Refex - refactoring expressions

Refex is a syntactically aware search-and-replace tool for Python, which allows you to specify code searches and rewrites using templates, or a more complex Clang-LibASTMatcher-like matcher interface.

Examples

Automatic parenthesis insertion: Refex will automatically insert parentheses to preserve the intended code structure:

$ echo "a = b.foo() * c" > test.py
$ refex --mode=py.expr '$x.foo()' --sub='$x.foo() + 1' -i test.py
...
$ cat test.py
a = (b.foo() + 1) * c

A naive regular expression replacement would have resulted in b.foo() + 1 * c, which is not equivalent, and is unrelated to the intended replacement.

Paired parentheses: Refex is aware of the full syntax tree, and will always match parentheses correctly:

$ echo "print(foo(bar(b))" > test.py
$ refex --mode=py.expr 'foo($x)' --sub='foo($x + 1)' -i test.py
...
$ cat test.py
a = print(foo(bar(b) + 1))

Here, a naive replacement using regular expressions could have resulted in either print(foo(bar(b)) + 1) or print(foo(bar(b) + 1)), depending on whether $x was matched greedily or non-greedily.

Combining replacements: you can pass multiple search/replace pairs to Refex which combine to do more complex rewrites. For example:

# Rewrites "self.assertTrue(x == False)" to "self.assertFalse(x)", even though
# that was not explicitly called out.
refex --mode=py.expr -i --iterate \
  --match='self.assertTrue($x == $y)'  --sub='self.assertEqual($x, $y)' \
  --match='self.assertEqual($x, False)' --sub='self.assertFalse($x)' \
  -R dir/

TODO: also describe --mode=py.

Getting started

Installation

TODO: Installation instructions will go here as soon as this gets up on PyPI, hang tight!

Current status

NOTE: Right now, the project is in the process of initial upload to github, PyPI, etc., so please don't publicize it widely. The initial release will be a 0.1 release.

Stable:

The APIs documented at https://refex.readthedocs.io/ are expected to remain mostly the same, except for trivial renames and moves.

These command-line interfaces are expected to remain roughly the same, without backwards-incompatible changes:

• --mode=py.expr
• --mode=fix
• --mode=re

Unstable

• All undocumented APIs (especially the API for creating a new matcher).
• --mode=py.stmt is missing many safety and convenience features.
• --mode=py, the matcher interface, will eventually need some fairly large restructuring to make it O(n), although simple uses should be unaffected.

(Also, all the stable parts are unstable too. This isn't a promise, just an expectation/statement of intent.)

Contributing

See the contribution guide

See Also

• asttokens: the token-preserving AST library that Refex is built on top of.
• Pasta: a code rewriting tool using AST mutation instead of string templates.
• Semgrep: cross-language AST search using a similar approach.
• lib2to3: the standard library's code rewriting tool based on the concrete syntax tree.

Disclaimer

You may have noticed Google copyright notices. This is not an officially supported Google product.