Re: [Tutor] isinstance versus 'is'?
Thanks everyone, that all makes more sense. I think I was indeed thinking of "is None", which is essentially the same as "== None" (I know there's a subtile difference, but they do the same thing). Of course, "is None" fits with this usage, as you're asking if the value is the literal None object. It seems it's the way None is handled, not an exception in the way 'is' works. Anyway, thanks for the explanations. > On Jul 5, 2016, at 20:54, Steven D'Aprano wrote: > > On Tue, Jul 05, 2016 at 03:05:45PM -0400, Alex Hall wrote: > > a = 5 > isinstance(a, int) >> True > a is int >> False >> >> What happened there? Don't these do the same thing? I thought I could use >> them interchangeably? > > You're probably thinking of "is a", as in, "5 is an int", "'Hello > World' is a str", "[1, 2, 3] is a list", etc. > > Python doesn't have an operator for testing "is a" relationships, it > uses isinstance(obj, type). There's also issubclass(), for testing > whether one class is a subclass of another. > > "x is y" checks whether the two operands x and y are the same object. > That's *not* the same as checking whether they are equal. You should > hardly ever use "is" in Python, with the exception of testing for None: > "if obj is None: ..." sort of thing. > > > -- > Steve > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] isinstance versus 'is'?
On Tue, Jul 05, 2016 at 03:05:45PM -0400, Alex Hall wrote: > >>> a = 5 > >>> isinstance(a, int) > True > >>> a is int > False > > What happened there? Don't these do the same thing? I thought I could use > them interchangeably? You're probably thinking of "is a", as in, "5 is an int", "'Hello World' is a str", "[1, 2, 3] is a list", etc. Python doesn't have an operator for testing "is a" relationships, it uses isinstance(obj, type). There's also issubclass(), for testing whether one class is a subclass of another. "x is y" checks whether the two operands x and y are the same object. That's *not* the same as checking whether they are equal. You should hardly ever use "is" in Python, with the exception of testing for None: "if obj is None: ..." sort of thing. -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] isinstance versus 'is'?
On 06/07/16 00:22, Alan Gauld via Tutor wrote: type(c) is C > True type(d) is type(C) > False The last one should of course be >>> type(d) is C False Apologies. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] isinstance versus 'is'?
On Tue, Jul 5, 2016 at 12:05 PM, Alex Hall wrote: a = 5 isinstance(a, int) > True a is int > False > > What happened there? Don't these do the same thing? I thought I could use > them interchangeably? 'isinstance' is something else from 'is'. isinstance will tell us if something is considered to be an "instance" of something: it knows how to categorize. This might be important if, when we're running our program, we have some data and want to do some case analysis based on what classification that data falls into. For example, True and False can be identified as instances of the booleans. ### >>> isinstance(True, bool) True >>> isinstance(False, bool) True ### If we're object oriented programming based on different classes of data, then 'isinstance' can be used to distinguish things based on their class. For example: # class C1(object): pass class C2(object): pass barry = C1() white = C2() print(isinstance(barry, C1)) print(isinstance(barry, C2)) ## if we have two classes, we can create instances of them, and use 'isinstance' to find out if they match a particular class-ification. I rarely use this operator in practice because usually, *how* a value operates and behaves is much more important than *what* strata that value belongs to. I suppose that's an ideal of meritocracy. :) In contrast, 'is' is a more low-level notion having to do with *equality*, not classification. These are both operations that compare two things, but their conceptual types are not the same: * 'isinstance' deals with a thing and a category. * 'is' works on two things that, presumably, might be the same type of thing. (Why? Because if we knew in advance that the the two things were of different types, then we already know that 'is' will return False, so we know our answer in advance.) ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] isinstance versus 'is'?
On 05/07/16 20:05, Alex Hall wrote: > I was double checking that I remembered the isinstance order of arguments > correctly by using the command line interpreter, and found something very > odd. > a = 5 isinstance(a, int) > True a is int > False > > What happened there? Don't these do the same thing? Evidently not! Think about it. isinstance() is asking if a is an instance of int. int is a type. 5 is an instance of that type but it's not the type itself. a is int is asking if a and int are the same things (are they actually the same object). Clearly 5 is not the type int. We can check this in the interpreter: >>> isinstance(5,int) True >>> int >>> 5 5 What you may be getting confused with is: >>> type(5) is int True >>> But even then this is not identical to isinstance, because the latter checks for subclass relationships too: >>> class C: pass ... >>> class D(C): pass ... >>> c = C() >>> d = D() >>> isinstance(c, C) True >>> isinstance(c, D) False >>> isinstance(d, C) True >>> isinstance(d, D) True >>> type(c) is C True >>> type(d) is type(C) False And to further complicate matters the above works differently in v2 from v3(above) The bottom line is that isinstance() is usually the best choice. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] isinstance versus 'is'?
On Tue, Jul 5, 2016 at 3:05 PM, Alex Hall wrote:
> Hi all,
> Thanks for all the help regarding decorators. It makes more sense now.
>
> I was double checking that I remembered the isinstance order of arguments
> correctly by using the command line interpreter, and found something very
> odd.
>
a = 5
isinstance(a, int)
> True
a is int
> False
>
> What happened there? Don't these do the same thing? I thought I could use
> them interchangeably?
>
> --
> Alex Hall
> Automatic Distributors, IT department
> ah...@autodist.com
> ___
> Tutor maillist - Tutor@python.org
> To unsubscribe or change subscription options:
> https://mail.python.org/mailman/listinfo/tutor
is denotes identity. 5 is not the type int. It is an int, but it is
not the same -- if it was 5('16') would be 16.
--
Joel Goldstick
http://joelgoldstick.com/blog
http://cc-baseballstats.info/stats/birthdays
___
Tutor maillist - Tutor@python.org
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[Tutor] isinstance versus 'is'?
Hi all, Thanks for all the help regarding decorators. It makes more sense now. I was double checking that I remembered the isinstance order of arguments correctly by using the command line interpreter, and found something very odd. >>> a = 5 >>> isinstance(a, int) True >>> a is int False What happened there? Don't these do the same thing? I thought I could use them interchangeably? -- Alex Hall Automatic Distributors, IT department ah...@autodist.com ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
