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POImproving error messages with pyparsing
 

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  1. POImproving error messages with pyparsing
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    <p><strong>Edit:</strong> I did a first version, which Eike helped me to advance quite a bit on it. I'm now stuck to a more specific problem, which I will describe bellow. You can have a look at the original question in the <a href="https://stackoverflow.com/posts/15897602/revisions">history</a></p> <hr> <p>I'm using pyparsing to parse a small language used to request specific data from a database. It features numerous keyword, operators and datatypes as well as boolean logic.</p> <p>I'm trying to improve the error message sent to the user when he does a syntax error, since the current one is not very useful. I designed a small example, similar to what I'm doing with the language aforementioned but much smaller:</p> <pre><code>#!/usr/bin/env python from pyparsing import * def validate_number(s, loc, tokens): if int(tokens[0]) != 0: raise ParseFatalException(s, loc, "number musth be 0") def fail(s, loc, tokens): raise ParseFatalException(s, loc, "Unknown token %s" % tokens[0]) def fail_value(s, loc, expr, err): raise ParseFatalException(s, loc, "Wrong value") number = Word(nums).setParseAction(validate_number).setFailAction(fail_value) operator = Literal("=") error = Word(alphas).setParseAction(fail) rules = MatchFirst([ Literal('x') + operator + number, ]) rules = operatorPrecedence(rules | error , [ (Literal("and"), 2, opAssoc.RIGHT), ]) def try_parse(expression): try: rules.parseString(expression, parseAll=True) except Exception as e: msg = str(e) print("%s: %s" % (msg, expression)) print(" " * (len("%s: " % msg) + (e.loc)) + "^^^") </code></pre> <p>So basically, the only things which we can do with this language, is writing series of <code>x = 0</code>, joined together with <code>and</code> and parenthesis.</p> <p>Now, there are cases, when <code>and</code> and parenthesis are used, where the error reporting is not very good. Consider the following examples:</p> <pre><code>&gt;&gt;&gt; try_parse("x = a and x = 0") # This one is actually good! Wrong value (at char 4), (line:1, col:5): x = a and x = 0 ^^^ &gt;&gt;&gt; try_parse("x = 0 and x = a") Expected end of text (at char 6), (line:1, col:1): x = 0 and x = a ^^^ &gt;&gt;&gt; try_parse("x = 0 and (x = 0 and (x = 0 and (x = a)))") Expected end of text (at char 6), (line:1, col:1): x = 0 and (x = 0 and (x = 0 and (x = a))) ^^^ &gt;&gt;&gt; try_parse("x = 0 and (x = 0 and (x = 0 and (x = 0)))") Expected end of text (at char 6), (line:1, col:1): x = 0 and (x = 0 and (x = 0 and (xxxxxxxx = 0))) ^^^ </code></pre> <p>Actually, it seems that if the parser can't <strong>parse</strong> (and <em>parse</em> here is important) something after a <code>and</code>, it doesn't produce good error messages anymore :(</p> <p>And I mean <strong>parse</strong>, since if it can parse 5 but the "validation" fails in the parse action, it still produces a good error message. But, if it can't parse a valid number (like <code>a</code>) or a valid keyword (like <code>xxxxxx</code>), it stops producing the right error messages.</p> <p>Any idea?</p>
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    1. COHave a validating parse action for the variable names too. Or have a catch all variable name like `Word(alphas)`, and put a parse action on it, that always raises an exception.
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    2. COAlternatively you could do the validation one level up. Have a parser `Word(alphas) - "==" - Word(nums)` and put a more complex parse action on it, that looks for legal variable names, and ensures the correctness of the numbers.
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    3. COAt the moment, that would be a last resort solution :)
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