bpo-38870: Expose a function to unparse an ast object in the ast module (GH-17302)

Add ast.unparse() as a function in the ast module that can be used to unparse an
ast.AST object and produce a string with code that would produce an equivalent ast.AST
object when parsed.

6 files changed, 772 insertions, 751 deletions

diff --git a/Doc/library/ast.rst b/Doc/library/ast.rst
index b468f42..a7e0729 100644
--- a/Doc/library/ast.rst
+++ b/Doc/library/ast.rst
@@ -161,6 +161,19 @@ and classes for traversing abstract syntax trees:
       Added ``type_comments``, ``mode='func_type'`` and ``feature_version``.
 
 
+.. function:: unparse(ast_obj)
+
+   Unparse an :class:`ast.AST` object and generate a string with code
+   that would produce an equivalent :class:`ast.AST` object if parsed
+   back with :func:`ast.parse`.
+
+   .. warning::
+      The produced code string will not necesarily be equal to the original
+      code that generated the :class:`ast.AST` object.
+
+   .. versionadded:: 3.9
+
+
 .. function:: literal_eval(node_or_string)
 
    Safely evaluate an expression node or a string containing a Python literal or
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index a3ad98d..9af5259 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -121,6 +121,11 @@ Added the *indent* option to :func:`~ast.dump` which allows it to produce a
 multiline indented output.
 (Contributed by Serhiy Storchaka in :issue:`37995`.)
 
+Added the :func:`ast.unparse` as a function in the :mod:`ast` module that can
+be used to unparse an :class:`ast.AST` object and produce a string with code
+that would produce an equivalent :class:`ast.AST` object when parsed.
+(Contributed by Pablo Galindo and Batuhan Taskaya in :issue:`38870`.)
+
 asyncio
 -------
 
diff --git a/Lib/ast.py b/Lib/ast.py
index 720dd48..97914eb 100644
--- a/Lib/ast.py
+++ b/Lib/ast.py
@@ -24,7 +24,9 @@
     :copyright: Copyright 2008 by Armin Ronacher.
     :license: Python License.
 """
+import sys
 from _ast import *
+from contextlib import contextmanager
 
 
 def parse(source, filename='<unknown>', mode='exec', *,
@@ -551,6 +553,697 @@ _const_node_type_names = {
     type(...): 'Ellipsis',
 }
 
