Re: returning True, False or None
nghoffma wrote: sorry, that should have been: pyimport sets pydef doit(thelist): ... s = sets.Set(thelist) ... if s == sets.Set([None]): ... return None ... else: ... return max(s - sets.Set([None])) Since a function that doesn't return is equivalent to one that returns None, you can write it as: def doit(lst): ... s = set(lst) - set([None]) ... if s: return max(s) that looks to me as the most elegant so far, but this is just because it's mine :-) You can also filter out Nones with a list/generator comprehension, but sets are just more elegant... -- Ciao, Matteo -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Matteo Dell'Amico wrote: Since a function that doesn't return is equivalent to one that returns None, you can write it as: def doit(lst): ... s = set(lst) - set([None]) ... if s: return max(s) that looks to me as the most elegant so far, but this is just because it's mine :-) Cool. I prefer to be explicit about returns (e.g. not counting on the automatic return None), and I'd rather not create the unnecessary None set, so I would probably write this like: py def f(lst): ... s = set(lst) ... s.discard(None) ... if s: ... return max(s) ... else: ... return None ... But it's definitely a very elegant solution. Thanks! Steve -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
I think it is evil to do something at your own risk ; I will certainly not embark some roller coaster at my own risk. I also think it is evil to scan the whole list (as max ought to do) when only scanning the first few elements would suffice most of the time. Regards Francis Girard Le vendredi 4 Février 2005 21:13, Steven Bethard a écrit : Fredrik Lundh wrote: Steven Bethard wrote: Raymond Hettinger wrote: return max(lst) Very clever! Thanks! too clever. boolean None isn't guaranteed by the language specification: Yup. I thought about mentioning that for anyone who wasn't involved in the previous thread discussing this behavior, but I was too lazy. ;) Thanks for pointing it out again. This implementation detail was added in Python 2.1a1, with the following note[1]: The outcome of comparing non-numeric objects of different types is not defined by the language, other than that it's arbitrary but consistent (see the Reference Manual). An implementation detail changed in 2.1a1 such that None now compares less than any other object. Code relying on this new behavior (like code that relied on the previous behavior) does so at its own risk. Steve [1] http://www.python.org/2.1/NEWS.txt -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
On Fri, 04 Feb 2005 13:04:16 -0500, rumours say that Steve Holden [EMAIL PROTECTED] might have written: [STeVe] For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. [Stevbe] If you wanted to get clever you could write something like for i in True, False: if i in lst: return i return False [!Steve] You mistyped None as False in the last line. Your typos are getting worse every day :) -- TZOTZIOY, I speak England very best. Be strict when sending and tolerant when receiving. (from RFC1958) I really should keep that in mind when talking with people, actually... -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Brian van den Broek [EMAIL PROTECTED] wrote: ... * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. ... for val in (x for x in lst if x is not None): return val return None ... These don't do what the OP desired. Ah, you're right, True should take precedence, point 2 of the specs. OK, let's take advantage of the fact that None False True: return max(lst) This fails when lst is empty (the specs presumably imply a None should be returned then). More robust: return max(lst or [None]) Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steve Holden wrote: Christos TZOTZIOY Georgiou wrote: On Fri, 04 Feb 2005 13:04:16 -0500, rumours say that Steve Holden [EMAIL PROTECTED] might have written: [STeVe] For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. [Stevbe] If you wanted to get clever you could write something like for i in True, False: if i in lst: return i return False [!Steve] You mistyped None as False in the last line. Your typos are getting worse every day :) That wasn't a *type*, it was a *thinko* regards Steve Sheesh, now I even make typos typing about typos ... giving-up-on-the-spill-chocker-ly y'rs - steve -- Meet the Python developers and your c.l.py favorites March 23-25 Come to PyCon DC 2005 http://www.pycon.org/ Steve Holden http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard [EMAIL PROTECTED] wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? What about...: for val in lst: if val is not None: return val return None or the somewhat fancy/clever: for val in (x for x in lst if x is not None): return val return None Alex -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. . . . Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? return max(lst) Raymond Hettinger -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
