pfst 0.2.6

Format-preserving high level AST editing for Python 3.10+.

pfst

High-level Python AST/CST manipulation that preserves formatting

Overview

This module exists in order to facilitate quick and easy high level editing of Python source in the form of an AST tree while preserving formatting. It is meant to allow you to change Python code functionality while not having to deal with the details of:

• Operator precedence and parentheses
• Indentation and line continuations
• Commas, semicolons, and tuple edge cases
• Comments and docstrings
• Various Python version-specific syntax quirks
• Lots more...

See Example Recipes for more in-depth examples.

>>> import fst  # pip install pfst, import fst

>>> ext_ast = fst.parse('''
... logger.info(  # just checking
...     f'not a {thing}', extra=extra,  # blah
... )'''.strip())

>>> ext_ast.f.body[0].value.insert('\nid=CID  # comment', -1, trivia=(False, False))

>>> print(fst.unparse(ext_ast))
logger.info(  # just checking
    f'not a {thing}',
    id=CID,  # comment
    extra=extra,  # blah
)

The tree is just normal AST with metadata, so if you know AST, you know FST.

>>> import ast

>>> print(ast.unparse(ext_ast))
logger.info(f'not a {thing}', id=CID, extra=extra)

fst works by adding FST nodes to existing standard Python AST nodes as an .f attribute (type-safe accessor castf() provided) which keep extra structure information, the original source, and provide the interface to format-preserving operations. Each operation through fst is a simultaneous edit of the AST tree and the source code and those are kept synchronized so that the current source will always parse to the current tree.

Links

• Repository
• Documentation
• PyPI

Install

From PyPI:

pip install pfst

From GitHub using pip:

pip install git+https://github.com/tom-pytel/pfst.git

From GitHub, after cloning for development:

pip install -e .[dev]

Example

Maybe you need to convert modern Python type annotations into older style type comments for something which doesn't understand annotations?

>>> from fst import *

>>> def type_annotations_to_type_comments(src: str) -> str:
...     fst_ = FST(src)  # same as "fst.parse(src).f"
...
...     # walk the whole tree but only yield AnnAssign nodes
...     for f in fst_.walk(AnnAssign):
...         # if just an annotation then skip it, alternatively could
...         # clean and store for later addition to __init__() assign in class
...         if not f.value:
...             continue
...
...         # own_src() gives us the original source exactly as written but dedented
...         target = f.target.own_src()
...         value = f.value.own_src()
...
...         # we use ast_src() for the annotation to get a clean type string
...         annotation = f.annotation.ast_src()
...
...         # preserve any existing end-of-line comment
...         comment = ' # ' + comment if (comment := f.get_line_comment()) else ''
...
...         # reconstruct the line using the PEP 484 type comment style
...         new_src = f'{target} = {value}  # type: {annotation}{comment}'
...
...         # replace the node, trivia=False preserves any leading comments
...         f.replace(new_src, trivia=False)
...
...     return fst_.src  # same as fst.unparse(fst_.a)

>>> print(type_annotations_to_type_comments("""
... def func():
...     normal = assign
...
...     x: int = 1
...
...     # y is such and such
...     y: float = 2.0  # more about y
...     # y was a good variable...
...
...     structure: tuple[
...         tuple[int, int],  # extraneous comment
...         dict[str, Any],   # could break stuff
...     ] | None = None# blah
...
...     call(  # invalid but just for demonstration purposes
...         some_arg,          # non-extraneous comment
...         some_kw=kw_value,  # will not break stuff
...     )[start : stop].attr: SomeClass = getthis()
... """.strip()))
def func():
    normal = assign

    x = 1  # type: int

    # y is such and such
    y = 2.0  # type: float # more about y
    # y was a good variable...

    structure = None  # type: tuple[tuple[int, int], dict[str, Any]] | None # blah

    call(  # invalid but just for demonstration purposes
        some_arg,          # non-extraneous comment
        some_kw=kw_value,  # will not break stuff
    )[start : stop].attr = getthis()  # type: SomeClass

Robust

Crazy syntax is handled correctly, which is a main goal of this module.

