New subject: tagged templates like in JavaScript

June 4, 2021

      Hi,

I have a crazy idea how to extend Python for web development.

What do you think? What are the next steps?

Original: https://github.com/guettli/python-custom-strings

Python Custom Strings

Python Custom Strings want to combine two great things:

   - Python's f-strings
   <https://docs.python.org/3/tutorial/inputoutput.html#formatted-string-literal...>
   - Django's conditional_escape()
   <https://docs.djangoproject.com/en/3.2/ref/utils/#django.utils.html.condition...>

Python Custom Strings is a proposal to enhance Python, to give developers
new ways to create escaped strings.

The Python Custom Strings proposal is not related to Django or HTML. This
text uses HTML and Django just as an example.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#motivat...>
Motivation

I could create HTML in Python with the help of Django's format_html()
<https://docs.djangoproject.com/en/3.2/ref/utils/#django.utils.html.format_ht...>
like
this:

html = format_html('''
 <h1>Hi {username}</h1>
 Your messages: {messages}''',
     username=username,
     messages=messages
     )

With "username" being a simple string that gets quoted. For example Mary &
Bob will get Mary & Bob.

With "messages" being a previously escaped string. For example
<ul><li>line1</li><li>line2</li></ul>. It does not get escaped, since it is
already escaped.

This "magic" detection whether escaping should be done or not gets handled
by conditional_escape()
<https://docs.djangoproject.com/en/3.2/ref/utils/#django.utils.html.condition...>
.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#goal>
Goal

I would like to shorten the above code to something similar to this:

html = h'''
 <h1>Hi {username}</h1>
 Your messages: {messages}'''

The developer should not need to type these line:

     username=username,
     messages=messages

These lines are meaningless and distracting. Avoiding them reduces the
cognitive load, especially for the reader of the code.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#customi...>
Customization

To make this customizable, the user needs to define the methods which
should get used.

In the above example the h prefix of this snippet:

html = h'''
 <h1>Hi {username}</h1>
 Your messages: {messages}'''

Should be executed like

html = mark_safe('''
 <h1>Hi {username}</h1>
 Your messages: {messages}''',
  username=conditional_escape(username),
  messages=conditional_escape(message))

Python does not know about these methods mark_safe() and
conditional_escape().

So we need a way to define that.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#defining-custom-strings>Defining
custom strings

Somehow, there needs to be a definition of the custom strings. Up to now I
open concerning "how to define custom strings?".

The syntax could look like this:

register_custom_string('h', pre_return, pre_insert)

With pre_return() and pre_insert() being methods which get executed to
create the custom string.

In the above example you would use this:

register_custom_string('h', mark_safe, conditional_escape)

This is just a first idea. I guess there are better ways to define both
methods. Please speak up if you have a better idea.

The letter h is just an example. The developer should be able to choose his
prefered character. Replacing the already used letteres like r, b, u is not
possible and results in an exception.

TODO: What should happen if register_custom_string() gets called twice for
the same character?
<https://github.com/guettli/python-custom-strings/blob/main/README.md#what-is-inside-curly-braces>What
is inside curly braces?

In the above example, we just use a variable name. But having more is
better.

Custom strings are meant for developers, so arbitrary code execution is
fine.

Arbitrary code in curly braces should be allowed:

html = h'''...{my_object.my_method(some_arg)}...'''

<https://github.com/guettli/python-custom-strings/blob/main/README.md#why-not-just-this-hack>Why
not just this hack?

Instead of h'...' you could use this hack to get the desired implementation:

def h(html):
    """
    Django's format_html() on steroids
    """
    def replacer(match):
        call_frame = sys._getframe(3)
        return conditional_escape(
            eval(match.group(1), call_frame.f_globals, call_frame.f_locals))
    return mark_safe(re.sub(r'{(.*?)}', replacer, html))

The above implementation has one big drawback:

IDEs and linters don't know that variables get used inside the custom
string.

This means IDEs and linters think variables (or imports) don't get used and
act accordingly.

Nevertheless it makes no sense to type myvar=mvar again and again, just to
make IDEs/linters happy.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#concern...>
Concerns

"Custom strings will downgrade Python to the level of PHP".

I think Separation of Concerns makes sense. But sometimes Locality of
Behaviour <https://htmx.org/essays/locality-of-behaviour/> makes more sense.

It really depends on the context. Sometimes it makes sense to have external
templates, sometimes it makes senes to have inline templates.

Don't judge this proposal just because your context never requires external
templates.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#backgro...>
Background

If you use the html-fragments-over-the-wire approach to web development
(for example with htmx <https://htmx.org/>), then you create many small
methods returning small html snippets.

If you have small methods, then keeping the HTML inside one file together
with your Python logic simplifies the development process. (See Locality of
Behaviour (LoB) <https://htmx.org/essays/locality-of-behaviour/>)

I guess a lot of people won't like this. Nevertheless some people like to
mix Python and HTML. Those people who don't like this, still can create
HTML in the way they like it.
<https://github.com/guettli/python-custom-strings/blob/main/README.md#pep>
PEP?

Do you think this could make it to a PEP? Please let me know: Just create
an issue here at Github.

Custom-Strings: Combine f-strings and condtional_escape()

Thomas Güttler

Chris Angelico

Matt del Valle

Guido van Rossum

Thomas Güttler

Guido van Rossum

Thomas Güttler

Thomas Güttler

Richard Damon

Thomas Güttler

Chris Angelico

Matt del Valle

Guido van Rossum

Thomas Güttler

Guido van Rossum

Thomas Güttler

Thomas Güttler

Richard Damon

Thomas Güttler

tags

participants (5)