On Fri, 04 Feb 2005 10:48:44 -0700, Steven Bethard wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None Yes, I see the smell, you are searching the list multiple times. You could bail out when you can: seenFalse = False for item in list: if item: return True if item is False: seenFalse = True if seenFalse: return False return None But I'd submit that if four item lists are your common case, that your original code is significantly easier to understand what it is doing. This can be alleviated with an appropriate comment on the chunk of code I gave you, though. -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? STeVe That code looks like a pretty solid implementation of the spec to me. There isn't a strict need for the last else, of course, which may be the smell you detect. If you wanted to get clever you could write something like for i in True, False: if i in lst: return i return False but frankly I think that's more obscure, and saves you pretty much nothing. regards Steve -- Meet the Python developers and your c.l.py favorites March 23-25 Come to PyCon DC 2005 http://www.pycon.org/ Steve Holden http://www.holdenweb.com/ -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? STeVe max(lst) ;-) Michael -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
This has a light code smell for me though -- can anyone see a simpler way of writing this? How's about: def ntf(x, y): if x is None and y is None: return None if x == True or y == True: return True return False # Then... reduce(ntf, list of stuff) You might need to make sure that you initialize your reduce call the right way with the optional third argument. HTH -- Stephen W. Juranich Science Applications Intl. Corp. (SAIC) Tucson, AZ -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Raymond Hettinger wrote: Steven Bethard For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. . . . Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? return max(lst) Very clever! Thanks! Steve -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Try: max(lst) Mick. -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Alex Martelli said unto the world upon 2005-02-04 13:02: Steven Bethard [EMAIL PROTECTED] wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: SNIP OP's code This has a light code smell for me though -- can anyone see a simpler way of writing this? What about...: for val in lst: if val is not None: return val return None or the somewhat fancy/clever: for val in (x for x in lst if x is not None): return val return None Alex These don't do what the OP desired. . test_case = [False, True, True, True ] . def alexs_funct(lst): .for val in lst: .if val is not None: .return val .return None alexs_funct(test_case) False But, by the 'spec', it ought return True. Best, Brian vdB A mere newbie, quite pleased with himself for finding a problem with 'bot code -- next scheduled to occur mid 2011 :-) -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard wrote: return max(lst) Very clever! Thanks! too clever. boolean None isn't guaranteed by the language specification: http://docs.python.org/ref/comparisons.html ... objects of different types always compare unequal, and are ordered consistently but arbitrarily. /.../ In the future, the comparison rules for objects of different types are likely to change. ... /F -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard wrote: I have lists containing values that are all either True, False or None, e.g.: [True, None, None, False] [None, False, False, None ] [False, True, True, True ] etc. For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. Right now, my code looks like: if True in lst: return True elif False in lst: return False else: return None This has a light code smell for me though -- can anyone see a simpler way of writing this? An attempt to short-circuit if possible: def tristate(iterable): it = iter(iterable) for item in it: if item is not None: return item or True in it Not as elegant as max(), but makes me wonder whether a max() that accepts an additional upper_bound argument and returns upper_bound as soon as it encounters a value = upper_bound would be worth the effort. Peter -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fredrik Lundh wrote: Steven Bethard wrote: Raymond Hettinger wrote: return max(lst) Very clever! Thanks! too clever. boolean None isn't guaranteed by the language specification: Yup. I thought about mentioning that for anyone who wasn't involved in the previous thread discussing this behavior, but I was too lazy. ;) Thanks for pointing it out again. This implementation detail was added in Python 2.1a1, with the following note[1]: The outcome of comparing non-numeric objects of different types is not defined by the language, other than that it's arbitrary but consistent (see the Reference Manual). An implementation detail changed in 2.1a1 such that None now compares less than any other object. Code relying on this new behavior (like code that relied on the previous behavior) does so at its own risk. Steve [1] http://www.python.org/2.1/NEWS.txt -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Steven Bethard For a given list: * If all values are None, the function should return None. * If at least one value is True, the function should return True. * Otherwise, the function should return False. One more approach, just for grins: s = set(lst) return True in s or s == set([None]) and None Raymond Hettinger -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
