How to parse over a Python expr?

Erik Max Francis max at alcyone.com
Fri Aug 30 22:28:53 CEST 2002


Stefan Franke wrote:

> 2. I guess any $(expr)-like string interpolation mechanism needs the
> same
> thing. As does - in a similar fashion, restricted to string literals -
> any CSV (comma
> separated value) parser.

In empy, Python expressions are set off with the @(...) notation.  In
order to pull out the expression, I walk along the substring ...,
keeping track of the depth (starting with zero) of the parentheses (an
open parenthesis increments the depth; a close parenthesis decrements
it) as I go, and ignoring any parentheses that appear inside of a string
literal.  That way I know when I'm done when I hit a close parenthesis
that marks the end of the whole expression, rather than just an internal
parenthesis, when the depth drops below zero.

I then leave it up to the Python interpreter itself, via eval, to
determine whether or not the expression is valid.  If all you're looking
for is a way to set off Python expressions in some embedded text
environment, that should be enough.

-- 
 Erik Max Francis / max at alcyone.com / http://www.alcyone.com/max/
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 A lambda calculus explorer in Python.



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