Re: checking if two things do not equal None
contact.tri...@gmail.com wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? I couldn't see anyone else give this, but I like if None not in (a, b): pass Jeremy -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
I couldn't see anyone else give this, but I like if None not in (a, b): I did. I am now considering: if None not in (a,b): or if (a is not None) and (b is not None): However, I decided to just turn the two parameters into one (sequence), since they were logically grouped anyhow. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 31/03/14 19:28, Abe wrote: I couldn't see anyone else give this, but I like if None not in (a, b): I did. I am now considering: if None not in (a,b): or if (a is not None) and (b is not None): That's just if not (a is None or b is None): but you seem to have found your way. However, I decided to just turn the two parameters into one (sequence), since they were logically grouped anyhow. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sat, 29 Mar 2014 19:54:09 -0700, Rustom Mody wrote: On Sunday, March 30, 2014 8:09:45 AM UTC+5:30, Roy Smith wrote: I have no particular problem with x 2 y because it fits the same pattern. But, if you show me a != None != b: my brain just goes into overload. Honestly, I don't even know what that means. My brain keeps trying to stick a, None, and b on Mrs. Albaum's number line and keeps walking into the wall. If you (the editorial you) tell me that my failure to grok that expression means I'm not fluent in Python, well then, guilty as charged. Math Terminology [...] So for != chained comparisons are not natural (or IMHO appropriate) I tend to agree they're not natural, although appropriate is another thing. The problem is that we tend to read something like: a != b != c as all of a, b and c are unequal, corresponding to: a == b == c as all of a, b and c are equal. But that's not what it means. It means that a != b and b != c, but it says nothing about a and c. And that was my mistake. The OP actually got it right in their first post, but sticking None in the middle to ensure it partakes of both comparisons. a is not None is not b Still, that's not easily extended to a third item, this would be wrong: a is not None is not b is not c since c only gets compared against b, not None. Better is to factor the not out: not (a is b is c is None) which now should be clear: you're testing whether or not *all* of a, b and c are None. If you prefer: not all(x is None for x in (a, b, c)) Which is more readable is a matter of personal preference. I think Johannes got it right: boolean logic is easier to reason about when there is a minimum of nots. -- Steven D'Aprano http://import-that.dreamwidth.org/ -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, Mar 30, 2014 at 4:54 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Sun, 30 Mar 2014 13:15:18 +1100, Chris Angelico wrote: Chained comparisons where you're checking a single variable against two constants make perfect sense: 2 x 5 Chained comparisons where you check a single constant against two variables don't, so much: x 2 y What exactly does that mean, and why is it written that way? It checks that 2 is strictly bounded between x on the left and y on the right, i.e. that 2 is inside the open interval x...y. I don't know why you think that's unclear. But then I do have a maths background and I'm used to chaining comparisons. Write it like this: low = x high = y a = 2 low a high Does that make more sense? Well-chosen names are good. The fact that a is a constant rather than a variable is no big deal: low 2 high The problem isn't that I can't see what the comparisons are. It makes very good sense to bound a variable within constants; but you already know exactly where 2 is on the number line, so asking Is 2 between these two variables seems a bit odd. Maybe it's less so with the strong mathematical background, but it seems odd to me. It'd be more useful but less clear if one of the conditions points the other way: x 2 y which checks that they're both less than two, which is quite different from what you wrote the first time. but IMO in a less-than-clear way. That's an understatement. If I saw code chaining comparisons in that fashion, I would assume the second operator was a typo. Chaining less-than and greater than operators should, for clarity, always be written in a single order. E.g. a = b c d, not a = b d c. (The second contains a subtle bug too.) Agreed. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Roy Smith wrote: But, if you show me a != None != b: my brain just goes into overload. Chained comparisons get weird with not-equal operators. If you see a == b == c then it implies that a == c, but a != b != c does *not* imply that a != c. At least it doesn't in Python; I've never seen any mathematicians write that, so I don't know what they would make of it. -- Greg -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Chris Angelico ros...@gmail.com writes: The problem isn't that I can't see what the comparisons are. It makes very good sense to bound a variable within constants; but you already know exactly where 2 is on the number line, so asking Is 2 between these two variables seems a bit odd. Maybe it's less so with the strong mathematical background, but it seems odd to me. I don't feel odd about asking the question “Is 2 between these two values?”