About "the backwards compatible option is probably necessary for some more 
advanced uses of templates out there, e.g. templating whitespace sensitive 
file formats." -- I'm not following why a similar find/replace approach 
wouldn't be sufficient to adapt those templates?

On Wednesday, January 4, 2017 at 6:10:12 PM UTC-5, Adam Johnson wrote:
>
> Whitespace control in templates is an exercise in frustration. In my 
>> experience more flexible tools such as {%-, {%+, -%}, and +%} in Jinja2 
>> increase the frustration.
>
>
> I really enjoy the {%- etc. operators in Jinja 2 in the context of 
> Ansible, there are often cases when templating obscure configuration files 
> that they come in useful.
>  
>
>> I'd like to know from Carl, Adam, and others, how much effort would be 
>> required to adapt the templates in your project for the new behavior. I 
>> imagine a script could be written to add newlines after all {% include %} 
>> in all plain text templates, for example.
>
>
> You're right, it's not too hard to fix for the text email template 
> usecase. But the backwards compatible option is probably necessary for some 
> more advanced uses of templates out there, e.g. templating whitespace 
> sensitive file formats.
>
> On 4 January 2017 at 22:53, Tim Graham <timog...@gmail.com <javascript:>> 
> wrote:
>
>> Aymeric, we have a difference of opinion. I feel that if {% include %} 
>> removed the trailing newline, it would result in far more intuitive 
>> whitespace control (which may be needed in plain text email templates, for 
>> example). I think the change has merits outside the context of this issue. 
>> For example, I checked view-source on a djangoproject.com page that uses 
>> a {% for ... %}{% include %}{% endfor %} construct and noticed that it's 
>> much shorter without all those extra blank lines. I'm not advocating for 
>> additional controls to craft well formatted HTML but I think there's some 
>> advantages to not having blank lines everywhere. (I don't like the 
>> keep_trailing_newline option either, I'd rather call it a bugfix and make a 
>> backwards-incompatible change but my projects are unaffected as far as I 
>> checked.)
>>
>> There are many questions on Stackoverflow about how to suppress newlines 
>> from {% include %} and even a django-include-strip-tag third-party app that 
>> offers the feature.
>>
>> In the context of multiple template engines, I think it would be useful 
>> if DTL templates could be written in a similar style to Jinja2.
>>
>> I'd like to know from Carl, Adam, and others, how much effort would be 
>> required to adapt the templates in your project for the new behavior. I 
>> imagine a script could be written to add newlines after all {% include %} 
>> in all plain text templates, for example.
>>
>> It won't be the end of the world if we can't get consensus to change the 
>> behavior -- I'm okay with removing the trailing newlines in the source 
>> files as needed, although it will add some inconvenience.
>>
>> On Wednesday, January 4, 2017 at 4:26:43 PM UTC-5, Aymeric Augustin wrote:
>>>
>>> If I understand correctly, we have to choose between:
>>>
>>> 1. breaking backwards compatibility for {% include %}
>>> 2. breaking backwards compatibility for widgets HTML
>>> 3. having a handful of single-line, non-newline-terminated files
>>>
>>> I don’t think option 1 is reasonable. Whitespace control in templates is 
>>> an exercise in frustration. In my experience more flexible tools such as 
>>> {%-, {%+, -%}, and +%} in Jinja2 increase the frustration. I don’t think 
>>> Django should get itself into this mess just for fixing the problem 
>>> discussed here.
>>>
>>> Option 2 seems acceptable to me. This is the "do nothing” option. I 
>>> don’t think the exact HTML string rendered by widgets is considered a 
>>> public API. We made changes to that output in the past, for example when we 
>>> switched to HTML5.
>>>
>>> Option 3 also seems reasonable to me. Files could end with {# no 
>>> newline, see issue #xxx #} and no newline. A test could validate that the 
>>> rendered output doesn’t contain a newline. This would be enough to catch 
>>> accidental regressions caused by editors automatically adding the newline.
>>>
>>> The quote from the POSIX standard says “should”, not “must”. If we 
>>> interpret it as if it was written in capitals in a RFC, this means we can 
>>> make an exception if we have a good reason.
>>>
>>> (At this point surely someone will suggest that we can write an 
>>> editorconfig to specify that these files mustn’t end with a newline. 
