Overview

This module exists in order to facilitate quick and easy editing of Python source while preserving formatting. It automatically deals with all the silly nonsense like indentation, parentheses, commas, comments, docstrings, semicolons, line continuations, precedence, else vs. elif, etc... And especially the many, many niche special cases of Python syntax.

fst provides its own format-preserving operations for AST trees, but also allows the AST tree to be changed by anything else outside of its control and can then reconcile the changes with what it knows to preserve formatting where possible. It works by adding FST nodes to existing AST nodes as an .f attribute which keep extra structure information, the original source, and provide the interface to the format-preserving operations.

The fact that it just extends existing AST nodes means that the AST tree can be used (and edited) as normal anywhere that AST is used, and later unparse() with formatting preserved where it can be. The degree to which formatting is preserved depends on how many operations are executed natively through fst mechanisms and how well FST.reconcile() works for those operations which are not.

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]

Examples

Format preserving parse and unparse:

>>> import fst

>>> a = fst.parse('''
... if a: func()  # comment
... else:
...     i = 1  # one
...     j = 2  # two
...     k = 3  # three
... '''.strip())  # drop-in ast.parse() replacement

>>> print(dump(a)[:80])  # normal AST
Module(body=[If(test=Name(id='a', ctx=Load()), body=[Expr(value=Call(func=Name(i

>>> print(fst.unparse(a))  # drop-in ast.unparse() replacement, with formatting
if a: func()  # comment
else:
    i = 1  # one
    j = 2  # two
    k = 3  # three

Basic operations:

>>> print(a.f.body[0].orelse[1].replace('call()  # something else').root.src)
if a: func()  # comment
else:
    i = 1  # one
    call()  # something else
    k = 3  # three

>>> print((old := a.f.body[0].orelse[1:].copy()).src)
call()  # something else
k = 3  # three

>>> a.f.body[0].put('if b:\n    pass  # noop', 'orelse')
>>> a.f.body[0].orelse[0].orelse = old

>>> print(a.f.src)
if a: func()  # comment
elif b:
    pass  # noop
else:
    call()  # something else
    k = 3  # three

Reconcile, edit AST outside fst control while preserving formatting:

>>> a = fst.parse('''
... def compute(x, y):
...     # Compute the weighted sum
...     result = (
...         x * 0.6  # x gets 60%
...         + y * 0.4  # y gets 40%
...     )
...
...     # Apply thresholding
...     if (
...         result > 10
...         # cap high values
...         and result < 100  # ignore overflow
...     ):
...         return result
...     else:
...         return 0
... '''.strip())

>>> m = a.f.mark()

>>> # pure AST manipulation
>>> a.body[0].body[0].value.left.right = Name(id='scalar1')
>>> a.body[0].body[0].value.right.right = Name(id='scalar2')
>>> a.body[0].body[-1].orelse[0] = (
...     If(test=Compare(left=Name(id='result'),
...                     ops=[Gt()],
...                     comparators=[Constant(value=1)]),
...        body=[a.body[0].body[-1].orelse[0]],
...        orelse=[Return(value=UnaryOp(op=USub(), operand=Constant(value=1)))]
...     )
... )

>>> print(a.f.reconcile(m).src)
def compute(x, y):
    # Compute the weighted sum
    result = (
        x * scalar1  # x gets 60%
        + y * scalar2  # y gets 40%
    )

    # Apply thresholding
    if (
        result > 10
        # cap high values
        and result < 100  # ignore overflow
    ):
        return result
    elif result > 1:
        return 0
    else:
        return -1

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

TODO

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

• Put one (non-raw) to FormattedValue / Interpolation conversion and format_spec, JoinedStr / TemplateStr values.

• Prescribed (non-raw) get / put slice from / to:

  • FunctionDef / AsyncFunctionDef / ClassDef.decorator_list
  • ClassDef.bases
  • Delete / Assign.targets
  • BoolOp.values
  • Call.args
  • comprehension.ifs
  • ListComp / SetComp / DictComp / GeneratorExp.generators
  • ClassDef / Call.keywords
  • Import / ImportFrom.names
  • With / AsyncWith.items
  • MatchMapping.keys / .patterns
  • MatchSequence / MatchClass / MatchOr.patterns
  • FunctionDef / AsyncFunctionDef / ClassDef / TypeAlias.type_params
  • Global.names / Nonlocal.names
  • JoinedStr / TemplateStr.values

• Improve comment handling and get/put specification and get rid of ugly trailing newlines.

• Lots of other stuff...

Trivia

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