rply | port David Beazley 's PLY to RPython , and give it a cooler

if I understood your question i think you mean to print an empty line if that is the question you have to type in :

The real problem is not in your parser; it's in the code which calls the parser in main.py, which looks roughly like this (with some irrelevant lines removed):

I found your question a bit hard to follow. First you say that the parse throws an exception, and then you show the parse working as expected (except that you apparently didn't expect the print function to print its argument).

But I guess you mean that when you modify the grammar to include a print statement, it throws errors on previously accepted inputs.

To start with the second question: None is the value returned by the eval method of the AST returned by parser.parse. Before returning, eval implemented the print statement by printing the argument. None is then printed because you print the value returned by the eval method. (You might want to suppress printing None instead of printing whatever eval returns.)

If you mean that the new grammar doesn't accept what the old grammar accepted, that is presumably because the old grammar accepted an expression as an input. Maybe the original first rule was Program : expression or maybe the first rule was a rule for the expression non-terminal. Hard to say without seeing the old grammar as well. But clearly, your new grammar specifies Program as the start symbol, and Program only matches an input which starts with the token print. If the first token is something else, such as the integer 5, you can expect a syntax error to be produced because there is no production for Program which applies.

If you want the generated parser to accept either a print statement or a simple expression, you'll need to add a production which allows that.

parser is a module in Python's standard library, so I believe your code is trying to import Parser from that file, not from your parser.py file. Since that object does not exist, you get the ImportError you see.

Your code indicates that you tested the lexer. However, the test indicates that the lexer is not producing the correct tokens: