How to stop `ast.parse` from converting numerical values into int/floats?

Question

For example::

>>> import ast
>>> print(type(ast.parse('1.2', mode='eval').body.n)
float

How do I let the parser convert a python source file into a syntax tree, while preserving the original values of nodes in str type? Because I need to convert for example '1.2' into exact values using fractions as precise as possible, without loosing any precision at all (the value 1.2 cannot be precisely represented in floating-point format).

Preferably I wish to do this without reimplementing the parser. Perhaps there are other parsers more suitable for this than the ast module.

BTW, I need to parse not only expressions but programs.

Answer 1

LibCST is a Python Concrete Syntax tree parser and toolkit which can be used to solve your problem. It provides a syntax tree looks like ast and preserve floating format as string. https://github.com/Instagram/LibCST/

https://libcst.readthedocs.io/en/latest/index.html

Here are some examples:

In [1]: import libcst as cst

In [2]: cst.parse_expression("1.2")
Out[2]:
Float(
    value='1.2',
    lpar=[],
    rpar=[],
)

In [3]: cst.parse_expression("1.2").value
Out[3]: '1.2'

In [4]: cst.parse_expression("5e-2").value
Out[4]: '5e-2'

Answer 2

The most advanced tool to work with AST that I know of is the former astng project that backed up pylint and similar tools from logilab. Now it is called Astroid and available from https://bitbucket.org/logilab/astroid/ If it not won't help, then probably nothing will do.