+# Large float and imaginary literals get turned into infinities in the AST.
+# We unparse those infinities to INFSTR.
+_INFSTR = "1e" + repr(sys.float_info.max_10_exp + 1)
+
+class _Unparser(NodeVisitor):
+    """Methods in this class recursively traverse an AST and
+    output source code for the abstract syntax; original formatting
+    is disregarded."""
+
+    def __init__(self):
+        self._source = []
+        self._buffer = []
+        self._indent = 0
+
+    def interleave(self, inter, f, seq):
+        """Call f on each item in seq, calling inter() in between."""
+        seq = iter(seq)
+        try:
+            f(next(seq))
+        except StopIteration:
+            pass
+        else:
+            for x in seq:
+                inter()
+                f(x)
+
+    def fill(self, text=""):
+        """Indent a piece of text and append it, according to the current
+        indentation level"""
+        self.write("\n" + "    " * self._indent + text)
+
+    def write(self, text):
+        """Append a piece of text"""
+        self._source.append(text)
+
+    def buffer_writer(self, text):
+        self._buffer.append(text)
+
+    @property
+    def buffer(self):
+        value = "".join(self._buffer)
+        self._buffer.clear()
+        return value
+
+    @contextmanager
+    def block(self):
+        """A context manager for preparing the source for blocks. It adds
+        the character':', increases the indentation on enter and decreases
+        the indentation on exit."""
+        self.write(":")
+        self._indent += 1
+        yield
+        self._indent -= 1
+
+    def traverse(self, node):
+        if isinstance(node, list):
+            for item in node:
+                self.traverse(item)
+        else:
+            super().visit(node)
+
+    def visit(self, node):
+        """Outputs a source code string that, if converted back to an ast
+        (using ast.parse) will generate an AST equivalent to *node*"""
+        self._source = []
+        self.traverse(node)
+        return "".join(self._source)
+
+    def visit_Module(self, node):
+        for subnode in node.body:
+            self.traverse(subnode)
+
+    def visit_Expr(self, node):
+        self.fill()
+        self.traverse(node.value)
+
+    def visit_NamedExpr(self, node):
+        self.write("(")
+        self.traverse(node.target)
+        self.write(" := ")
+        self.traverse(node.value)
+        self.write(")")
+
+    def visit_Import(self, node):
+        self.fill("import ")
+        self.interleave(lambda: self.write(", "), self.traverse, node.names)
+
+    def visit_ImportFrom(self, node):
+        self.fill("from ")
+        self.write("." * node.level)
+        if node.module:
+            self.write(node.module)
+        self.write(" import ")
+        self.interleave(lambda: self.write(", "), self.traverse, node.names)
+
+    def visit_Assign(self, node):
+        self.fill()
+        for target in node.targets:
+            self.traverse(target)
+            self.write(" = ")
+        self.traverse(node.value)
+
+    def visit_AugAssign(self, node):
+        self.fill()
+        self.traverse(node.target)
+        self.write(" " + self.binop[node.op.__class__.__name__] + "= ")
+        self.traverse(node.value)
+
+    def visit_AnnAssign(self, node):
+        self.fill()
+        if not node.simple and isinstance(node.target, Name):
+            self.write("(")
+        self.traverse(node.target)
+        if not node.simple and isinstance(node.target, Name):
+            self.write(")")
+        self.write(": ")
+        self.traverse(node.annotation)
+        if node.value:
+            self.write(" = ")
+            self.traverse(node.value)
+
+    def visit_Return(self, node):
+        self.fill("return")
+        if node.value:
+            self.write(" ")
+            self.traverse(node.value)
+
+    def visit_Pass(self, node):
+        self.fill("pass")
+
+    def visit_Break(self, node):
+        self.fill("break")
+
+    def visit_Continue(self, node):
+        self.fill("continue")
+
+    def visit_Delete(self, node):
+        self.fill("del ")
+        self.interleave(lambda: self.write(", "), self.traverse, node.targets)
+
+    def visit_Assert(self, node):
+        self.fill("assert ")
+        self.traverse(node.test)
+        if node.msg:
+            self.write(", ")
+            self.traverse(node.msg)
+
+    def visit_Global(self, node):
+        self.fill("global ")
+        self.interleave(lambda: self.write(", "), self.write, node.names)
+
+    def visit_Nonlocal(self, node):
+        self.fill("nonlocal ")
+        self.interleave(lambda: self.write(", "), self.write, node.names)
+
+    def visit_Await(self, node):
+        self.write("(")
+        self.write("await")
+        if node.value:
+            self.write(" ")
+            self.traverse(node.value)
+        self.write(")")
+
+    def visit_Yield(self, node):
+        self.write("(")
+        self.write("yield")
+        if node.value:
+            self.write(" ")
+            self.traverse(node.value)
+        self.write(")")
+
+    def visit_YieldFrom(self, node):
+        self.write("(")
+        self.write("yield from")
+        if node.value:
+            self.write(" ")