. It's straightforward and concise. Can you explain better why you find it odd? -- \ “You are welcome to visit the cemetery where famous Russian and | `\Soviet composers, artists, and writers are buried daily except | _o__) Thursday.” —Russian orthodox monastery, Moscow | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, Mar 30, 2014 at 5:52 PM, Ben Finney ben+pyt...@benfinney.id.au wrote: Chris Angelico ros...@gmail.com writes: The problem isn't that I can't see what the comparisons are. It makes very good sense to bound a variable within constants; but you already know exactly where 2 is on the number line, so asking Is 2 between these two variables seems a bit odd. Maybe it's less so with the strong mathematical background, but it seems odd to me. I don't feel odd about asking the question “Is 2 between these two values?”. It's straightforward and concise. Can you explain better why you find it odd? Possibly because the variable between two constants is something I've done often (usually in the more explicit form of x min x max in a language without chained comparisons), usually bounds-checking some value. I've never had to ask whether a single constant has two variables, one on either side. But that's just that I've personally never done it; it doesn't mean nobody does it, by any means. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Gregory Ewing greg.ew...@canterbury.ac.nz: a != b != c does *not* imply that a != c. At least it doesn't in Python; I've never seen any mathematicians write that, so I don't know what they would make of it. Any resemblance between mathematics notation and Python is purely coincidental. I must admit I had missed Python's chained comparisons until this discussion, but now I looked up the definition: comparison::= or_expr ( comp_operator or_expr )* comp_operator ::= | | == | = | = | != | is [not] | [not] in [...] Formally, if a, b, c, ..., y, z are expressions and op1, op2, ..., opN are comparison operators, then a op1 b op2 c ... y opN z is equivalent to a op1 b and b op2 c and ... y opN z, except that each expression is evaluated at most once. That means, in my opinion, that you should feel free to use chaining any way you see fit. Also, the rule is crystal-clear and easy to grasp: there's an implicit and there. It's another thing, then, if it was a good idea to include chaining there in the first place, but I trust the idea was properly vetted and double checked against possible parsing ambiguities. Even without chaining is not is a bit suspect: False is not 0 True False is (not 0) False False is not not not 0 File stdin, line 1 False is not not not 0 ^ SyntaxError: invalid syntax Marko -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
In article 5337b4e4$0$29994$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I think Johannes got it right: boolean logic is easier to reason about when there is a minimum of nots. I used to do a lot of digital logic design. In certain logic families, it's easier to build a NAND gate than an AND gate (and similarly, NOR is easier than OR). This leads to lots of inverted logic. Adding to the confusion, many designs would use active low logic, which means a 1 was represented by a low voltage, and a 0 by a high voltage. So, you quickly end up with gibberish like, not active low clear nand not active low enable clock. I'm glad I don't do that stuff any more. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 2014-03-30 13:21, Roy Smith wrote: In article 5337b4e4$0$29994$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: I think Johannes got it right: boolean logic is easier to reason about when there is a minimum of nots. I used to do a lot of digital logic design. In certain logic families, it's easier to build a NAND gate than an AND gate (and similarly, NOR is easier than OR). This leads to lots of inverted logic. Adding to the confusion, many designs would use active low logic, which means a 1 was represented by a low voltage, and a 0 by a high voltage. So, you quickly end up with gibberish like, not active low clear nand not active low enable clock. I'm glad I don't do that stuff any more. When you're building with discrete logic chips, NAND gates are useful because you can use them as inverters too, which helps to keep the chip count down. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Roy Smith wrote: Adding to the confusion, many designs would use active low logic, which means a 1 was represented by a low voltage, and a 0 by a high voltage. So, you quickly end up with gibberish like, not active low clear nand not active low enable clock. There are ways of dealing with that in schematic diagrams. For exammple, if you have two active-low signals A and B and want to express A is active or B is active, you draw an OR gate symbol with inversion circles on the inputs. That's equivalent to a NAND gate, but makes the intention clear. Schematics drawn that way are much easier to follow than ones that only use the inverted-output versions of the symbols. -- Greg -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sat, 29 Mar 2014 11:56:50 -0700, contact.trigon wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? Do you actually want to check for arbitrary objects which may claim to equal None, or do you want to check for objects which are None? Nearly always when people test for == to None, they don't really mean it. They actually want to use an identity test. I'm going to assume the same holds here. if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... -- Steven D'Aprano http://import-that.dreamwidth.org/ -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Steven D'Aprano steve+comp.lang.pyt...