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pasta: Python AST Augmentation

This is still a work-in-progress; there is much more to do. Existing functionality may not be perfect.


Enable python source code refactoring through AST modifications.

Sample use cases:

  • Facilitate moving or renaming python modules by rewriting import statements.
  • Refactor code to enforce a certain style, such as reordering function definitions.
  • Safely migrate code from one API to another.

Design Goals

  • Symmetry: Given any input source, it should hold that pasta.dump(pasta.parse(src)) == src.
  • Mutability: Any changes made in the AST are reflected in the code generated from it.
  • Standardization: The syntax tree parsed by pasta will not introduce new nodes or structure that the user must learn.

Python Version Support

Supports python 2.7 and up to 3.8.


pasta depends on six.

Basic Usage

import pasta
tree = pasta.parse(source_code)

# ... Augment contents of tree ...

source_code = pasta.dump(tree)

Built-in Augmentations

Pasta includes some common augmentations out-of-the-box. These can be used as building blocks for more complex refactoring actions.

There will be more of these basic augmentations added over time. Stay tuned!

Rename an imported name

Rewrites references to an imported name, module or package. For some more examples, see pasta/augment/

# Rewrite references from one module to another
rename.rename_external(tree, 'pkg.subpkg.module', 'pkg.other_module')

# Rewrite references from one package to another
rename.rename_external(tree, 'pkg.subpkg', 'pkg.other_pkg')

# Rewrite references to an imported name in another module
rename.rename_external(tree, 'pkg.module.Query', 'pkg.module.ExecuteQuery')

Known issues and limitations

  • Changing the indentation level of a block of code is not supported. This is not an issue for renames, but would cause problems for refactors like extracting a method.

  • pasta works under the assumption that the python version that the code is written for and the version used to run pasta are the same. This is because pasta relies on ast.parse

  • Some python features are not fully supported, including global.


This project uses setuptools to facilitate testing and packaging.

# Run all tests
python test

# Run a single test suite
python test -s pasta.base.annotate_test.suite


This is not an official Google product.