+            self.traverse(node.value)
+        self.write(")")
+
+    def visit_Raise(self, node):
+        self.fill("raise")
+        if not node.exc:
+            if node.cause:
+                raise ValueError(f"Node can't use cause without an exception.")
+            return
+        self.write(" ")
+        self.traverse(node.exc)
+        if node.cause:
+            self.write(" from ")
+            self.traverse(node.cause)
+
+    def visit_Try(self, node):
+        self.fill("try")
+        with self.block():
+            self.traverse(node.body)
+        for ex in node.handlers:
+            self.traverse(ex)
+        if node.orelse:
+            self.fill("else")
+            with self.block():
+                self.traverse(node.orelse)
+        if node.finalbody:
+            self.fill("finally")
+            with self.block():
+                self.traverse(node.finalbody)
+
+    def visit_ExceptHandler(self, node):
+        self.fill("except")
+        if node.type:
+            self.write(" ")
+            self.traverse(node.type)
+        if node.name:
+            self.write(" as ")
+            self.write(node.name)
+        with self.block():
+            self.traverse(node.body)
+
+    def visit_ClassDef(self, node):
+        self.write("\n")
+        for deco in node.decorator_list:
+            self.fill("@")
+            self.traverse(deco)
+        self.fill("class " + node.name)
+        self.write("(")
+        comma = False
+        for e in node.bases:
+            if comma:
+                self.write(", ")
+            else:
+                comma = True
+            self.traverse(e)
+        for e in node.keywords:
+            if comma:
+                self.write(", ")
+            else:
+                comma = True
+            self.traverse(e)
+        self.write(")")
+
+        with self.block():
+            self.traverse(node.body)
+
+    def visit_FunctionDef(self, node):
+        self.__FunctionDef_helper(node, "def")
+
+    def visit_AsyncFunctionDef(self, node):
+        self.__FunctionDef_helper(node, "async def")
+
+    def __FunctionDef_helper(self, node, fill_suffix):
+        self.write("\n")
+        for deco in node.decorator_list:
+            self.fill("@")
+            self.traverse(deco)
+        def_str = fill_suffix + " " + node.name + "("
+        self.fill(def_str)
+        self.traverse(node.args)
+        self.write(")")
+        if node.returns:
+            self.write(" -> ")
+            self.traverse(node.returns)
+        with self.block():
+            self.traverse(node.body)
+
+    def visit_For(self, node):
+        self.__For_helper("for ", node)
+
+    def visit_AsyncFor(self, node):
+        self.__For_helper("async for ", node)
+
+    def __For_helper(self, fill, node):
+        self.fill(fill)
+        self.traverse(node.target)
+        self.write(" in ")
+        self.traverse(node.iter)
+        with self.block():
+            self.traverse(node.body)
+        if node.orelse:
+            self.fill("else")
+            with self.block():
+                self.traverse(node.orelse)
+
+    def visit_If(self, node):
+        self.fill("if ")
+        self.traverse(node.test)
+        with self.block():
+            self.traverse(node.body)
+        # collapse nested ifs into equivalent elifs.
+        while node.orelse and len(node.orelse) == 1 and isinstance(node.orelse[0], If):
+            node = node.orelse[0]
+            self.fill("elif ")
+            self.traverse(node.test)
+            with self.block():
+                self.traverse(node.body)
+        # final else
+        if node.orelse:
+            self.fill("else")
+            with self.block():
+                self.traverse(node.orelse)
+
+    def visit_While(self, node):
+        self.fill("while ")
+        self.traverse(node.test)
+        with self.block():
+            self.traverse(node.body)
+        if node.orelse:
+            self.fill("else")
+            with self.block():
+                self.traverse(node.orelse)
+
+    def visit_With(self, node):
+        self.fill("with ")
+        self.interleave(lambda: self.write(", "), self.traverse, node.items)
+        with self.block():
+            self.traverse(node.body)
+
+    def visit_AsyncWith(self, node):
+        self.fill("async with ")
+        self.interleave(lambda: self.write(", "), self.traverse, node.items)
+        with self.block():
+            self.traverse(node.body)
+
+    def visit_JoinedStr(self, node):
+        self.write("f")
+        self._fstring_JoinedStr(node, self.buffer_writer)
+        self.write(repr(self.buffer))
+
+    def visit_FormattedValue(self, node):
+        self.write("f")
+        self._fstring_FormattedValue(node, self.buffer_writer)
+        self.write(repr(self.buffer))
+
+    def _fstring_JoinedStr(self, node, write):
+        for value in node.values:
+            meth = getattr(self, "_fstring_" + type(value).__name__)
+            meth(value, write)
+
+    def _fstring_Constant(self, node, write):
+        if not isinstance(node.value, str):
+            raise ValueError("Constants inside JoinedStr should be a string.")
+        value = node.value.replace("{", "{{").replace("}", "}}")
+        write(value)
+