@pearwood.info writes: if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... 1 is not 1 is not None False So definitely the former! ciao, lele. -- nickname: Lele Gaifax | Quando vivrò di quello che ho pensato ieri real: Emanuele Gaifas | comincerò ad aver paura di chi mi copia. l...@metapensiero.it | -- Fortunato Depero, 1929. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Do you actually want to check for arbitrary objects which may claim to equal None, or do you want to check for objects which are None? Arbitrary objects are not a concern. if not (a is b is None): ... if a is not b is not None: ... Thanks for the examples. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 29.03.2014 20:05, Steven D'Aprano wrote: On Sat, 29 Mar 2014 11:56:50 -0700, contact.trigon wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... Is this an obfuscated coding contest? Why do you opt for a solution that one has to at least think 2 seconds about when the simplest solution: if (a is not None) or (b is not None): is immediately understandable by everyone? Cheers, Johannes -- Wo hattest Du das Beben nochmal GENAU vorhergesagt? Zumindest nicht öffentlich! Ah, der neueste und bis heute genialste Streich unsere großen Kosmologen: Die Geheim-Vorhersage. - Karl Kaos über Rüdiger Thomas in dsa hidbv3$om2$1...@speranza.aioe.org -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
In article lh7cb4$ntu$2...@news.albasani.net, Johannes Bauer dfnsonfsdu...@gmx.de wrote: On 29.03.2014 20:05, Steven D'Aprano wrote: On Sat, 29 Mar 2014 11:56:50 -0700, contact.trigon wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... Is this an obfuscated coding contest? Why do you opt for a solution that one has to at least think 2 seconds about when the simplest solution: if (a is not None) or (b is not None): is immediately understandable by everyone? I agree with that. But if (a, b) != (None, None): seems pretty straight-forward to me too. In fact, if anything, it seems easier to understand than if (a is not None) or (b is not None): I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Code gets read a lot more often than it get written. Make it dead-ass simple to understand, and future generations of programmers who inherit your code will thank you for it. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Roy Smith r...@panix.com Wrote in message: In article lh7cb4$ntu$2...@news.albasani.net, Johannes Bauer dfnsonfsdu...@gmx.de wrote: On 29.03.2014 20:05, Steven D'Aprano wrote: On Sat, 29 Mar 2014 11:56:50 -0700, contact.trigon wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... Is this an obfuscated coding contest? Why do you opt for a solution that one has to at least think 2 seconds about when the simplest solution: if (a is not None) or (b is not None): is immediately understandable by everyone? I agree with that. But if (a, b) != (None, None): seems pretty straight-forward to me too. In fact, if anything, it seems easier to understand than if (a is not None) or (b is not None): I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Code gets read a lot more often than it get written. Make it dead-ass simple to understand, and future generations of programmers who inherit your code will thank you for it. The other advantage to keeping it simple is it's more than likely to be right. If we take the original form as the spec, we'll find that two of the alternatives are not even equivalent. def trigon1(a, b): return (a,b) != (None, None) #master def trigon2(a, b): return a != None != b. #different def steven1(a, b): return not(a is b is None) def steven2(a, b): return a is not b is not None #different def johannes(a, b): return (a is not None) or (b is not None) table = [ trigon1, trigon2, steven1, steven2, johannes ] for func in table: print func.__name__ print func(None, None), func(None, 42), func(42, None), func(42, 42), func(42, never) -- DaveA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 29.03.2014 22:07, Roy Smith wrote: I agree with that. But if (a, b) != (None, None): seems pretty straight-forward to me too. In fact, if anything, it seems easier to understand than if (a is not None) or (b is not None): Yes, probably. I liked the original, too. If I were writing the code, I'd probably try to aim to invert the condition though and simply do if (a is None) and (b is None) Which is pretty easy to understand for even a rookie programmer. I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Code gets read a lot more often than it get written. Make it dead-ass simple to understand, and future generations of programmers who inherit your code will thank you for it. Absolutely. Cheers, Johannes -- Wo hattest Du das Beben nochmal GENAU vorhergesagt? Zumindest nicht öffentlich! Ah, der neueste und bis heute genialste Streich unsere großen Kosmologen: Die Geheim-Vorhersage. - Karl Kaos über Rüdiger Thomas in dsa hidbv3$om2$1...