Compatible AST construction and ast.unparse for Python 3.5-3.13.

• backward compatibility for constructing ASTs

  For instance, ast.arguments is changed since Python 3.8, that another field posonlyarg is introduced, hence you should construct an ast.arguments with an additional argument posonlyarg=... since Python 3.8 and it wouldn't work if your code is written before Python 3.8.

  Through ast_compat, you don't have to worry about above question if a new argument is optional or a list.

  Note that you should use ast_compat.XXX instead of ast.XXX to construct ASTs.

• Support ast.Constant before Python 3.6.

  ast.Constant is convenient, and things like ast.Num are redundant according to this observation and improvement: https://bugs.python.org/issue32892

  However, ast.Constant is not available in 3.5 or earlier versions, thus we backport ast.Constant in this library.

  To access the content of ast.Constant in a compatible way, use ast_compat.get_constant instead of .value.

• Support dumping AST to string with ast_compat.unparse, which synchronizes the tooling code provided by CPython official repo.

import ast_compat as astc
from ast_compat import get_constant

assert get_constant(astc.Constant((1, 2))) == (1, 2)

empty_args = astc.arguments() # work for all of Python 3.5-3.13

The compatibility is following the specification of Python ASTs, i.e, the ADSL file you can find at

https://github.com/python/cpython/blob/<branch/tag>/Parser/Python.asdl.

We parse the file, and generate verifications and default argument factories, check ast_compat/compat3k*_{ast|unparse}.py. To explain, ast_compat/compat3k5_*.py is for Python 3.5, and ast_compat/compat3k9_*.py is for Python 3.9.

The file of generator is generate_ast_compat.py, and the use of generator API is in this way:

from generate_ast_compat import compat
compat((3, 5)) # generate ast_compat/compat3k5_ast.py and  ast_compat/compat3k5_unparse.py
compat((3, 6))
compat((3, 7))
compat((3, 8))
prerelease_url = "https://raw.githubusercontent.com/python/cpython/v3.9.0a3/parser/python.asdl"
compat((3, 9), prerelease_url)

The code generation needs Python 3.7+, though this library works for Python 3.5 and 3.5+.