Re: [Python-ideas] True and False are singletons
On 18Mar2019 08:10, Eric Fahlgren wrote: On Mon, Mar 18, 2019 at 7:04 AM Rhodri James wrote: On 18/03/2019 12:19, Richard Damon wrote: > On 3/18/19 7:27 AM, Greg Ewing wrote: >> Juancarlo Añez wrote: >> >>> if settings[MY_KEY] is True: >>> ... >> >> If I saw code like this, it would take a really good argument to >> convince me that it shouldn't be just >> >> if settings[MY_KEY]: >> ... >> > That means something VERY different. The first asks if the item is > specifically the True value, while the second just asks if the value is > Truthy, it wold be satisfied also for values like 1. Yes. And the latter is what people almost always mean. No, it depends heavily on the context. In GUI code, Oleg's example (tri-state checkbox) is a pervasive idiom. There's lots of code that says "if x is True" or "if x is False" or "if x is None" and that's a very clear indicator that you are dealing with these "booleans that can also be 'unset'". Yeah, but on a personal basis I would generally write such an idiom as "if x is None: ... elif x: truthy-action else: falsey-action" i.e. only rely on a singleton for the "not set" sentinel (usually None, occasionally something else). Cheers, Cameron Simpson ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
18.03.19 22:52, Wes Turner пише: >>> True = 1 File "", line 1 SyntaxError: can't assign to keyword The error message will be changed in 3.8. >>> True = 1 File "", line 1 SyntaxError: cannot assign to True ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
18.03.19 22:58, Greg Ewing пише: Oleg Broytman wrote: Three-way (tri state) checkbox. You have to distinguish False and None if the possible valuse are None, False and True. In that case the conventional way to write it would be if settings[MY_KEY] == True: ... It's not a major issue, but I get nervous when I see code that assumes True and False are unique, because things weren't always that way. "x == True" looks more dubious to me than "x is True". The latter can be intentional (see for example the JSON serializer), the former is likely was written by a newbie and contains a bug. For example, 1 and 1.0 will pass this test, but 2 and 1.2 will not. Python 3.8 will emit a syntax warning for "x is 1" but not for "x is True", because the latter is well defined and have valid use cases. ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Tue, Mar 19, 2019 at 11:32:56AM +1300, Greg Ewing wrote: > Tim Delaney wrote: > >I would argue the opposite - the use of "is" shows a clear knowledge > >that True and False are each a singleton and the author explicitly > >intended to use them that way. > > I don't think you can infer that. It could equally well be someone who's > *not* familiar with Python truth rules and really just meant "if x". > Or someone who's unfamiliar with booleans in general and thinks that > every "if" statement has to have a comparison in it. This! Writing "if some_bool = true" in static typed languages is pretty common. I used to see it a lot in my Pascal days. In fairness that was because I used to write some of it myself :-( For some reason it rarely seems to happen when the flag being tested is itself a boolean expression: if x > 0: # this if (x > 0) is True: # but never this which gives credence to your idea that people expect that people are (consciously or unconsciously) expecting every if to include a comparison. -- Steven ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Mon, Mar 18, 2019 at 3:42 PM Greg Ewing
wrote:
> Tim Delaney wrote:
> > I would argue the opposite - the use of "is" shows a clear knowledge
> > that True and False are each a singleton and the author explicitly
> > intended to use them that way.
>
> I don't think you can infer that. It could equally well be someone who's
> *not* familiar with Python truth rules and really just meant "if x".
> Or someone who's unfamiliar with booleans in general and thinks that
> every "if" statement has to have a comparison in it.
>
Regardless of whether it's idiomatic Python code or not, this pattern ("is
True:") can be found all over Python code in the wild.
If CPython ever broke this guarantee, quite a few popular libraries on pypi
would be broken, including pandas, sqlalchemy, attrs and even Python's own
standard library:
https://github.com/python/cpython/blob/c183444f7e2640b054956474d71aae6e8d31a543/Lib/textwrap.py#L175
___
Python-ideas mailing list
Python-ideas@python.org
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
Tim Delaney wrote: I would argue the opposite - the use of "is" shows a clear knowledge that True and False are each a singleton and the author explicitly intended to use them that way. I don't think you can infer that. It could equally well be someone who's *not* familiar with Python truth rules and really just meant "if x". Or someone who's unfamiliar with booleans in general and thinks that every "if" statement has to have a comparison in it. -- Greg ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Tue, Mar 19, 2019 at 09:58:55AM +1300, Greg Ewing wrote:
> Oleg Broytman wrote:
> > Three-way (tri state) checkbox. You have to distinguish False and
> >None if the possible valuse are None, False and True.