>>> f = FST(r'''
... if True:
...     @decorator1
...
...     # pre-comment
...     \
...  @ \
...   ( decorator2 )(
...         a,
...     ) \
...     # post-comment
...
...     @ \
...     decorator3()
...
...     def func(): weird\
...  ; \
... \
... stuff()
...
...     pass
... '''.strip())

>>> deco = f.body[0].get_slice(1, 2, 'decorator_list', cut=True, trivia=('all-', 'all-'))

>>> deco.dump('stmt+')
0: # pre-comment
1: \
2: @ \
3: ( decorator2 )(
4:     a,
5: ) \
6: # post-comment
_decorator_list - ROOT 0,0..8,0
  .decorator_list[1]
   0] Call - 4,0..6,1
     .func Name 'decorator2' Load - 4,2..4,12
     .args[1]
      0] Name 'a' Load - 5,4..5,5

>>> print(f.src)
if True:
    @decorator1
    @ \
    decorator3()

    def func(): weird\
 ; \
\
stuff()

    pass

>>> f.body[0].put_slice(deco, 'decorator_list', trivia=('all-', 'all-'))

>>> f.body[0].body[0] = 'good'

>>> print(f.src)
if True:
    # pre-comment
    \
    @ \
    ( decorator2 )(
        a,
    ) \
    # post-comment
    def func():
        good
        stuff()

    pass

Misc

Familiar AST structure and pythonic operations.

>>> f = FST('if a:\n    print(a)')

>>> f.test = 'not a'

>>> f.body.append('return a')

>>> print(f.src)
if not a:
    print(a)
    return a

Higher level slice abstraction.

>>> print(FST('a < b < c')._all[:2].copy().src)
a < b

>>> f = FST('case {1: a, 2: b, **c}: pass')  # match_case

>>> print(f.pattern.get_slice(1, 3, '_all').src)
{2: b, **c}

>>> f = FST('call(a, b=c, *d)')

>>> f[1:] = '*e, f=g, **h'

>>> print(f.src)
call(a, *e, f=g, **h)

>>> print(FST('def f(a, /, b=2, *c, d=4, **e): pass').args[-2:].copy().src)
*, d=4, **e

Use native AST.

>>> f = FST('i = [a, b, c]')

>>> f.targets[0] = Subscript(Name('j'), Slice(Name('x'), Name('y')))

>>> f.value.elts[1:] = Name('d')

>>> print(f.src)
j[x:y] = [a, d]

Traversal is in syntactic order.

>>> list(f.src for f in FST('call(a, x=1, *b, y=2, **c)').walk())[1:]
['call', 'a', 'x=1', '1', '*b', 'b', 'y=2', '2', '**c', 'c']

>>> list(f.src for f in FST('def func[T](a=1, b=2) -> int: pass').walk())[1:]
['T', 'a=1, b=2', 'a', '1', 'b', '2', 'int', 'pass']

>>> list(f.src for f in FST('{key1: val1, **val2, key3: val3}').walk())[1:]
['key1', 'val1', 'val2', 'key3', 'val3']

Locations are zero based in character units, not bytes. Most nodes have a location, including ones which don't in AST nodes.

>>> FST('蟒=Æ+д').dump()
Assign - ROOT 0,0..0,5
  .targets[1]
   0] Name '蟒' Store - 0,0..0,1
  .value BinOp - 0,2..0,5
    .left Name 'Æ' Load - 0,2..0,3
    .op Add - 0,3..0,4
    .right Name 'д' Load - 0,4..0,5

For more examples see the documentation in docs/, or if you're feeling particularly masochistic have a look at the tests in the tests/ directory.

TODO

This module is not finished but functional enough that it can be useful.

• Put one to:

  • FormattedValue.conversion
  • FormattedValue.format_spec
  • Interpolation.str
  • Interpolation.conversion
  • Interpolation.format_spec

• Prescribed get / put slice from / to:

  • MatchClass.patterns+kwd_attrs:kwd_patterns
  • JoinedStr.values
  • TemplateStr.values

• Improve comment and whitespace handling, especially allow get / put comments in single element non-statement operations where it may apply (where comment may belong to expression instead of statement). Allow specify insert line. Direct comment manipulation functions.

• Indentation of multiline sequences should be better, tree search / match, decide between primitive or node ops on primitive fields, different source encodings, code cleanups, API additions for real-world use, optimization, testing, bughunting, etc...

• Finish reconcile(). Proper comment handling, locations and deduplication. Make it use all slice operations to preserve more formatting.

Trivia

The "F" in FST stands for "Fun".