@speranza.aioe.org -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 29.03.2014 22:55, Johannes Bauer wrote: if (a is not None) or (b is not None): Yes, probably. I liked the original, too. If I were writing the code, I'd probably try to aim to invert the condition though and simply do if (a is None) and (b is None) Which is pretty easy to understand for even a rookie programmer. Let me expand on that thought one or two more sentences: Although it may seem really trivial, inversions (not) in my opinion can really make code unreadable. One thing that I regularly see when peer-reviewing code is something like: if not feature_disabled: or one that I've seen in-field (modulo the programming language and the variable names): if (not no_delayed_commit) and (not data_unchanged): instead of: if immediate_commit and data_changed: Enough of my two cents for today :-) Cheers, Johannes -- Wo hattest Du das Beben nochmal GENAU vorhergesagt? Zumindest nicht öffentlich! Ah, der neueste und bis heute genialste Streich unsere großen Kosmologen: Die Geheim-Vorhersage. - Karl Kaos über Rüdiger Thomas in dsa hidbv3$om2$1...@speranza.aioe.org -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 2014-03-29 17:07, Roy Smith wrote: if (a is not None) or (b is not None): is immediately understandable by everyone? I agree with that. But if (a, b) != (None, None): seems pretty straight-forward to me too. In fact, if anything, it seems easier to understand than And for cases where you have more than one or two things to test for None-itude, you could use if all(x is None for x in [a, b, c, d]): do_something_if_theyre_all_None() or if all(x is not None for x in [a, b, c, d]): do_something_if_no_Nones() or if not any(x is None for x in [a, b, c, d]): do_something_if_no_Nones() which I find *much* more readable from a maintenance point of view. -tkc -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Mar 29, 2014, at 6:36 PM, Tim Chase wrote: And for cases where you have more than one or two things to test for None-itude, you could use if all(x is None for x in [a, b, c, d]): do_something_if_theyre_all_None() I might have written that as: if set([a, b, c, d]) == set(None) That's even clearer if you happen to already have the items in an iterable: if set(conditions) == set(None) -- Roy Smith r...@panix.com -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 2014-03-29 18:41, Roy Smith wrote: On Mar 29, 2014, at 6:36 PM, Tim Chase wrote: And for cases where you have more than one or two things to test for None-itude, you could use if all(x is None for x in [a, b, c, d]): do_something_if_theyre_all_None() I might have written that as: if set([a, b, c, d]) == set(None) That's even clearer if you happen to already have the items in an iterable: if set(conditions) == set(None) Though am I correct that your iteration tests for equality, while mine tests for identity? Also, my version bails early in the event quitting early is possible. That's particularly useful in the case of doing something like if all(x() is None for x in [func1, func2, func3, costly_func]): do_something() -tkc -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
In article mailman.8703.1396133206.18130.python-l...@python.org, Tim Chase t...@thechases.com wrote: On 2014-03-29 18:41, Roy Smith wrote: On Mar 29, 2014, at 6:36 PM, Tim Chase wrote: And for cases where you have more than one or two things to test for None-itude, you could use if all(x is None for x in [a, b, c, d]): do_something_if_theyre_all_None() I might have written that as: if set([a, b, c, d]) == set(None) That's even clearer if you happen to already have the items in an iterable: if set(conditions) == set(None) Though am I correct that your iteration tests for equality, while mine tests for identity? Hmmm, you're almost certainly correct on that, but you would have to have a perversely designed class for that to make a difference. I'll take the increased readability. Also, my version bails early in the event quitting early is possible. That's particularly useful in the case of doing something like if all(x() is None for x in [func1, func2, func3, costly_func]): do_something() Again, you're correct. But, I'll take the increased readability over the premature optimization :-) -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 3/29/2014 2:56 PM, contact.tri...@gmail.com wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? if a is not None is not b == if a is not None and None is not b == if a is not None and b is not None which is what I would write if not trying to be cute. a x b is more readable as a chained comparison than the double is not. -- Terry Jan Reedy -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, Mar 30, 2014 at 9:46 AM, Tim Chase t...@thechases.com wrote: Though am I correct that your iteration tests for equality, while mine tests for identity? Also, my version bails early in the event quitting early is possible. That's particularly useful in the case of doing something like if all(x() is None for x in [func1, func2, func3, costly_func]): do_something() Presumably you mean to actually call those functions, as checking the identity of a costly function is still cheap :) ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