>
> In that case the conventional way to write it would be
>
> if settings[MY_KEY] == True:
> ...
For a tri-state setting, I would always check for None (or
whatever third state was used) first:
setting = settings[MY_KEY]
if setting is None:
# handle third state
elif setting:
# handle true state
else:
# handle false state
If for some strange reason I required the flags to be precisely True or
False rather than arbitrary truthy values, that's a *four* state flag
where the fourth state is an error condition.
setting = settings[MY_KEY]
if setting is None:
# handle third state
if not isinstance(setting, bool):
raise TypeError("not a bool! (but why do I care???)")
if setting:
# handle true state
else:
# handle false state
> It's not a major issue, but I get nervous when I see code
> that assumes True and False are unique, because things
> weren't always that way.
Do you also guard against True and False not being defined at all?
As long as True and False have been builtins, it has been a language
guarantee that they will be unique.
--
Steven
___
Python-ideas mailing list
Python-ideas@python.org
https://mail.python.org/mailman/listinfo/python-ideas
Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Tue, 19 Mar 2019 at 08:42, Greg Ewing wrote: > Oleg Broytman wrote: > >Three-way (tri state) checkbox. You have to distinguish False and > > None if the possible valuse are None, False and True. > > In that case the conventional way to write it would be > > if settings[MY_KEY] == True: > ... > > It's not a major issue, but I get nervous when I see code > that assumes True and False are unique, because things > weren't always that way. I would argue the opposite - the use of "is" shows a clear knowledge that True and False are each a singleton and the author explicitly intended to use them that way. Use of == in the same context is more likely to indicate a programmer who is unfamiliar with Python's truth rules. Tim Delaney ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
There are few cases where I would approve of 'if x is True'. However, the names used in the example suggest it could be one of those rare cases. Settings of True/False/None (i.e. not set) seem like a reasonable pattern. In fact, in code like that, merely "truthy" values are probably a bug that should not pass silently. Obviously this depends on the surrounding code to decide. On Mon, Mar 18, 2019, 5:44 PM Greg Ewing wrote: > Richard Damon wrote: > > On 3/18/19 7:27 AM, Greg Ewing wrote: > > > >>if settings[MY_KEY]: > >>... > > > > That means something VERY different. > > Yes, but there needs to be justification for why the difference > matters and why this particular way is the best way to deal > with it. > > Whenever you write 'x is True' or 'x == True', you are putting > a burden on all code that assigns to x to ensure that the > value is actually an instance of bool rather than just a > truthy or falsy value. That's an unusual requiremebt that > can lead to obscure bugs. > > In the tri-state example, the way I would do it is to guard > uses of it with 'if x is not None' and then treat the other > values as truthy or falsy. > > -- > Greg > ___ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
It was a VERY long time ago when True and False were not singletons. I don't think we should still try to write code based on rules that stopped applying more than a decade ago. On Mon, Mar 18, 2019, 5:42 PM Greg Ewing wrote: > Oleg Broytman wrote: > >Three-way (tri state) checkbox. You have to distinguish False and > > None if the possible valuse are None, False and True. > > In that case the conventional way to write it would be > > if settings[MY_KEY] == True: > ... > > It's not a major issue, but I get nervous when I see code > that assumes True and False are unique, because things > weren't always that way. > > -- > Greg > ___ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
Richard Damon wrote: On 3/18/19 7:27 AM, Greg Ewing wrote: if settings[MY_KEY]: ... > That means something VERY different. Yes, but there needs to be justification for why the difference matters and why this particular way is the best way to deal with it. Whenever you write 'x is True' or 'x == True', you are putting a burden on all code that assigns to x to ensure that the value is actually an instance of bool rather than just a truthy or falsy value. That's an unusual requiremebt that can lead to obscure bugs. In the tri-state example, the way I would do it is to guard uses of it with 'if x is not None' and then treat the other values as truthy or falsy. -- Greg ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
Oleg Broytman wrote: Three-way (tri state) checkbox. You have to distinguish False and None if the possible valuse are None, False and True. In that case the conventional way to write it would be if settings[MY_KEY] == True: ... It's not a major issue, but I get nervous when I see code that assumes True and False are unique, because things weren't always that way. -- Greg ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Tue, Mar 19, 2019 at 7:53 AM Wes Turner wrote: > > 'True' is a keyword. (Which is now immutable in Python 3.X?) > > >>> True = 1 > File "", line 1 > SyntaxError: can't assign to keyword In Python 3, the source code token "True" is a keyword literal that always represents the bool value True. > In Python 2: > > >>> True > True > >>> True is True > True > >>> True = 1 > >>> True is 1 > True > >>> True is None > False > >>> True = None > >>> True is None > True In Python 2, the source code token "True" is simply a name, and there is a built-in of that name. Before it became a built-in, it was common for scripts to have their own definitions of True and False [1], so to avoid unnecessary breakage, they were made assignable in the normal way. Python 3 simplifies this by making them keywords. But either way, the *values* True and False are special, and are the only two instances of the bool type that will ever exist. ChrisA [1] Note that I learned about this in history class; it was before my time. ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