I'm seeing a consistent problem in most of these approaches. Verbalized, the logic of the OP's original code reads as such: If True is in the list *at all*, return True. Otherwise, if False is in the list *at all*, return False. Otherwise, return None. So if we used Alex Martelli's code: for val in lst: if val is not None: return val return None and the list was: [ False , False , True , None ] False would be returned upon inspection of the first index, even though True was in fact in the list. The same is true of the code of Jeremy Bowers, Steve Juranich, and Jeff Shannon. As for Raymond Hettinger, I can't even be sure ;) The original OP's code, on the other hand, inadvertently searches through the list twice where once would have sufficed, causing a needless performance pitfall. The following applies the OP's initial logic while only iterating once: def boolhunt( items ): falseExists = False for item in items: if item is True: return True elif item is False and not falseExists: falseExists = True if falseExists: return False l1 = [ True , None , None , False ] l2 = [ None , False , False , None ] l3 = [ False , True , True , True ] boolhunt( l1 ) True boolhunt( l2 ) False boolhunt( l3 ) True It isn't elegant or clever, but it gets the job done :) -- Daniel Bickett dbickett at gmail.com http://heureusement.org/ -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
On Fri, 04 Feb 2005 16:44:48 -0500, Daniel Bickett wrote: [ False , False , True , None ] False would be returned upon inspection of the first index, even though True was in fact in the list. The same is true of the code of Jeremy Bowers, Steve Juranich, and Jeff Shannon. As for Raymond Hettinger, I can't even be sure ;) Nope. To recall, my code was: seenFalse = False for item in list: if item: return True if item is False: seenFalse = True if seenFalse: return False return None So, turning that into a real function and not a sketch: Python 2.3.4 (#1, Jan 25 2005, 21:29:33) [GCC 3.4.3 (Gentoo Linux 3.4.3, ssp-3.4.3-0, pie-8.7.6.6)] on linux2 Type help, copyright, credits or license for more information. def thingy(l): ... seenFalse = False ... for item in l: ... if item: return True ... if item is False: seenFalse = True ... if seenFalse: ... return False ... return None ... thingy([ False , False , True , None ]) True The defense rests, your honor. :-) (I like the later use of returnValue and the reduce solution was cute and quite educational (very appropriate here). I deliberately eschewed fanciness, though.) -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Jeremy Bowers wrote: The defense rests, your honor. :-) I stand corrected :-) My apologies. -- Daniel Bickett dbickett at gmail.com http://heureusement.org/ -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fredrik Lundh wrote: Steven Bethard wrote: Raymond Hettinger wrote: return max(lst) Very clever! Thanks! too clever. boolean None isn't guaranteed by the language specification: http://docs.python.org/ref/comparisons.html ... objects of different types always compare unequal, and are ordered consistently but arbitrarily. /.../ In the future, the comparison rules for objects of different types are likely to change. ... Then maybe like this: def max_of_two_with_None_less_than_any_other_object(e1, e2): ... if e1 == None: ... return e2 ... elif e2 == None: ... return e1 ... else: ... return max(e1, e2) reduce(max_of_two_with_None_less_than_any_other_object, lst) Mick. -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Jeremy Bowers wrote: On Fri, 04 Feb 2005 16:44:48 -0500, Daniel Bickett wrote: [ False , False , True , None ] False would be returned upon inspection of the first index, even though True was in fact in the list. The same is true of the code of Jeremy Bowers, Steve Juranich, and Jeff Shannon. As for Raymond Hettinger, I can't even be sure ;) Nope. Indeed. Similarly for mine, which was really just a slight transform of Jeremy's (setting a return variable directly, instead of setting a flag that's later used to decide what to return): def tfn(lst): ... answer = None ... for item in lst: ... if item is True: return True ... if item is False: answer = False ... return answer ... list = [False, False, True, None] tfn(list) 1 list = [None, False, False, None] tfn(list) 0 list = [None, None, None, None] print tfn(list) None The noted logical flaw *has* been present in a number of proposed solutions, however. The key point to note is that one *must* examine the entire list *unless* you find a True; short-circuiting on False means that you may miss a later True. Jeff Shannon Technician/Programmer Credit International -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Daniel