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A pure Python ECMAScript parser and engine.



A pure Python ECMAScript 5.1 parser and engine.


Installation from Source

Unpack the archive, cd into the source directory, and run the following command:

python install

Installation with pip and git

Assuming you have pip and git installed, run the following command to install from the GitHub repository:

pip install git+git://

Basic Usage

Evaluating ECMAScript

The installer will install a script named bigrig that provides a basic shell for executing scripts. With no arguments the script will launch an interactive read-evaluate-print loop:

$ bigrig
> (function() { return "Hello World!"; })()
Hello World!

Press Ctrl+D to exit the shell, or Ctrl+C to reset the prompt.

Positional filename arguments may be given, corresponding to script files that will be executed in the given order in the same execution context:

$ bigrig ./script.js ...

The console object provides a single log method that will print the toString representations of the given arguments:

$ bigrig
> console.log("Hello World!");
Hello World!

The --eval or -e flag can be specified to execute a string:

$ bigrig -e "console.log('test');"

Parsing ECMAScript

The main interface for using the parsing library is found in the bigrig.parser module. To parse an ECMAScript file into an abstract syntax tree, the utility function parse_file is provided:

from bigrig import parser
ast = parser.parse_file('/path/to/an/ecmascript/file.js', encoding='utf-8')

Upon encountering unparseable source the parser will throw a bigrig.parser.ParseException exception with what is hopefully a useful error message. Note that parse_file accepts the keyword arguments line, which is the line number of the start of the source file, column, which is the character offset on the current line at which the source file begins, and encoding, which is the character encoding of the source file so that it may be converted to unicode internally.

The utility function bigrig.parser.parse_string works in a similar fashion to parse_file except that it accepts source as a string instead of the path to a file. If you'd like to ascribe some kind of file name for location tracking information it accepts one in the keyword argument filename.

Lower-Level Parsing

If you would like more control over parsing productions, you can use the parser building utility functions found in bigrig.parser in the form of make_file_parser and make_string_parser. These utilities simply build a parser for the given inputs without attempting to parse anything. This might be useful to you if you want to see what the result of parsing a production other than Program is by calling one of the parse prefixed parsing methods. Here's a quick example of parsing a function declaration using a Parser object:

from bigrig import parser
source = 'function example() { console.log("example"); }'
parser_obj = parser.make_string_parser(source)
function_node = parser_obj.parse_function_declaration()

Playing with the Abstract Syntax Tree

The abstract syntax tree is comprised of bigrig.parser.node.Node objects, with some terminals being expressed as list, None and unicode objects. To navigate the tree, nodes provide a simple fields and attributes interface. Fields represent child nodes in the parse tree and attributes are metadata about the node. To examine a node's fields, an iterable of available field attributes is stored in the node_object.fields attribute and may be examined using the iter_fields generator method, which returns (name, value) pairs. If you simply want to iterate over the child values, nodes provide an iter_children generator method.

To see the available node types that are built by the default Parser class, have a look over the bigrig.parser.ast module. If these nodes types are insufficient for your needs, have a look at the bigrig.parser.factory module, which contains the base node building mixin-class that the default Parser class uses to build the abstract syntax tree. Making your own node factory parser mixin class will allow you to customize the abstract syntax tree that the parser will build.

Tokenizing ECMAScript

The ECMAScript tokenizing class is found in the bigrig.parser.scanner module. This module provides the utility functions make_file_scanner and make_string_scanner to quickly build tokenizers for ECMAScript source files and strings respectively. The Token types are defined within the bigrig.token module, so look there to see what the various lexical tokens are. The public interface of the scanner class consists simply of a next method, which produces the next lexical token from the input. To facilitate parsing source with lookahead, the bigrig.parser.scanner.TokenStream class provides a light buffering wrapper around Scanner objects, adding the peek method which returns the next Token in the source without advancing the stream state. Here's a quick example of tokenizing an ECMAScript string:

from bigrig.parser import make_string_scanner
source = 'if (token) { console.log(token); } else { console.log("error!"); }'
scanner_obj = scanner.make_string_scanner(source)
while True:
    token =
    if token.type == 'EOF':
    print token
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