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Chapter 4
Usage

4.1 Package content

TPG is a package which main function is to take a grammar and return a parser1. You only need to import TPG and use these four objects:

tpg.compile(grammar):
This function takes a grammar in a string and produces a parser in Python (also in a string). You can call exec to actually build it.
tpg.LexerError:
This exception is raised when the lexer fails.
tpg.ParserError:
This exception is raised when the parser fails.
tpg.SemanticError:
This exception is raised by the grammar itself when some semantic properties fail.

The grammar must be in a string (see figure 4.1).


Figure 4.1: Grammar embeding example
    my_grammar = r"""  
 
    parser Foo:  
 
        START/x -> Bar/x .  
 
        Bar/x -> 'bar'/x .  
 
    """

The tpg.compile function produces Python code from the grammar (see figure 4.2).


Figure 4.2: Parser compilation example
    exec(tpg.compile(my_grammar))    # Compiles my_grammar

Then you can use the new generated parser. The parser is now simply a Python class (see figure 4.3).


Figure 4.3: Parser usage example
    test = "bar"  
    my_parser = Foo()  
    x = my_parser(test)               # Uses the START symbol  
    print x  
    x = my_parser.parse('Bar', test)  # Uses the Bar symbol  
    print x

4.2 Command line usage

The tpg script is just a wrapper for the package. It reads a grammar in a file and write the generated code in a Python script. To produce a Python script from a grammar you can use tpg as follow:
    tpg [-v|-vv] grammar.g [-o parser.py]

tpg accepts some options on the command line:

-v
turns tpg into a verbose mode (it displays parser names).
-vv
turns tpg into a more verbose mode (it displays parser names and simplified rules).
-o file.py
tells tpg to generate the parser in file.py. The default output file is grammar.py if -o option is not provided and grammar.g is the name of the grammar.

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