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-literals> - Django's conditional_escape() <https://docs.djangoproject.com/en/3.2/ref/utils/#django.utils.html.conditional_escape> 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#motivation> 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_html> 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.conditional_escape> . <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#customization> 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#concerns> 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#background> 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.
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