[Python-Dev] A minimal Python interpreter written in Python for experimenting with language changes

[asrp]

> Message-ID: <20180206034013.GZ26553 at ando.pearwood.info>
> 
> On Sat, Feb 03, 2018 at 11:45:15AM +0100, asrp wrote:
> 
> > > Can you give an example of how you would do that? I don't mean the 
> > > mechanism used, I mean how would a developer implement a new syntactic 
> > > feature. Suppose I wanted to add a new clause to for...else, let's say:
> > >
> > >     for ... :
> > >        block
> > >     otherwise:
> > >        # runs only if the for-loop was empty
> > > 
> > > How would do I do that?
> [...]
> > If you tell me a bit more about the intended behaviour of "otherwise", 
> > I'd be happy to do an example with that clause.
> 
> 
> Here's a faked session showing the sort of thing I am referring to. 
> (Note that this is just an example, not a proposal for a new language 
> feature.)
> 
> for x in [1, 2, 3]:
>     print(x)
> otherwise:
>     print("nothing there")
> 
> 
> prints 1, 2, 3.
> 
> for x in []:
>     print(x)
> otherwise:
>     print("nothing there")
> 
> prints "nothing there". In other words, the otherwise block runs if, and 
> only if, the loop iterable is empty and the for block does NOT run.
> 
> Can you do something like that?
> 

Oh, I see. Yes, definitely. This time I'll change for_stmt in lib/simple_ast.py beforehand (but runtime reload also works).

def for_stmt(index_var, iterable, block, else_block, otherwise_block):
    iterator = iter(evaluate(iterable))
    try:
        assignment(index_var, iterator.next())
    except StopIteration:
        evaluate(otherwise_block)
        return
    while_true:
        __caller__['__continue__'] = __continue__
        __caller__['__break__'] = __break__
        evaluate(block)
        try:
            assignment(index_var, iterator.next())
        except StopIteration:
            evaluate(else_block)
            return

Then start the interpreter

$ ipython -i test/python_repl.py 
p>> simport simple_ast
p>> ^D
[...]
In [1]: grammar = python_grammar.full_definition + python_grammar.extra

In [2]: grammar += r"""
   ...: for_stmt = "for" {exprlist} "in" {testlist} ":" {suite} {((SAME_INDENT "else" ":" {suite}) | void=pass_stmt) ((SAME_INDENT "otherwise" ":" {suite}) | void=pass_stmt)}
   ...: """

In [3]: pyterp.parser = python.Interpreter(i3.parse("grammar", grammar))

In [4]: pyterp.repl()
p>> for x in [1, 2]:
...   print(x)
... otherwise:
...   print("Nothing there")
... 
1
2
p>> for x in []:
...   print(x)
... otherwise:
...   print("Nothing there")
... 
Nothing there
p>> for x in [1, 2]:
...   print(x)
... else:
...   print("Something there")
... otherwise:
...   print("Nothing there")
... 
1
2
Something there


I noticed since my last post that the else and otherwise blocks don't need to be named if they are positional in for_stmt. The grammar change here reflects that.

I've also posted an example with `until_stmt` and I talk a bit more about debugging changes made there.

    http://blog.asrpo.com/adding_new_statement

(Nothing here is a proposed language change, just demos.)

asrp

> 
> 
> -- 
> Steve
> 
>