On Nov 7, 7:48 am, [EMAIL PROTECTED] wrote: > I am trying to simulate the execution of some PLC ladder logic in > python. > > I manually modified the rungs and executed this within python as a > proof of concept, but I'd like to be able to skip the modification > step. My thought was that this might be able to be completed via > overloading, but I am not sure if (or how) it could be done. > > overloadings: > + ==> OR > * ==> AND > / ==> NOT > > Example original code: > A=/B+C*D > translates to: > A=not B or C and D > > I tried > def __add__ (a,b): > return (a or b) > > which gives me this: > > >>> x=False > >>> y=True > >>> x+y > 1 > >>> x=True > >>> x+y > 2 > > How can this be done?
This reminds me of a little project I wrote a long time ago to use operator overloading to compute the overall resistance of a network of resistors - I used - for series connections and | for parallel. You can see the code here: http://pastebin.com/m1e89aae9 If you are going to design a mini-DSL using overloading, you'll first have to choose which operators correspond to your syntax, working within those offered by Python. For NOT, you have only have two unary operators to choose from, ~ (__invert__) or - (__neg__). / is not supported as a unary operator, only as binary division (__div__ or __truediv__). For AND and OR, you have a wealth of binary operators from which to pick. But first, think about any precedence of operations. As I recall from my ladder diagramming days, logic was strictly left-to-right, with no precedence for one operation over the other (as opposed to common arithmetic operator precedence, in which in 4+2*3 evaluates as 4+(2*3), as opposed to (4+2)*3, which would be strict left-to-right evaluation). When you implement your DSL, you will still be subject to Python's definitions for operator precedence. So if you want strict left-to-right evaluation, then choose two operators at the same level of precedence, such as + and -, or * and /. On the other hand, if you want AND evaluated before OR (which is typical precedence in programming), then pick operators from two different levels of precedence. -- Paul -- http://mail.python.org/mailman/listinfo/python-list