+    def _fstring_FormattedValue(self, node, write):
+        write("{")
+        expr = type(self)().visit(node.value).rstrip("\n")
+        if expr.startswith("{"):
+            write(" ")  # Separate pair of opening brackets as "{ {"
+        write(expr)
+        if node.conversion != -1:
+            conversion = chr(node.conversion)
+            if conversion not in "sra":
+                raise ValueError("Unknown f-string conversion.")
+            write(f"!{conversion}")
+        if node.format_spec:
+            write(":")
+            meth = getattr(self, "_fstring_" + type(node.format_spec).__name__)
+            meth(node.format_spec, write)
+        write("}")
+
+    def visit_Name(self, node):
+        self.write(node.id)
+
+    def _write_constant(self, value):
+        if isinstance(value, (float, complex)):
+            # Substitute overflowing decimal literal for AST infinities.
+            self.write(repr(value).replace("inf", _INFSTR))
+        else:
+            self.write(repr(value))
+
+    def visit_Constant(self, node):
+        value = node.value
+        if isinstance(value, tuple):
+            self.write("(")
+            if len(value) == 1:
+                self._write_constant(value[0])
+                self.write(",")
+            else:
+                self.interleave(lambda: self.write(", "), self._write_constant, value)
+            self.write(")")
+        elif value is ...:
+            self.write("...")
+        else:
+            if node.kind == "u":
+                self.write("u")
+            self._write_constant(node.value)
+
+    def visit_List(self, node):
+        self.write("[")
+        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
+        self.write("]")
+
+    def visit_ListComp(self, node):
+        self.write("[")
+        self.traverse(node.elt)
+        for gen in node.generators:
+            self.traverse(gen)
+        self.write("]")
+
+    def visit_GeneratorExp(self, node):
+        self.write("(")
+        self.traverse(node.elt)
+        for gen in node.generators:
+            self.traverse(gen)
+        self.write(")")
+
+    def visit_SetComp(self, node):
+        self.write("{")
+        self.traverse(node.elt)
+        for gen in node.generators:
+            self.traverse(gen)
+        self.write("}")
+
+    def visit_DictComp(self, node):
+        self.write("{")
+        self.traverse(node.key)
+        self.write(": ")
+        self.traverse(node.value)
+        for gen in node.generators:
+            self.traverse(gen)
+        self.write("}")
+
+    def visit_comprehension(self, node):
+        if node.is_async:
+            self.write(" async for ")
+        else:
+            self.write(" for ")
+        self.traverse(node.target)
+        self.write(" in ")
+        self.traverse(node.iter)
+        for if_clause in node.ifs:
+            self.write(" if ")
+            self.traverse(if_clause)
+
+    def visit_IfExp(self, node):
+        self.write("(")
+        self.traverse(node.body)
+        self.write(" if ")
+        self.traverse(node.test)
+        self.write(" else ")
+        self.traverse(node.orelse)
+        self.write(")")
+
+    def visit_Set(self, node):
+        if not node.elts:
+            raise ValueError("Set node should has at least one item")
+        self.write("{")
+        self.interleave(lambda: self.write(", "), self.traverse, node.elts)
+        self.write("}")
+
+    def visit_Dict(self, node):
+        self.write("{")
+
+        def write_key_value_pair(k, v):
+            self.traverse(k)
+            self.write(": ")
+            self.traverse(v)
+
+        def write_item(item):
+            k, v = item
+            if k is None:
+                # for dictionary unpacking operator in dicts {**{'y': 2}}
+                # see PEP 448 for details
+                self.write("**")
+                self.traverse(v)
+            else:
+                write_key_value_pair(k, v)
+
+        self.interleave(
+            lambda: self.write(", "), write_item, zip(node.keys, node.values)
+        )
+        self.write("}")
+
+    def visit_Tuple(self, node):
+        self.write("(")
+        if len(node.elts) == 1:
+            elt = node.elts[0]
+            self.traverse(elt)
+            self.write(",")
+        else:
+            self.interleave(lambda: self.write(", "), self.traverse, node.elts)
+        self.write(")")
+
+    unop = {"Invert": "~", "Not": "not", "UAdd": "+", "USub": "-"}
+
+    def visit_UnaryOp(self, node):
+        self.write("(")
+        self.write(self.unop[node.op.__class__.__name__])
+        self.write(" ")
+        self.traverse(node.operand)
+        self.write(")")
+
+    binop = {
+        "Add": "+",
+        "Sub": "-",
+        "Mult": "*",
+        "MatMult": "@",
+        "Div": "/",
+        "Mod": "%",
+        "LShift": "<<",
+        "RShift": ">>",
+        "BitOr": "|",
+        "BitXor": "^",
+        "BitAnd": "&",
+        "FloorDiv": "//",
+        "Pow": "**",
+    }
+
+    def visit_BinOp(self, node):
+        self.write("(")
+        self.traverse(node.left)
+        self.write(" " + self.binop[node.op.__class__.__name__] + " ")
+        self.traverse(node.right)
+        self.write(")")
+
+    cmpops = {
+        "Eq": "==",
