[New-bugs-announce] [issue35143] Annotations future requires unparse, but not accessible from Pythpn

Kay Hayen report at bugs.python.org
Fri Nov 2 04:28:53 EDT 2018

New submission from Kay Hayen <kay.hayen at gmail.com>:


in trying to know what to put into "__annotations__" at run time, the "from __future__ import annotations" pose a new problem. It is now necessary to "unparse" to ast code that is still there. 

Code to do so, is in C API "_PyAST_ExprAsUnicode", but won't work with the Python level ast objects.

I have a kludge, using "compile" and "exec", to get past that. It's pretty ugly to do it like this, and Nuitka cannot be alone in trying to predict the value of "__annotations__". 

This code is my "ast.unparse" replacement:

def unparse(node):
        _host_node = ast.parse("x:1")

        _host_node.body[0].annotation = node

        r = compile(_host_node, "<annotations>", "exec", 1048576, dont_inherit = True)

        # Using exec here, to compile the ast node tree back to string,
        # there is no accessible "ast.unparse", and this works as a hack
        # to convert our node to a string annotation, pylint: disable=exec-used
        m = {}
        exec(r, m) 

        return m["__annotations__"]['x']

I am caching "_host_node" in the real code.

Having a real "ast.unparse" would be better however. It seems that the building blocks are all there, just not in that form.


messages: 329114
nosy: kayhayen
priority: normal
severity: normal
status: open
title: Annotations future requires unparse, but not accessible from Pythpn
type: enhancement
versions: Python 3.7

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