Quenelle

A basic semantic patching tool for Python 2 code, similar to Coccinelle. Semantic patching allows generic patches to be written which ignore incidental formatting, such as whitespace:

$ cat examples/lambda.rule
replace:
map(lambda V1: E1(V1), E2)
with:
map(E1, E2)
~/code/quenelle
$ ./quenelle examples/lambda.rule examples/lambda.py
====[ examples/lambda.py:8 ]================================
SREBMUN = map(lambda x: reverse_upper(x), numbers.split())
============================================================
  - map(lambda x: reverse_upper(x), numbers.split())
  + map(reverse_upper, numbers.split())


====[ examples/lambda.py:9 ]================================
srebmun = map(lambda y: reverse_lower(y), reversed(numbers.split()))
============================================================
  - map(lambda y: reverse_lower(y), reversed(numbers.split()))
  + map(reverse_lower, reversed(numbers.split()))

Building

Build in a cabal sandbox:

cabal sandbox init
cabal install --enable-tests --only-dependencies
cabal build && cabal test

Testing is a mixture of unit-tests using HUnit and QuickCheck tests. Example rule and source files are contained in the examples directory.

Example

A Quenelle rule consists of a replace stanza followed by a with stanza. Stanzas support all Python 2 syntax that is supported by language-python. Expressions and variables can be bound in the replace stanza and used in the with stanza using the E<n> and V<n> syntax respectively. If an expression or variable is not needed in the with stanza it can be bound with E or V instead.

$ cat examples/filter.rule
replace:
[E1 for E1 in E2 if E3(E1)]
with:
filter(E3, E2)
$ ./quenelle examples/filter.rule examples/filter.py
====[ examples/filter.py:2 ]================================
ys = [x for x in xs if len(x)]
============================================================
  - [x for x in xs if len(x)]
  + filter(len, xs)

This rule captures three expressions, E1, E2 and E3. All instances of a bound expression must be equal, so this rule would not match [x+1 for x in xs if f(x)] because not all occurrences of E1 do not match.

Numeric literals match based on their value, so a f(E1 + 0) stanza will match f(10 + 0x00000000).

Limitations

Quenelle only currently supports matching and replacing expressions. Statements cannot be transformed.