain examples work because of the Python implementation. c = 1 d = 1 c is d # This only works because CPython caches small values. True c == d True a = 10 b = 10 a is b False a == b True 10 is 10 True '10 is 10' works because the interpreter caches the number because it is on the same line. Only use 'is' if you are looking for objects like True, False, None or something that MUST be exactly the same object. In general, use '=='. Ramit Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology 712 Main Street | Houston, TX 77002 work phone: 713 - 216 - 5423 -- This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. -- http://mail.python.org/mailman/listinfo/python-list
Re: Style question (Poll)
On 14 March 2012 22:15, Prasad, Ramit ramit.pra...@jpmorgan.com wrote: Only use 'is' if you are looking for objects like True, False, None or something that MUST be exactly the same object. I've rarely seen valid uses of 'is True' or 'is False'. -- Arnaud -- http://mail.python.org/mailman/listinfo/python-list
RE: Style question (Poll)
Only use 'is' if you are looking for objects like True, False, None or something that MUST be exactly the same object. I've rarely seen valid uses of 'is True' or 'is False'. It can be useful when you think something might be None or False. Although, I suppose you could always just use 'is None' instead. 1 == True True 1 is True False 0 == False True 0 is False False Granted, the above example is a pretty facetious case; not sure I can come up with a reasonably real world use case. Ramit Ramit Prasad | JPMorgan Chase Investment Bank | Currencies Technology 712 Main Street | Houston, TX 77002 work phone: 713 - 216 - 5423 -- This email is confidential and subject to important disclaimers and conditions including on offers for the purchase or sale of securities, accuracy and completeness of information, viruses, confidentiality, legal privilege, and legal entity disclaimers, available at http://www.jpmorgan.com/pages/disclosures/email. -- http://mail.python.org/mailman/listinfo/python-list
Re: Style question (Poll)
On Thu, Mar 15, 2012 at 7:37 AM, Croepha croe...@gmail.com wrote: Which is preferred: for value in list: if not value is another_value: value.do_something() break --or-- if list and not list[0] is another_value: list[0].do_something() Comments are welcome, Thanks General principle: Make the code look like what it's doing. I don't mean text art and making code in the shape of pi that prints the digits of pi (although that can be awesome too), but more that a loop should not be used when you don't intend for it to loop. Consider the For-Case Paradigm[1] and the amazing confusion value that it can offer. A loop needn't execute more than once (it needn't even execute the first time), but it should at least have the _potential_ to execute the same code multiple times, otherwise it's hardly a loop. I had a particularly nasty example of a loop-that-wasn't-a-loop at work a while ago; it was PHP, not Python, so I won't share it here, but it had a do-while loop and an insidious bug in it. Very tricky. ChrisA [1] http://thedailywtf.com/Articles/The_FOR-CASE_paradigm.aspx -- http://mail.python.org/mailman/listinfo/python-list
