PyDev Grammar

This page shows the steps needed for modifying and creating a new grammar.

Where to start?

The org.python.pyev.parser plugin contains all the grammars supported by PyDev. JavaCC knowledge is needed to edit those files (most notably the python.jjt_template files -- python.jjt files should not be directly edited).

The most relevant packages are:


This package contains the AST (Abstract Syntax Tree) structure used by PyDev. It implements a visitor pattern so that clients can traverse the structure.

All the grammars generate the same AST. This means it must remain compatible with all versions of Python -- that isn't much a problem as new features arrive, as it'd be just a matter of creating new AST nodes which will only exist on certain versions and not others or simply creating new fields in the nodes.

For example, the 'with' construct was introduced on Python 2.5, so there is a 'With' AST node, but the Python 2.4 grammar doesn't generate it.

To change this structure, the file to be changed is Python.asdl and should be run after it's changed (with the Python.asdl file as a parameter) so that the nodes are regenerated (and the is also the place to be edited if more features are needed in the nodes structure).


Each of the grammar packages provides a specific implementation for grammar. Note that the PythonGrammarXXXXX classes are all automatically generated.

The TreeBuilder class is responsible for actually generating the nodes, and there's a base class to reuse code among many tree builders.

The python.jjt file is also automatically generated from the python.jjt_template file. To generate it, run the file at the org.python.pydev.parser.grammarcommon package.

If you want to generate the files only for this grammar (usually while testing), the ant build.xml can be used. To regenerate for all the grammars at the same time, use the ant build_all.xml (at the org.python.pydev.parser.grammarcommon package).


This package contains the classes that are common among all the grammars and provides a to generate the python.jjt files and an ant build_all.xml to regenerate all the PythonGrammar classes (note that ant build_all.xml doesn't call the

The can be edited to provide constructs that are common among more than one grammar.


One thing essential for code to get into PyDev is that it has to be properly tested. For example, on tests for the grammar, see the PyParserXXTest classes under tests/org.python.pydev.parser.


Note that the grammar is a fork of the Jython structure, but it has several changes to support the features needed in PyDev:


  • Provides a way to get comments and other tokens (specialsBefore and specialsAfter in SimpleNode, containing comments and other syntax tokens such as 'if', 'with', etc... which are needed for a more accurate pretty-printing process)
  • Faster (there were lots of optimizations in this area, such as a faster char stream, use of FastStringBuffer instead of StringBuffer, declaration of some classes as final, no creation of intermediary nodes, less memory use using a pool of strings during the parse, and so forth).
  • Structure changed so that multiple grammars can reuse the code that creates the tree structure and the grammar itself
  • Error handling is done in the grammar so that an AST can be properly generated even in the face of errors (although it's not 100% fail-proof).

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