Thanks everyone; it has been very educational. Dave Angel: ...we'll find that two of the alternatives are not even equivalent. That helped me realize (a,b) != (None, None) is not correct for the function. It's a case where two parameters have None as the default argument. What I want is to make sure that both are not None. I am now considering: if None not in (a,b): or if (a is not None) and (b is not None): -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 03/29/2014 02:01 PM, Johannes Bauer wrote: On 29.03.2014 20:05, Steven D'Aprano wrote: On Sat, 29 Mar 2014 11:56:50 -0700, contact.trigon wrote: if (a, b) != (None, None): or if a != None != b: Preference? Pros? Cons? Alternatives? if not (a is b is None): ... Or if you prefer: if a is not b is not None: ... Is this an obfuscated coding contest? Why do you opt for a solution that one has to at least think 2 seconds about when the simplest solution: if (a is not None) or (b is not None): is immediately understandable by everyone? +1 -- ~Ethan~ -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On 2014-03-30 10:17, Chris Angelico wrote: On Sun, Mar 30, 2014 at 9:46 AM, Tim Chase t...@thechases.com wrote: Though am I correct that your iteration tests for equality, while mine tests for identity? Also, my version bails early in the event quitting early is possible. That's particularly useful in the case of doing something like if all(x() is None for x in [func1, func2, func3, costly_func]): ^^^ do_something() Presumably you mean to actually call those functions, as checking the identity of a costly function is still cheap :) Which is what I do...calling only those necessary until the all/any condition has been met. :-) If you create the list of things to iterate over by calling them as you create the list, then you don't save much of anything. If you only call until one of them breaks the any/all construct, you save all the subsequent function calls. -tkc -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, Mar 30, 2014 at 12:19 PM, Tim Chase python.l...@tim.thechases.com wrote: On 2014-03-30 10:17, Chris Angelico wrote: On Sun, Mar 30, 2014 at 9:46 AM, Tim Chase t...@thechases.com wrote: Though am I correct that your iteration tests for equality, while mine tests for identity? Also, my version bails early in the event quitting early is possible. That's particularly useful in the case of doing something like if all(x() is None for x in [func1, func2, func3, costly_func]): ^^^ do_something() Presumably you mean to actually call those functions, as checking the identity of a costly function is still cheap :) Which is what I do...calling only those necessary until the all/any condition has been met. :-) If you create the list of things to iterate over by calling them as you create the list, then you don't save much of anything. If you only call until one of them breaks the any/all construct, you save all the subsequent function calls. *facepalm* Yep, you do indeed. My bad! Take no notice of the man behind the curtain... ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sat, 29 Mar 2014 17:07:20 -0400, Roy Smith wrote: I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Apart from the fact that I got it wrong (that's what happens when I post at 6am after being up all night, thanks for the correction Lele), if you consider chained comparisons to be obfuscated, I think you're not really fluent at Python. The OP even suggested `a != None != b` so I think that (s)he at least understands chained comparisons. However, I agree with Johannes that inverted conditions (using not) are sometimes harder to reason about than regular conditions. -- Steven D'Aprano http://import-that.dreamwidth.org/ -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, Mar 30, 2014 at 1:04 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Sat, 29 Mar 2014 17:07:20 -0400, Roy Smith wrote: I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Apart from the fact that I got it wrong (that's what happens when I post at 6am after being up all night, thanks for the correction Lele), if you consider chained comparisons to be obfuscated, I think you're not really fluent at Python. The OP even suggested `a != None != b` so I think that (s)he at least understands chained comparisons. However, I agree with Johannes that inverted conditions (using not) are sometimes harder to reason about than regular conditions. Chained comparisons where you're checking a single variable against two constants make perfect sense: 2 x 5 Chained comparisons where you check a single constant against two variables don't, so much: x 2 y What exactly does that mean, and why is it written that way? We can figure out how the interpreter will parse that, but does that correspond to the programmer's intention? It'd be more useful but less clear if one of the conditions points the other way: x 2 y which checks that they're both less than two, but IMO in a less-than-clear way. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sat, 29 Mar 2014 17:36:55 -0500, Tim Chase wrote: And for cases where you have more than one or two things to test for None-itude, you could use if all(x is None for x in [a, b, c, d]): do_something_if_theyre_all_None() or if all(x is not None for x in [a, b, c, d]): do_something_if_no_Nones() or if not any(x is None for x in [a, b, c, d]): do_something_if_no_Nones() which I find *much* more readable from a maintenance point of view. With one or two things, I would stick to a regular comparison (skipping the not): a is None a is b is None With three, I would consider either idiom: a is b is c is None all(x is None for x in (a, b, c)) but lean towards the use of all(). From four onwards I would definitely use all(), and of course if there is an arbitrary number of items, I would definitely use all(). -- Steven D'Aprano http://import-that.dreamwidth.org/ -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
