As discussed in https://mail.python.org/archives/list/python-dev@python.org/thread/JAQDBMC23HW2PQ27HQNJ7G244T423IDD/ I propose to expose the unparse.py tool as part of the standard library in the ast module.

The exposed function will have the interface:

ast.unparse(ast_obj, *, option1, option2,...)

and will return a string with the unparsed version of ast_obj.

The unparse tool will need some cosmetic improvements that will be tracked separately in this issue.

After PR17302 is merged we need to fix the following cosmetic issues indicated by Victor:

(*) unparse adds many useless parentheses. The algorithm seems naive.
For example, it adds "()" to "class _AddedDllDirectory():". It also
adds parenthesis around yield, like "(yield (top, dirs, nondirs))",
whereas the AST node was at "top level": it isn't a sub-expression.
Maybe this algortihm should be made smarter.

(*) newlines in docstring are rendered as two characters: "\" + "n"
(escaped newline: \n), rather than a newline character. I would expect
a newline, it's more common that \n... But it may "break" inline
doctests rendering... Maybe it should be an option (render newlines as
newline character or escape them?), or maybe even let the user choose
how to render them using a callback (that's related to the "pluggable
formatter" question).

(*) Indentation is hardcoded to 4 spaces. What if I want 2 spaces or
something different? Should it become an option?

(*) Float infinity is replaces with 1e309. Again, maybe someone wants
to render this differently? It sounds like an arbitrary choice (which
"works" as expected).

After PR 17302 is accepted, I'll work on refactorings including a precedence algorithm to find when to parentheses.

@gvanrossum are you OK with adding type comments support? Current version loses type comment information so if typed_ast parses this, they wont be the same in AST representation.

> @gvanrossum are you OK with adding type comments support? Current version loses type comment information so if typed_ast parses this, they wont be the same in AST representation.

Good catch, definitely do that!

New changeset 27fc3b6f3fc49a36d3f962caac5c5495696d12ed by Pablo Galindo in branch 'master':
bpo-38870: Expose a function to unparse an ast object in the ast module (GH-17302)
https://github.com/python/cpython/commit/27fc3b6f3fc49a36d3f962caac5c5495696d12ed

New changeset ded8888fbc33011dd39b7b1c86a5adfacc4943f3 by Miss Islington (bot) (Pablo Galindo) in branch 'master':
bpo-38870: Remove dependency on contextlib to avoid performance regression on import (GH-17376)
https://github.com/python/cpython/commit/ded8888fbc33011dd39b7b1c86a5adfacc4943f3