+        "NotEq": "!=",
+        "Lt": "<",
+        "LtE": "<=",
+        "Gt": ">",
+        "GtE": ">=",
+        "Is": "is",
+        "IsNot": "is not",
+        "In": "in",
+        "NotIn": "not in",
+    }
+
+    def visit_Compare(self, node):
+        self.write("(")
+        self.traverse(node.left)
+        for o, e in zip(node.ops, node.comparators):
+            self.write(" " + self.cmpops[o.__class__.__name__] + " ")
+            self.traverse(e)
+        self.write(")")
+
+    boolops = {And: "and", Or: "or"}
+
+    def visit_BoolOp(self, node):
+        self.write("(")
+        s = " %s " % self.boolops[node.op.__class__]
+        self.interleave(lambda: self.write(s), self.traverse, node.values)
+        self.write(")")
+
+    def visit_Attribute(self, node):
+        self.traverse(node.value)
+        # Special case: 3.__abs__() is a syntax error, so if node.value
+        # is an integer literal then we need to either parenthesize
+        # it or add an extra space to get 3 .__abs__().
+        if isinstance(node.value, Constant) and isinstance(node.value.value, int):
+            self.write(" ")
+        self.write(".")
+        self.write(node.attr)
+
+    def visit_Call(self, node):
+        self.traverse(node.func)
+        self.write("(")
+        comma = False
+        for e in node.args:
+            if comma:
+                self.write(", ")
+            else:
+                comma = True
+            self.traverse(e)
+        for e in node.keywords:
+            if comma:
+                self.write(", ")
+            else:
+                comma = True
+            self.traverse(e)
+        self.write(")")
+
+    def visit_Subscript(self, node):
+        self.traverse(node.value)
+        self.write("[")
+        self.traverse(node.slice)
+        self.write("]")
+
+    def visit_Starred(self, node):
+        self.write("*")
+        self.traverse(node.value)
+
+    def visit_Ellipsis(self, node):
+        self.write("...")
+
+    def visit_Index(self, node):
+        self.traverse(node.value)
+
+    def visit_Slice(self, node):
+        if node.lower:
+            self.traverse(node.lower)
+        self.write(":")
+        if node.upper:
+            self.traverse(node.upper)
+        if node.step:
+            self.write(":")
+            self.traverse(node.step)
+
+    def visit_ExtSlice(self, node):
+        self.interleave(lambda: self.write(", "), self.traverse, node.dims)
+
+    def visit_arg(self, node):
+        self.write(node.arg)
+        if node.annotation:
+            self.write(": ")
+            self.traverse(node.annotation)
+
+    def visit_arguments(self, node):
+        first = True
+        # normal arguments
+        all_args = node.posonlyargs + node.args
+        defaults = [None] * (len(all_args) - len(node.defaults)) + node.defaults
+        for index, elements in enumerate(zip(all_args, defaults), 1):
+            a, d = elements
+            if first:
+                first = False
+            else:
+                self.write(", ")
+            self.traverse(a)
+            if d:
+                self.write("=")
+                self.traverse(d)
+            if index == len(node.posonlyargs):
+                self.write(", /")
+
+        # varargs, or bare '*' if no varargs but keyword-only arguments present
+        if node.vararg or node.kwonlyargs:
+            if first:
+                first = False
+            else:
+                self.write(", ")
+            self.write("*")
+            if node.vararg:
+                self.write(node.vararg.arg)
+                if node.vararg.annotation:
+                    self.write(": ")
+                    self.traverse(node.vararg.annotation)
+
+        # keyword-only arguments
+        if node.kwonlyargs:
+            for a, d in zip(node.kwonlyargs, node.kw_defaults):
+                if first:
+                    first = False
+                else:
+                    self.write(", ")
+                self.traverse(a),
+                if d:
+                    self.write("=")
+                    self.traverse(d)
+
+        # kwargs
+        if node.kwarg:
+            if first:
+                first = False
+            else:
+                self.write(", ")
+            self.write("**" + node.kwarg.arg)
+            if node.kwarg.annotation:
+                self.write(": ")
+                self.traverse(node.kwarg.annotation)
+
+    def visit_keyword(self, node):
+        if node.arg is None:
+            self.write("**")
+        else:
+            self.write(node.arg)
+            self.write("=")
+        self.traverse(node.value)
+
+    def visit_Lambda(self, node):
+        self.write("(")
+        self.write("lambda ")
+        self.traverse(node.args)
+        self.write(": ")
+        self.traverse(node.body)
+        self.write(")")
+
+    def visit_alias(self, node):
+        self.write(node.name)
+        if node.asname:
+            self.write(" as " + node.asname)
+
+    def visit_withitem(self, node):
+        self.traverse(node.context_expr)
+        if node.optional_vars:
+            self.write(" as ")
+            self.traverse(node.optional_vars)
+
+def unparse(ast_obj):
+    unparser = _Unparser()
+    return unparser.visit(ast_obj)
+
 