>>> Whatever works.)
>>>
>>> I don’t have a strong preference for 2 or 3, however, I have a strong 
>>> distaste for anything more complicated.
>>>
>>> Best regards,
>>>
>>> -- 
>>> Aymeric.
>>>
>>> On 4 Jan 2017, at 20:58, Tim Graham <timog...@gmail.com> wrote:
>>>
>>> Shortly after template widget rendering was merged, an issue about extra 
>>> newlines appearing in the rendered output was reported [0]:
>>>
>>> For example, from django-money:
>>>
>>> <option value="XFU" \n>UIC-Franc</option>
>>> \n\n
>>> <option value="USD" selected\n>US Dollar</option>
>>> \n\n
>>>
>>> The newlines aren't useful and they break assertions like this:
>>>
>>> <option value="USD" selected>US Dollar</option> in form.as_p
>>>
>>> --- end report---
>>>
>>> The reporter suggested removing the trailing newline in the attrs.html 
>>> template but Adam Johnson reported: "POSIX states that all text files 
>>> should end with a newline, and some tools break when operating on text 
>>> files missing the final newline. That's why git has the warning \ No 
>>> newline at end of file and Github has a warning symbol for the missing 
>>> newline."
>>>
>>> I suggested that perhaps {% include %} could do .rstrip('\n') on 
>>> whatever it renders.
>>>
>>> Preston pointed out that Jinja2 does something similar:
>>>
>>> http://jinja.pocoo.org/docs/dev/templates/#whitespace-control
>>>
>>>    - a single trailing newline is stripped if present
>>>    - other whitespace (spaces, tabs, newlines etc.) is returned 
>>>    unchanged
>>>
>>> I noted that the issue of {% include %} adding extra newlines was raised 
>>> in #9198 [1] but Malcolm said, "For backwards compatibility reasons we 
>>> cannot change the default behaviour now (there will be templates that rely 
>>> on the newline being inserted)." I was skeptical this would be a burdensome 
>>> change.
>>>
>>> Carl replied:  "It seems quite likely to me that there are templates in 
>>> the wild relying on preservation of the final newline. People render 
>>> preformatted text (e.g. text emails) using DTL. I probably have some text 
>>> email templates in older projects myself that would break with that change. 
>>>
>>> We could add a new option to {% include %}, though." Adam also said, "I 
>>> too have used DTL for text emails that would break under that behaviour. 
>>> New option to include sounds good to me."
>>>
>>>
>>> Me again: In the long run, having {% include %} remove the trailing 
>>> newline seems like a more sensible default. For example, I wouldn't expect 
>>> this code to have a newline inserted between it:
>>>
>>>
>>> {% include "foo.txt" %}
>>> {% include "bar.txt" %}
>>>  
>>>
>>> An option for {% include %} seems superfluous given that if you were 
>>> writing
>>>
>>>
>>> {% include "foo.txt" %}{% include "bar.txt" %}
>>>  
>>>
>>> now you can write the first thing which is much more intuitive.
>>>
>>>
>>> How about a keep_trailing_newline TEMPLATES option for backwards 
>>> compatibility for those who don't want to adapt their templates for the new 
>>> behavior? Jinja2 has that option.
>>>
>>> Carl replied: An engine option may be better than an option to {% 
>>> include %}, though it doesn't allow us to ensure that we strip the 
>>> newline in the specific case of attrs.
>>>
>>> How we default the engine option I guess just depends on how seriously 
>>> we take backwards compatibility. If we default it to strip and make people 
>>> add the config to preserve the old behavior, that's not really backwards 
>>> compatible. Historically (as seen in Malcolm's comment) we would choose to 
>>> err on the side of actual backwards compatibility in a case like this, even 
>>> if it didn't result in the ideal future behavior. But the adaptation isn't 
>>> hard in this case, so I won't object if the choice is to break back-compat.
>>>
>>>
>>> If it's not a per-include choice, of course, we have to break overall 
>>> back-compat to preserve form-attr-rendering back-compat.
>>> ----
>>>
>>> What do you think?
>>>
>>> [0] https://github.com/django/django/pull/7769
>>> [1] https://code.djangoproject.com/ticket/9198
>>>
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>
>
>
> -- 
> Adam
>

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