'True' is a keyword. (Which is now immutable in Python 3.X?) >>> True = 1 File "", line 1 SyntaxError: can't assign to keyword https://docs.python.org/3/reference/datamodel.html#the-standard-type-hierarchy https://docs.python.org/3/search.html?q=singleton - "Since None is a singleton, testing for object identity (using == in C) is sufficient. There is no PyNone_Check() function for the same reason." https://docs.python.org/3/c-api/none.html?highlight=singleton - "Using a trailing comma for a singleton tuple: a, or (a,)" (?) https://docs.python.org/3/library/stdtypes.html?highlight=singleton#tuple https://docs.python.org/3/library/stdtypes.html#boolean-values https://docs.python.org/3/library/stdtypes.html#truth-value-testing In Python 2: >>> True True >>> True is True True >>> True = 1 >>> True is 1 True >>> True is None False >>> True = None >>> True is None True On Mon, Mar 18, 2019 at 7:34 AM Chris Angelico wrote: > On Mon, Mar 18, 2019 at 10:14 PM Juancarlo Añez wrote: > > > > It came to my attention that: > > > > In the original PEP True and False are said to be singletons > https://www.python.org/dev/peps/pep-0285/, but it's not in the Data Model > https://docs.python.org/3/reference/datamodel.html > > > > > > This came to my attention by code wanting to own the valid values in a > dict's key: > > > > if settings[MY_KEY] is True: > >... > > > > > > If True and False are singletons in the spec (and not only in the > CPython implementation), it should be prominent and well known. > > > > "Singleton" technically means that there is only one such object. > 'None' is a singleton, by language specification; if type(x) is > type(None), you can safely assume that x is None. Booleans are a bit > more tricky; there will only ever be those two, but they're two. IMO > the PEP is minorly inaccurate to use the word "singleton" there, but > it's no big deal. As Remi says, the two built-in ones are the only two > instances of that type. > > ChrisA > ___ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ > ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On 18/03/2019 15:10, Eric Fahlgren wrote: On Mon, Mar 18, 2019 at 7:04 AM Rhodri James wrote: On 18/03/2019 12:19, Richard Damon wrote: On 3/18/19 7:27 AM, Greg Ewing wrote: Juancarlo Añez wrote: if settings[MY_KEY] is True: ... If I saw code like this, it would take a really good argument to convince me that it shouldn't be just if settings[MY_KEY]: ... That means something VERY different. The first asks if the item is specifically the True value, while the second just asks if the value is Truthy, it wold be satisfied also for values like 1. Yes. And the latter is what people almost always mean. No, it depends heavily on the context. In GUI code, Oleg's example (tri-state checkbox) is a pervasive idiom. There's lots of code that says "if x is True" or "if x is False" or "if x is None" and that's a very clear indicator that you are dealing with these "booleans that can also be 'unset'". I would still contend that even in that case, testing "x is True" is asking to be hit with subtle bugs. -- Rhodri James *-* Kynesim Ltd ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Mon, Mar 18, 2019 at 7:04 AM Rhodri James wrote: > On 18/03/2019 12:19, Richard Damon wrote: > > On 3/18/19 7:27 AM, Greg Ewing wrote: > >> Juancarlo Añez wrote: > >> > >>> if settings[MY_KEY] is True: > >>> ... > >> > >> If I saw code like this, it would take a really good argument to > >> convince me that it shouldn't be just > >> > >> if settings[MY_KEY]: > >> ... > >> > > That means something VERY different. The first asks if the item is > > specifically the True value, while the second just asks if the value is > > Truthy, it wold be satisfied also for values like 1. > > Yes. And the latter is what people almost always mean. > No, it depends heavily on the context. In GUI code, Oleg's example (tri-state checkbox) is a pervasive idiom. There's lots of code that says "if x is True" or "if x is False" or "if x is None" and that's a very clear indicator that you are dealing with these "booleans that can also be 'unset'". ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On 18/03/2019 12:19, Richard Damon wrote: On 3/18/19 7:27 AM, Greg Ewing wrote: Juancarlo Añez wrote: if settings[MY_KEY] is True: ... If I saw code like this, it would take a really good argument to convince me that it shouldn't be just if settings[MY_KEY]: ... That means something VERY different. The first asks if the item is specifically the True value, while the second just asks if the value is Truthy, it wold be satisfied also for values like 1. Yes. And the latter is what people almost always mean. -- Rhodri James *-* Kynesim Ltd ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Tue, Mar 19, 2019 at 12:27:04AM +1300, Greg Ewing wrote: > Juancarlo A?ez wrote: > > >if settings[MY_KEY] is True: > >... > > If I saw code like this, it would take a really good argument to > convince me that it shouldn't be just > > if settings[MY_KEY]: > ... Three-way (tri state) checkbox. You have to distinguish False and None if the possible valuse are None, False and True. > -- > Greg Oleg. -- Oleg Broytmanhttps://phdru.name/p...