 def main():
     import argparse
diff --git a/Lib/test/test_tools/test_unparse.py b/Lib/test/test_unparse.py
index a958ebb..9197c8a 100644
--- a/Lib/test/test_tools/test_unparse.py
+++ b/Lib/test/test_unparse.py
@@ -3,19 +3,12 @@
 import unittest
 import test.support
 import io
-import os
+import pathlib
 import random
 import tokenize
 import ast
+import functools
 
-from test.test_tools import basepath, toolsdir, skip_if_missing
-
-skip_if_missing()
-
-parser_path = os.path.join(toolsdir, "parser")
-
-with test.support.DirsOnSysPath(parser_path):
-    import unparse
 
 def read_pyfile(filename):
     """Read and return the contents of a Python source file (as a
@@ -26,6 +19,7 @@ def read_pyfile(filename):
         source = pyfile.read()
     return source
 
+
 for_else = """\
 def f():
     for x in range(10):
@@ -119,18 +113,21 @@ with f() as x, g() as y:
     suite1
 """
 
+
 class ASTTestCase(unittest.TestCase):
     def assertASTEqual(self, ast1, ast2):
         self.assertEqual(ast.dump(ast1), ast.dump(ast2))
 
-    def check_roundtrip(self, code1, filename="internal"):
-        ast1 = compile(code1, filename, "exec", ast.PyCF_ONLY_AST)
-        unparse_buffer = io.StringIO()
-        unparse.Unparser(ast1, unparse_buffer)
-        code2 = unparse_buffer.getvalue()
-        ast2 = compile(code2, filename, "exec", ast.PyCF_ONLY_AST)
+    def check_roundtrip(self, code1):
+        ast1 = ast.parse(code1)
+        code2 = ast.unparse(ast1)
+        ast2 = ast.parse(code2)
         self.assertASTEqual(ast1, ast2)
 
