Re: Overriding True and False ?
It's fixed already in Python 3. Please use Python 3 when teaching to students. $ python3 Python 3.6.0 (default, Dec 24 2016, 00:01:50) [GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> True = "foo" File "", line 1 SyntaxError: can't assign to keyword -- https://mail.python.org/mailman/listinfo/python-list
RE: Overriding True and False ?
Irv Kalb wrote, on Sunday, January 29, 2017 9:04 PM
>
> I teach intro to programming using Python. In my first
> assignment, students are asked to assign variables of
> different types and print out the values.
>
> One student (who really did not understand Booleans) turned
> in the following for his/her interpretation of Booleans (Python 2.7):
>
> True = 'shadow'
> False = 'light'
> print "If the sun is behind a cloud, there is", True
> print "If it is a clear day, there is", False
>
> And it printed:
>
> If the sun is behind a cloud, there is shadow
> If it is a clear day, there is light
>
>
> It seems very odd that Python allows you to override the
> values of True and False. In the code, True and False were
> clearly recognized as keywords as they were colored purple.
> But there was no error message.
>
> You cannot assign new values to other keywords. Simple tests
> of things like:
>
> for = 5
>
> while = 2
>
> not = 3
>
> As expected, all result in SyntaxError: invalid syntax. Why
> would Python allow you to override the values of True and
> False? I wonder if this is some sort of historical thing as
> these are the only keywords besides None that are uppercased.
> This line:
>
> None = 5
>
> Even gives a special SyntaxError: cannot assign to None
>
> Just curious,
>
> Irv
Just guessing, but in the examples you give in Python 2.7, substitute
strings are syntactically correct in print statements, but:
5 in list('abc'):
2 True:
if a 3 b:
would all be syntactical errors.
As is 'None = 5'.
Looks like the moral of the story is that in Python 2.7 you can redefine
keywords, so long as you don't get any syntax errors after (or during)
redefinition.
--
https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
Irv Kalb writes: > I teach intro to programming using Python. […] Thank you for teaching Python to beginners! > It seems very odd that Python allows you to override the values of > True and False. Yes, it is. That's why Python 3 forbids it:: >>> True = "shadow" File "", line 1 SyntaxError: can't assign to keyword >>> False = "light" File "", line 1 SyntaxError: can't assign to keyword When teaching Python, please do not teach Python 2. Your students should learn Python 3 first, primarily, and for most of the course. Python 2 is a legacy that will never gain new features, and will only slip further behind the current supported Python version. That makes Python 2 a poor choice for teaching to beginners. -- \ “The entertainment industry calls DRM "security" software, | `\ because it makes them secure from their customers.” —Cory | _o__) Doctorow, 2014-02-05 | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Mon, Jan 30, 2017 at 6:50 PM, Deborah Swanson wrote: > Looks like the moral of the story is that in Python 2.7 you can redefine > keywords, so long as you don't get any syntax errors after (or during) > redefinition. The moral is actually that "True" and "False" aren't keywords in Py2. They're just built-ins. Compare: rosuav@sikorsky:~$ python3 Python 3.7.0a0 (default:cebc9c7ad195, Jan 24 2017, 06:55:19) [GCC 6.2.0 20161027] on linux Type "help", "copyright", "credits" or "license" for more information. >>> import dis >>> dis.dis(lambda: True) 1 0 LOAD_CONST 1 (True) 2 RETURN_VALUE >>> rosuav@sikorsky:~$ python Python 2.7.12+ (default, Sep 1 2016, 20:27:38) [GCC 6.2.0 20160927] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> import dis >>> dis.dis(lambda: True) 1 0 LOAD_GLOBAL 0 (True) 3 RETURN_VALUE >>> In Python 2, returning "True" involves a name lookup; in Python 3, it's simply a constant - a literal. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Mon, Jan 30, 2017 at 4:03 PM, Irv Kalb wrote: > It seems very odd that Python allows you to override the values of True and > False. In the code, True and False were clearly recognized as keywords as > they were colored purple. But there was no error message. > > You cannot assign new values to other keywords. Simple tests of things like: > > for = 5 > > while = 2 > > not = 3 > > As expected, all result in SyntaxError: invalid syntax. Why would Python > allow you to override the values of True and False? I wonder if this is some > sort of historical thing as these are the only keywords besides None that are > uppercased. This line: > > None = 5 > > Even gives a special SyntaxError: cannot assign to None There are slightly different things going on here. Trying to assign to a piece of syntax like "while" makes absolutely no sense, but trying to assign to "None" is structurally sane, yet disallowed. IIRC there's only one non-assignable name in Python 2 (None), but as mentioned, Python 3 adds True and False to that. (Interestingly, Ellipsis is not included in that.) > I teach intro to programming using Python. May I please request that you consider teaching Python 3? Python 2 isn't going anywhere (for better or for worse), and Py3 is a superior language in many ways, not least of which is that it keeps text and bytes separate, giving text the full power that it should have. For a beginning programmer, this is very helpful; there's nothing to un-learn when going international. (There will be new nuances to be learned, such as RTL text, but nothing to unlearn.) Python 3 also fixes a number of other problems that Python 2 inherited from C, making it altogether a better language for teaching with. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Jan 30, 2017 2:00 AM, "Chris Angelico" wrote: (Interestingly, Ellipsis is not included in that.) Perhaps because it's rather unusual for a program to depend upon the value of Ellipsis. -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Jan 30, 2017 1:32 AM, "Irv Kalb" wrote: I teach intro to programming using Python. In my first assignment, students are asked to assign variables of different types and print out the values. One student (who really did not understand Booleans) turned in the following for his/her interpretation of Booleans (Python 2.7): True = 'shadow' False = 'light' print "If the sun is behind a cloud, there is", True print "If it is a clear day, there is", False And it printed: If the sun is behind a cloud, there is shadow If it is a clear day, there is light It seems very odd that Python allows you to override the values of True and False. In the code, True and False were clearly recognized as keywords as they were colored purple. But there was no error message. Plenty of people have remarked that this is fixed in Python 3, but nobody that I see has explained yet the reason for this behavior in the first place. True and False are not keywords in Python 2 for backward compatibility. Many versions ago, Python did not define True and False at all. It was common in that era to lead scripts with: False = 0 True = 1 To define useful boolean constants. When the built-in constants were added, the developers did not want to break programs that followed this pattern. This is also the reason why bool is a subclass of int -- so that False == 0 and True == 1 remain true for programs that depend on this. -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Tue, 31 Jan 2017 03:12 am, Ian Kelly wrote: > On Jan 30, 2017 1:32 AM, "Irv Kalb" wrote: > > I teach intro to programming using Python. In my first assignment, > students are asked to assign variables of different types and print out > the values. [...] Hey Ian, Your news reader or mail client has stopped quoting the text you are quoting, so it appears as if you have written it. See: https://mail.python.org/pipermail/python-list/2017-January/719015.html -- Steve “Cheer up,” they said, “things could be worse.” So I cheered up, and sure enough, things got worse. -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Jan 30, 2017 6:07 PM, "Steve D'Aprano" wrote: > Hey Ian, > > Your news reader or mail client has stopped quoting the text you are > quoting, so it appears as if you have written it. > > See: > > https://mail.python.org/pipermail/python-list/2017-January/719015.html Well, nuts. It looks fine in my client (the Gmail Android app) so I guess this must be an issue of HTML versus text content. Unfortunately the mobile client doesn't have any option for text-only that I can find, so my options appear to be to laboriously replace the quoting as I've done here, or just stop using the mobile client. -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
Ian Kelly writes: > Well, nuts. It looks fine in my client (the Gmail Android app) so I > guess this must be an issue of HTML versus text content. Unfortunately > the mobile client doesn't have any option for text-only that I can > find, so my options appear to be to laboriously replace the quoting as > I've done here, or just stop using the mobile client. For composing discussion posts, yes, I recommend people stop using mobile clients. It can wait until you're at a real keyboard and a decent authoring environment. -- \ “Very few things happen at the right time, and the rest do not | `\ happen at all. The conscientious historian will correct these | _o__) defects.” —Mark Twain, _A Horse's Tale_ | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
Am 31.01.17 um 04:27 schrieb Ian Kelly: On Jan 30, 2017 6:07 PM, "Steve D'Aprano" wrote: Hey Ian, Your news reader or mail client has stopped quoting the text you are quoting, so it appears as if you have written it. See: https://mail.python.org/pipermail/python-list/2017-January/719015.html Well, nuts. It looks fine in my client (the Gmail Android app) so I guess this must be an issue of HTML versus text content. Unfortunately the mobile client doesn't have any option for text-only that I can find, so my options appear to be to laboriously replace the quoting as I've done here, or just stop using the mobile client. This one looks fine, FYI. Christian -- https://mail.python.org/mailman/listinfo/python-list
Re: Overriding True and False ?
On Sunday, January 29, 2017 at 11:29:29 PM UTC-8, Irv Kalb wrote: > It seems very odd that Python allows you to override the values of > True and False. Hi Irv, Let me join the chorus of people who are encouraging you to switch to Python3, which corrects this problem. Overriding builtins has been judged harmful, and now it is a lot harder to do. There are sometimes good reasons for shadowing builtins, but more than a few newcomers to Python (once upon a time, myself included) wrote buggy code by accident because of this surprising flexibility. That being said, I can't resist re-posting a story from 14 years ago: https://groups.google.com/forum/#!original/comp.lang.python/xEtUYsxLnFE/6yYgvhvkggMJ == From: lad...@my-deja.com (John Ladasky) Newsgroups: comp.lang.python Subject: Re: Python 2.3 True = False Date: 24 Mar 2003 12:29:04 -0800 an...@calbay.com (Anand) wrote in message news:... > >>> True = False > >>> True > False Shhh! Anand! You're not supposed to show anyone that! Now every politician on Earth will want to program in Python! == -- https://mail.python.org/mailman/listinfo/python-list
