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 <type '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