@phdru.name Programmers don't die, they just GOSUB without RETURN. ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On 3/18/19 7:27 AM, Greg Ewing wrote: > Juancarlo Añez wrote: > >> if settings[MY_KEY] is True: >> ... > > If I saw code like this, it would take a really good argument to > convince me that it shouldn't be just > > if settings[MY_KEY]: > ... > That means something VERY different. The first asks if the item is specifically the True value, while the second just asks if the value is Truthy, it wold be satisfied also for values like 1. -- Richard Damon ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On 3/18/19 7:32 AM, Chris Angelico wrote: > On Mon, Mar 18, 2019 at 10:14 PM Juancarlo Añez wrote: >> It came to my attention that: >> >> In the original PEP True and False are said to be singletons >> https://www.python.org/dev/peps/pep-0285/, but it's not in the Data Model >> https://docs.python.org/3/reference/datamodel.html >> >> >> This came to my attention by code wanting to own the valid values in a >> dict's key: >> >> if settings[MY_KEY] is True: >>... >> >> >> If True and False are singletons in the spec (and not only in the CPython >> implementation), it should be prominent and well known. >> > "Singleton" technically means that there is only one such object. > 'None' is a singleton, by language specification; if type(x) is > type(None), you can safely assume that x is None. Booleans are a bit > more tricky; there will only ever be those two, but they're two. IMO > the PEP is minorly inaccurate to use the word "singleton" there, but > it's no big deal. As Remi says, the two built-in ones are the only two > instances of that type. > > ChrisA Which says that the type of True or False isn't a singleton, but those particular values are. There may be other objects with values that are Truthy or Falsey, but if the value actually IS True or False, the object WILL be those particular objects. As a comparison, if the Tuple (1,2) was described as a Singleton, then any computation that generates that value would all need to return the exact same object, but they don't (the language could make that a true statement). When converting a Truthy or Falsey value to True or False, Python will ALWAYS grab those particular objects, and not create a new object with the same value, so they are Singletons, even if the type itself isn't Yes, in many languages the term Singleton is used for Types and not values, and in part that difference is due to that fact that in Python ALL values are objects, and names are just bound to some object, so the idea of Singleton primitive values actually makes sense. -- Richard Damon ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
Juancarlo Añez wrote: if settings[MY_KEY] is True: ... If I saw code like this, it would take a really good argument to convince me that it shouldn't be just if settings[MY_KEY]: ... -- Greg ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
On Mon, Mar 18, 2019 at 10:14 PM Juancarlo Añez wrote: > > It came to my attention that: > > In the original PEP True and False are said to be singletons > https://www.python.org/dev/peps/pep-0285/, but it's not in the Data Model > https://docs.python.org/3/reference/datamodel.html > > > This came to my attention by code wanting to own the valid values in a dict's > key: > > if settings[MY_KEY] is True: >... > > > If True and False are singletons in the spec (and not only in the CPython > implementation), it should be prominent and well known. > "Singleton" technically means that there is only one such object. 'None' is a singleton, by language specification; if type(x) is type(None), you can safely assume that x is None. Booleans are a bit more tricky; there will only ever be those two, but they're two. IMO the PEP is minorly inaccurate to use the word "singleton" there, but it's no big deal. As Remi says, the two built-in ones are the only two instances of that type. ChrisA ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
Re: [Python-ideas] True and False are singletons
Le 18 mars 2019 à 12:15:05, Juancarlo Añez (apal...@gmail.com(mailto:apal...@gmail.com)) a écrit: > It came to my attention that: > > > In the original PEP True and False are said to be singletons > > https://www.python.org/dev/peps/pep-0285/, but it's not in the Data Model > > https://docs.python.org/3/reference/datamodel.html > > This came to my attention by code wanting to own the valid values in a dict's > key: > > > if settings[MY_KEY] is True: > > ... > > > > > If True and False are singletons in the spec (and not only in the CPython > implementation), it should be prominent and well known. I think it’s what "The two objects representing the values False and True are the only Boolean objects." mean. Rémi > Cheers, > > -- > Juancarlo Añez ___ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ ___ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