Bickett wrote: def boolhunt( items ): ... falseExists = False ... for item in items: ... if item is True: ... return True ... elif item is False and not falseExists: ... falseExists = True ... if falseExists: ... return False Or even shorter: def boolhunt(items): ...result = None ...for item in items: ...if item: ...return True ...elif result is None and item is False: ...result = False ...return result Or like the Melmacians would do it: def boolgen(items): ... result = None ... for item in items: ... if result: ... raise StopIteration ... elif item is not None: ... result = item ... yield result [item for item in boolgen(a_list)][-1] Mick. -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Is it cheating to use a Set? pydef doit(thelist): ... s = sets.Set(thelist) ... if s == sets.Set([None]): ... return None ... else: ... return max(s) ... pyprint doit([ True , None , None , False ] ) True pyprint doit([ None , False , False , None ] ) False pyprint doit([ False , True , True , True ] ) True pyprint doit( [None, None, None, None] ) None -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
reduce(lambda x, y: x or y, lst) works but when I tried import operator reduce(operator.or_, lst) this did not work. It pukes Traceback (most recent call last): File interactive input, line 1, in ? TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' Any comments? Fahri -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) works but when I tried import operator reduce(operator.or_, lst) this did not work. It pukes Traceback (most recent call last): File interactive input, line 1, in ? TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' Any comments? Fahri TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' operator.or_ is | i.e., bitwise, not logical or Michael -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Michael Spencer [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) works but when I tried import operator reduce(operator.or_, lst) this did not work. It pukes Traceback (most recent call last): File interactive input, line 1, in ? TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' Any comments? Fahri TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' operator.or_ is | i.e., bitwise, not logical or Michael That explains it. Is there a logical or we can use with reduce? Fahri -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fahri Basegmez wrote: Michael Spencer [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) works but when I tried import operator reduce(operator.or_, lst) this did not work. It pukes Traceback (most recent call last): File interactive input, line 1, in ? TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' Any comments? Fahri TypeError: unsupported operand type(s) for |: 'NoneType' and 'bool' operator.or_ is | i.e., bitwise, not logical or Michael That explains it. Is there a logical or we can use with reduce? Fahri Yes, but it's not quite the same as the 'or' operator bool.__or__(True, False) True bool.__or__(False, False) False bool.__or__(False, None) NotImplemented this may not be intentional... Michael -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) This doesn't solve the OPs problem since reduce(lambda x, y: x or y, [False, None]) returns None instead of False. Mick. -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Mick Krippendorf [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) This doesn't solve the OPs problem since reduce(lambda x, y: x or y, [False, None]) returns None instead of False. Mick. You are right. I tested None or False and it worked. I assumed order did not matter for or operator. None or False returns False False or None returns None You know what they say about assumptions. Live and learn. Fahri -- http://mail.python.org/mailman/listinfo/python-list
Re: returning True, False or None
Fahri Basegmez said unto the world upon 2005-02-04 23:14: Mick Krippendorf [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED] Fahri Basegmez wrote: reduce(lambda x, y: x or y, lst) This doesn't solve the OPs problem since reduce(lambda x, y: x or y, [False, None]) returns None instead of False. Mick. You are right. I tested None or False and it worked. I assumed order did not matter for or operator. None or False returns False False or None returns None You know what they say about assumptions. Live and learn. Fahri Hi Fahri, I don't have a reference at hand, but you might want to check the docs' index or do a google for short circuit python or something similar. or works by evaluating the first value and returning it if it evaluates to True. Otherwise it returns the second. 0 or 42 42 Likewsie, and returns the first if it evaluates to False, otherwise it returns the second. [] and 42 [] The idea is that the evaluation breaks out as soon as it has seen enough to determine the result. Hence, short circuit. And, instead of returning a Boolean, it returns the actual object flanking the operator. Hence, the behaviour observed. HTH, Brian vdB -- http://mail.python.org/mailman/listinfo/python-list