+    def check_invalid(self, node, raises=ValueError):
+        self.assertRaises(raises, ast.unparse, node)
+
+
 class UnparseTestCase(ASTTestCase):
     # Tests for specific bugs found in earlier versions of unparse
 
@@ -174,8 +171,8 @@ class UnparseTestCase(ASTTestCase):
         self.check_roundtrip("-1e1000j")
 
     def test_min_int(self):
-        self.check_roundtrip(str(-2**31))
-        self.check_roundtrip(str(-2**63))
+        self.check_roundtrip(str(-(2 ** 31)))
+        self.check_roundtrip(str(-(2 ** 63)))
 
     def test_imaginary_literals(self):
         self.check_roundtrip("7j")
@@ -265,54 +262,67 @@ class UnparseTestCase(ASTTestCase):
         self.check_roundtrip(r"""{**{'y': 2}, 'x': 1}""")
         self.check_roundtrip(r"""{**{'y': 2}, **{'x': 1}}""")
 
+    def test_invalid_raise(self):
+        self.check_invalid(ast.Raise(exc=None, cause=ast.Name(id="X")))
+
+    def test_invalid_fstring_constant(self):
+        self.check_invalid(ast.JoinedStr(values=[ast.Constant(value=100)]))
+
+    def test_invalid_fstring_conversion(self):
+        self.check_invalid(
+            ast.FormattedValue(
+                value=ast.Constant(value="a", kind=None),
+                conversion=ord("Y"),  # random character
+                format_spec=None,
+            )
+        )
+
+    def test_invalid_set(self):
+        self.check_invalid(ast.Set(elts=[]))
+
 
 class DirectoryTestCase(ASTTestCase):
     """Test roundtrip behaviour on all files in Lib and Lib/test."""
-    NAMES = None
 
-    # test directories, relative to the root of the distribution
-    test_directories = 'Lib', os.path.join('Lib', 'test')
+    lib_dir = pathlib.Path(__file__).parent / ".."
+    test_directories = (lib_dir, lib_dir / "test")
+    skip_files = {"test_fstring.py"}
 
-    @classmethod
-    def get_names(cls):
-        if cls.NAMES is not None:
-            return cls.NAMES
+    @functools.cached_property
+    def files_to_test(self):
+        # bpo-31174: Use cached_property to store the names sample
+        # to always test the same files. It prevents false alarms
+        # when hunting reference leaks.
 
-        names = []
-        for d in cls.test_directories:
-            test_dir = os.path.join(basepath, d)
-            for n in os.listdir(test_dir):
-                if n.endswith('.py') and not n.startswith('bad'):
-                    names.append(os.path.join(test_dir, n))
+        items = [
+            item.resolve()
+            for directory in self.test_directories
+            for item in directory.glob("*.py")
+            if not item.name.startswith("bad")
+        ]
 
         # Test limited subset of files unless the 'cpu' resource is specified.
         if not test.support.is_resource_enabled("cpu"):
-            names = random.sample(names, 10)
-        # bpo-31174: Store the names sample to always test the same files.
-        # It prevents false alarms when hunting reference leaks.
-        cls.NAMES = names
-        return names
+            items = random.sample(items, 10)
+        return items
 
     def test_files(self):
-        # get names of files to test
-        names = self.get_names()
-
-        for filename in names:
+        for item in self.files_to_test:
             if test.support.verbose:
-                print('Testing %s' % filename)
+                print(f"Testing {item.absolute()}")
 
             # Some f-strings are not correctly round-tripped by
-            #  Tools/parser/unparse.py.  See issue 28002 for details.
-            #  We need to skip files that contain such f-strings.
-            if os.path.basename(filename) in ('test_fstring.py', ):
+            # Tools/parser/unparse.py.  See issue 28002 for details.
+            # We need to skip files that contain such f-strings.
+            if item.name in self.skip_files:
                 if test.support.verbose:
-                    print(f'Skipping {filename}: see issue 28002')
+                    print(f"Skipping {item.absolute()}: see issue 28002")
                 continue
 
-            with self.subTest(filename=filename):
-                source = read_pyfile(filename)
+            with self.subTest(filename=item):
+                source = read_pyfile(item)
                 self.check_roundtrip(source)
 
