Mailman 3 March 2011 - Doc-SIG

Shouldn't tutorial follow the pep8?
by INADA Naoki March 7, 2011

March 7, 2011

In http://docs.python.org/py3k/tutorial/classes.html#random-remarks > Possible conventions include capitalizing method names, prefixing > data attribute names with a small unique string (perhaps just an underscore), > or using verbs for methods and nouns for data attributes. Capitalizing methd names doesn't follows pep8. Many sample codes in tutorial doesn't follow pep8 too. For example, in http://docs.python.org/py3k/tutorial/introduction.html#numbers there are no spaces next to operators. -- INADA Naoki <songofacandy(a)gmail.com>

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Re: [Doc-SIG] Does the "is" operator only matter for mutable object?
by Laura Creighton March 7, 2011

March 7, 2011

In a message of Mon, 07 Mar 2011 10:30:08 +1000, Nick Coghlan writes: >On Mon, Mar 7, 2011 at 9:08 AM, Laura Creighton <lac(a)openend.se> wrote: >> Singleton objects such as ``True``, ``False``, and ``None`` are always >> the same object. The canonical way to test whether an object is >> a singleton is to test for object identity, not equality. So: >> >>>>> if x is None: >> ... do something ... >> >> not >> >>>>> if x == None: >> ... do something ... > >Careful with that - "if x is True:" and "if x is False:" (along with >s/is/==/) are almost always the wrong thing to do, but a reader could >easily generalise the above to cover those two cases as well. > >Cheers, >Nick. Good point. Indeed, that may be where people who write code like that get the idea. I've always wondered. Ok. Change to: Testing the object identity of unmutable objects with the ``is`` or ``is not`` operators is rarely something you want to do, because the result is implementation dependent. But the canonical way to test whether an object is None is to test for object identity, not equality. so >>> if x is None: # this is how we write it ... do something ... not >>> if x == None: # don't do this. ... do something ... ----------- I figure that the fact that None is a singleton is not actually relevant to this. Laura

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Re: [Doc-SIG] Does the "is" operator only matter for mutable object?
by Laura Creighton March 7, 2011

March 7, 2011

In a message of Sun, 06 Mar 2011 10:59:55 +1100, "Steven D'Aprano" writes: <snip> >The more I think about this the more I agree with Fred Drake that we >should keep this simple. The documentation for the ``is`` operator is >not the place for a discussion of implementation-specific optimizations. > > The operator ``is`` tests whether the two operands are the > same object. ``is not`` tests that they are different objects. > >If we have to mention singletons here, and I don't think we do, then a >single example is enough: > > Singleton objects such as ``None`` are, by definition, > always the same object. > > > >-- >Steven While the point is well taken that objects that are the same do not have to occupy the same memory location, and don't in PyPy, the problem with explaining the 'is' and 'is not' operator is explaining what 'is the same object' means. I think the greatest cause of confusion is the fact that Cpython interns the intergers. Thus: [GCC 4.4.5] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> x = 5 >>> x == 5 True >>> x is 5 True while: Python 2.5.2 (e503e483e9ac, Dec 21 2010, 12:02:29) [PyPy 1.4.1] on linux2 Type "help", "copyright", "credits" or "license" for more information. And now for something completely different: ``dystopian and utopian chairs'' >>>> x = 5 >>>> x == 5 True >>>> x is 5 False ------------------ So maybe what we need to do is to explictly state that some implementations have chosen to represent objects which are equivalent as the exact same object, but that you should not rely on this behaviour because it is an implementation detail? Laura

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Re: [Doc-SIG] Does the "is" operator only matter for mutable object?
by Laura Creighton March 7, 2011

March 7, 2011

I've been doing more thinking, and I think the problem is more deeply rooted than this. From the original doc: > The operators is and is not compare whether two objects are really the same > object; > this only matters for mutable objects like lists. This is actually wrong. If x is y, then you _aren't_ comparing two objects, you are comparing one object with itself, and that is the whole point. You are comparing two names, and seeing if they are bound to the same object. What I really want to tell the perplexed to do is read: http://effbot.org/zone/python-objects.htm However: Given that this is in the section on comparing sequences and other types: The operators ``is`` and ``is not`` test for object identity. If two different names are bound to the same object, then they compare the same: ``x is y`` . >>> v = [1,2,3] >>> w = v >>> v is w True >>> v.append(4) >>> v [1, 2, 3, 4] >>> w [1, 2, 3, 4] Confusion arises because implementors are free to cache immutable objects with the same value and type to the same object for reasons of efficiency. [PyPy 1.4.1] Type "help", "copyright", "credits" or "license" for more information. >>>> w = 1 >>>> y = 1 >>>> w is y False Python 2.6.6 #(I don't have a 3.x around , I assume it works the same way) Type "help", "copyright", "credits" or "license" for more information. >>> w = 1 >>> y = 1 >>> w is y # this might surprise you True Testing the object identity of unmutable objects with the ``is`` or ``is not`` operators is rarely something you want to do, because the result is implementation dependent. Singleton objects such as ``True``, ``False``, and ``None`` are always the same object. The canonical way to test whether an object is a singleton is to test for object identity, not equality. So >>> if x is None: ... do something ... not >>> if x == None: ... do something ... -------------- I think this will fix the confusion, but maybe it belongs someplace other than the comparing sequence section. http://docs.python.org/py3k/tutorial/datastructures.html#comparing-sequence…

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Re: [Doc-SIG] Does the "is" operator only matter for mutable object?
by Laura Creighton March 6, 2011

March 6, 2011

In a message of Mon, 07 Mar 2011 00:08:38 +0100, Laura Creighton writes: <snip> oops, that one escaped before I was done. The other point was that I wondered about the section title itself. 5.8. Comparing Sequences and Other Types This reads as if we are comparing types, when that is not what this section is about at all. Laura

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Does the "is" operator only matter for mutable object?
by INADA Naoki March 5, 2011

March 5, 2011

In http://docs.python.org/py3k/tutorial/datastructures.html#comparing-sequence… , The operators is and is notcompare whether two objects are really the same > object; > this only matters for mutable objects like lists. I think this notation may confuse readers of the tutorial. For example, pep8 recommands using is operator for immutable singleton objects like None. Shall we remove letter part? -- INADA Naoki <songofacandy(a)gmail.com>

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Re: [Doc-SIG] Does the "is" operator only matter for mutable object?
by Laura Creighton March 5, 2011

March 5, 2011

In a message of Sat, 05 Mar 2011 09:06:22 PST, Aahz writes: >+1 -- here's my rewrite for a bit more clarity: > >The operators ``is`` and ``is not`` compare whether two objects are >really the same object (have the same memory location). Immutable >objects with the same value and type may be cached to the same object for >efficiency. For example, ``'spam' is 'spam'`` is either ``True`` or >``False`` depending on Python implementation. Singleton objects >(``True``, ``False``, ``None``) are always the same object. I like Aahz's version. Laura

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