In article mailman.8709.1396145720.18130.python-l...@python.org, Chris Angelico ros...@gmail.com wrote: Chained comparisons where you're checking a single variable against two constants make perfect sense: 2 x 5 Chained comparisons where you check a single constant against two variables don't, so much: x 2 y To me, chained comparisons make intuitive sense when they're all (or =). I just think back to junior high school algebra class, with the big number line above the blackboard. Thus, a b c means if you put a, b, and c on the number line, a is to the left of b, which is to the left of c. I have no problem extending that to more than three values: a b c d e still makes intuitive sense. I have no particular problem with x 2 y because it fits the same pattern. But, if you show me a != None != b: my brain just goes into overload. Honestly, I don't even know what that means. My brain keeps trying to stick a, None, and b on Mrs. Albaum's number line and keeps walking into the wall. If you (the editorial you) tell me that my failure to grok that expression means I'm not fluent in Python, well then, guilty as charged. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
In article 5337807b$0$29994$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: a is b is c is None And we are all together. See how they run like pigs from a gun, see how they fly. -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sunday, March 30, 2014 8:09:45 AM UTC+5:30, Roy Smith wrote: I have no particular problem with x 2 y because it fits the same pattern. But, if you show me a != None != b: my brain just goes into overload. Honestly, I don't even know what that means. My brain keeps trying to stick a, None, and b on Mrs. Albaum's number line and keeps walking into the wall. If you (the editorial you) tell me that my failure to grok that expression means I'm not fluent in Python, well then, guilty as charged. Math Terminology A relation that is reflexive antisymmetric and transitive is a partial order Strict order: Irreflexive asymmetric and transitive Both are strongly related For general R (partial) S (strict) S from R xSy = xRy ∧ x ≠ y R from S xRy = xSy ∨ x=y /Math Terminology For both these chained comparisons are natural != is not transitive: 2 != 3 and 3 != 2 ⊬ 2 == 2 So for != chained comparisons are not natural (or IMHO appropriate) -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On March 29, 2014 9:43:00 PM CDT, Roy Smith r...@panix.com wrote: In article 5337807b$0$29994$c3e8da3$54964...@news.astraweb.com, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: a is b is c is None And we are all together. See how they run like pigs from a gun, see how they fly. I'm cryin'. (Really, that was terrible.) Walrus-ly y'rs, Zach -- https://mail.python.org/mailman/listinfo/python-list
Re: checking if two things do not equal None
On Sun, 30 Mar 2014 13:15:18 +1100, Chris Angelico wrote: On Sun, Mar 30, 2014 at 1:04 PM, Steven D'Aprano steve+comp.lang.pyt...@pearwood.info wrote: On Sat, 29 Mar 2014 17:07:20 -0400, Roy Smith wrote: I certainly agree that things like if a is not b is not None: ... belong in an obfuscated coding contest. Apart from the fact that I got it wrong (that's what happens when I post at 6am after being up all night, thanks for the correction Lele), if you consider chained comparisons to be obfuscated, I think you're not really fluent at Python. The OP even suggested `a != None != b` so I think that (s)he at least understands chained comparisons. However, I agree with Johannes that inverted conditions (using not) are sometimes harder to reason about than regular conditions. Chained comparisons where you're checking a single variable against two constants make perfect sense: 2 x 5 Chained comparisons where you check a single constant against two variables don't, so much: x 2 y What exactly does that mean, and why is it written that way? It checks that 2 is strictly bounded between x on the left and y on the right, i.e. that 2 is inside the open interval x...y. I don't know why you think that's unclear. But then I do have a maths background and I'm used to chaining comparisons. Write it like this: low = x high = y a = 2 low a high Does that make more sense? Well-chosen names are good. The fact that a is a constant rather than a variable is no big deal: low 2 high We can figure out how the interpreter will parse that, but does that correspond to the programmer's intention? That applies to just about anything: (x % 2 == 1) or (x 0) What that my intention, or did I intend to write (x % 2 == 0) and (x 0) At some point you just have to accept that, in the absence of clearly nonsensical code or a contradiction between the documentation and the behaviour (i.e. a bug), the programmer will have written what she intended to write. It'd be more useful but less clear if one of the conditions points the other way: x 2 y which checks that they're both less than two, which is quite different from what you wrote the first time. but IMO in a less-than-clear way. That's an understatement. If I saw code chaining comparisons in that fashion, I would assume the second operator was a typo. Chaining less-than and greater than operators should, for clarity, always be written in a single order. E.g. a = b c d, not a = b d c. (The second contains a subtle bug too.) -- Steven D'Aprano http://import-that.dreamwidth.org/ -- https://mail.python.org/mailman/listinfo/python-list
