Idea for pure-python templates using AST.

Terry Reedy tjreedy at
Tue Aug 16 11:47:58 EDT 2011

On 8/16/2011 7:33 AM, Paul Wray wrote:
> Hello all
> Ive had what I think is a great idea for pure-python templates (I can
> almost hear the groans, bear with me...)
> For the impatient, proof of concept is at
> demonstrating simple substitution, balanced tags using context manager,
> subtemplates, and template inheritance.
> I'm posting here to get opinions on:
> * the merits of the idea, (or otherwise!)
> * whether there are any established/mature templating systems that use
> this approach, or whether its come up before,
> * ideas for improvements and advice on other aspects such as sandboxing
> * of course, to share the idea in case others want to use it
> Background: Ive been working on an application that recursively renders
> html/xml documents from heterogenoeus trees, with separate classes for
> each document component. First I built my own renderer but was
> dissatisfied with the repetitive code. Then looked at Mako and Jinja,
> and used Jinja but was still disatisfied, because I still needed a
> render method in each class to do preparation, and also the template
> which was centrally managed by the Jinja loader and environment. I found
> a way to call templates recursively via Jinja filters, but was not sure
> that it wouldnt blow up in my face, so I also had separate calls to
> render the children of each node, and then feed the value to the parent
> template. I kept thinking that there must be a way to get the brevity
> and clarity of declarative templates, plus the simplicity of pure python
> loops, tests and function calls.
> The idea:
> Python syntax allows a statement to be a bare literal or identifier.
> These have no effect on the program.

More generally, Python has expression statements, with the result of the 
expression ignored. These are usually function calls with side-effects. 
"print('x')" has no effect on the program and the return value is 
usually ignored.

> So the function below is legal python:
> def myFunc():
> 'a'
> x = 45
> 'b'; 'c'; x
> So is this (within the appropriate class context of course):
> def body(self, r):
> '<h1>'; self.heading; '</h1>'
> '<ul>'
> for itm in self.items:
> '<li>'; itm; '</li>'
> '</ul>'
> The idea is simply to use python ASTs to transform this code so that it
> accumulates the values of the bare expressions.

Interesting idea, though I have no experience for comparison.

Terry Jan Reedy

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