 
-if __name__ == '__main__':
+if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2019-11-20-22-43-48.bpo-38870.rLVZEv.rst b/Misc/NEWS.d/next/Library/2019-11-20-22-43-48.bpo-38870.rLVZEv.rst
new file mode 100644
index 0000000..61af368
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-11-20-22-43-48.bpo-38870.rLVZEv.rst
@@ -0,0 +1,4 @@
+Expose :func:`ast.unparse` as a function of the :mod:`ast` module that can
+be used to unparse an :class:`ast.AST` object and produce a string with code
+that would produce an equivalent :class:`ast.AST` object when parsed. Patch
+by Pablo Galindo and Batuhan Taskaya.
diff --git a/Tools/parser/unparse.py b/Tools/parser/unparse.py
deleted file mode 100644
index a5cc000..0000000
--- a/Tools/parser/unparse.py
+++ /dev/null
@@ -1,704 +0,0 @@
-"Usage: unparse.py <path to source file>"
-import sys
-import ast
-import tokenize
-import io
-import os
-
-# Large float and imaginary literals get turned into infinities in the AST.
-# We unparse those infinities to INFSTR.
-INFSTR = "1e" + repr(sys.float_info.max_10_exp + 1)
-
-def interleave(inter, f, seq):
-    """Call f on each item in seq, calling inter() in between.
-    """
-    seq = iter(seq)
-    try:
-        f(next(seq))
-    except StopIteration:
-        pass
-    else:
-        for x in seq:
-            inter()
-            f(x)
-
-class Unparser:
-    """Methods in this class recursively traverse an AST and
-    output source code for the abstract syntax; original formatting
-    is disregarded. """
-
-    def __init__(self, tree, file = sys.stdout):
-        """Unparser(tree, file=sys.stdout) -> None.
-         Print the source for tree to file."""
-        self.f = file
-        self._indent = 0
-        self.dispatch(tree)
-        print("", file=self.f)
-        self.f.flush()
-
-    def fill(self, text = ""):
-        "Indent a piece of text, according to the current indentation level"
-        self.f.write("\n"+"    "*self._indent + text)
-
-    def write(self, text):
-        "Append a piece of text to the current line."
-        self.f.write(text)
-
-    def enter(self):
-        "Print ':', and increase the indentation."
-        self.write(":")
-        self._indent += 1
-
-    def leave(self):
-        "Decrease the indentation level."
-        self._indent -= 1
-
-    def dispatch(self, tree):
-        "Dispatcher function, dispatching tree type T to method _T."
-        if isinstance(tree, list):
-            for t in tree:
-                self.dispatch(t)
-            return
-        meth = getattr(self, "_"+tree.__class__.__name__)
-        meth(tree)
-
-
-    ############### Unparsing methods ######################
-    # There should be one method per concrete grammar type #
-    # Constructors should be grouped by sum type. Ideally, #
-    # this would follow the order in the grammar, but      #
-    # currently doesn't.                                   #
-    ########################################################
-
-    def _Module(self, tree):
-        for stmt in tree.body:
-            self.dispatch(stmt)
-
-    # stmt
-    def _Expr(self, tree):
-        self.fill()
-        self.dispatch(tree.value)
-
-    def _NamedExpr(self, tree):
-        self.write("(")
-        self.dispatch(tree.target)
-        self.write(" := ")
-        self.dispatch(tree.value)
-        self.write(")")
-
-    def _Import(self, t):
-        self.fill("import ")
-        interleave(lambda: self.write(", "), self.dispatch, t.names)
-
-    def _ImportFrom(self, t):
-        self.fill("from ")
-        self.write("." * t.level)
-        if t.module:
-            self.write(t.module)
-        self.write(" import ")
-        interleave(lambda: self.write(", "), self.dispatch, t.names)
-
-    def _Assign(self, t):
-        self.fill()
-        for target in t.targets:
-            self.dispatch(target)
-            self.write(" = ")
-        self.dispatch(t.value)
-
-    def _AugAssign(self, t):
-        self.fill()
-        self.dispatch(t.target)
-        self.write(" "+self.binop[t.op.__class__.__name__]+"= ")
-        self.dispatch(t.value)
-
-    def _AnnAssign(self, t):
-        self.fill()
-        if not t.simple and isinstance(t.target, ast.Name):
-            self.write('(')
-        self.dispatch(t.target)
-        if not t.simple and isinstance(t.target, ast.Name):
-            self.write(')')
-        self.write(": ")
-        self.dispatch(t.annotation)
-        if t.value:
-            self.write(" = ")
-            self.dispatch(t.value)
-
-    def _Return(self, t):
-        self.fill("return")
-        if t.value:
-            self.write(" ")
-            self.dispatch(t.value)
-
-    def _Pass(self, t):
-        self.fill("pass")
-
-    def _Break(self, t):
-        self.fill("break")
-
-    def _Continue(self, t):
-        self.fill("continue")
-
-    def _Delete(self, t):
-        self.fill("del ")
-        interleave(lambda: self.write(", "), self.dispatch, t.targets)
-
-    def _Assert(self, t):
-        self.fill("assert ")
-        self.dispatch(t.test)
-        if t.msg:
-            self.write(", ")
-            self.dispatch(t.msg)
-
-    def _Global(self, t):
-        self.fill("global ")
-        interleave(lambda: self.write(", "), self.write, t.names)
-
-    def _Nonlocal(self, t):
-        self.fill("nonlocal ")
-        interleave(lambda: self.write(", "), self.write, t.names)
-
-    def _Await(self, t):
-        self.write("(")
-        self.write("await")
-        if t.value:
-            self.write(" ")
-            self.dispatch(t.value)
-        self.write(")")
-
-    def _Yield(self, t):
-        self.write("(")
-        self.write("yield")
-        if t.value:
-            self.write(" ")
-            self.dispatch(t.value)
-        self.write(")")
-
-    def _YieldFrom(self, t):
-        self.write("(")
-        self.write("yield from")
-        if t.value:
-            self.write(" ")
-            self.dispatch(t.value)
-        self.write(")")
-
-    def _Raise(self, t):
-        self.fill("raise")
-        if not t.exc:
-            assert not t.cause
-            return
-        self.write(" ")
-        self.dispatch(t.exc)
-        if t.cause:
-            self.write(" from ")
-            self.dispatch(t.cause)
-
-    def _Try(self, t):
-        self.fill("try")
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-        for ex in t.handlers:
-            self.dispatch(ex)
-        if t.orelse:
-            self.fill("else")
-            self.enter()
-            self.dispatch(t.orelse)
-            self.leave()
-        if t.finalbody:
-            self.fill("finally")
-            self.enter()
-            self.dispatch(t.finalbody)
-            self.leave()
-
-    def _ExceptHandler(self, t):
-        self.fill("except")
-        if t.type:
-            self.write(" ")
-            self.dispatch(t.type)
-        if t.name:
-            self.write(" as ")
-            self.write(t.name)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-
-    def _ClassDef(self, t):
-        self.write("\n")
-        for deco in t.decorator_list:
-            self.fill("@")
-            self.dispatch(deco)
-        self.fill("class "+t.name)
-        self.write("(")
-        comma = False
-        for e in t.bases:
-            if comma: self.write(", ")
-            else: comma = True
-            self.dispatch(e)
-        for e in t.keywords:
-            if comma: self.write(", ")
-            else: comma = True
-            self.dispatch(e)
-        self.write(")")
-
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-
-    def _FunctionDef(self, t):
-        self.__FunctionDef_helper(t, "def")
-
-    def _AsyncFunctionDef(self, t):
-        self.__FunctionDef_helper(t, "async def")
-
-    def __FunctionDef_helper(self, t, fill_suffix):
-        self.write("\n")
-        for deco in t.decorator_list:
-            self.fill("@")
-            self.dispatch(deco)
-        def_str = fill_suffix+" "+t.name + "("
-        self.fill(def_str)
-        self.dispatch(t.args)
-        self.write(")")
-        if t.returns:
-            self.write(" -> ")
-            self.dispatch(t.returns)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-
-    def _For(self, t):
-        self.__For_helper("for ", t)
-
-    def _AsyncFor(self, t):
-        self.__For_helper("async for ", t)
-
-    def __For_helper(self, fill, t):
-        self.fill(fill)
-        self.dispatch(t.target)
-        self.write(" in ")
-        self.dispatch(t.iter)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-        if t.orelse:
-            self.fill("else")
-            self.enter()
-            self.dispatch(t.orelse)
-            self.leave()
-
-    def _If(self, t):
-        self.fill("if ")
-        self.dispatch(t.test)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-        # collapse nested ifs into equivalent elifs.
-        while (t.orelse and len(t.orelse) == 1 and
-               isinstance(t.orelse[0], ast.If)):
-            t = t.orelse[0]
-            self.fill("elif ")
-            self.dispatch(t.test)
-            self.enter()
-            self.dispatch(t.body)
-            self.leave()
-        # final else
-        if t.orelse:
-            self.fill("else")
-            self.enter()
-            self.dispatch(t.orelse)
-            self.leave()
-
-    def _While(self, t):
-        self.fill("while ")
-        self.dispatch(t.test)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-        if t.orelse:
-            self.fill("else")
-            self.enter()
-            self.dispatch(t.orelse)
-            self.leave()
-
-    def _With(self, t):
-        self.fill("with ")
-        interleave(lambda: self.write(", "), self.dispatch, t.items)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-
-    def _AsyncWith(self, t):
-        self.fill("async with ")
-        interleave(lambda: self.write(", "), self.dispatch, t.items)
-        self.enter()
-        self.dispatch(t.body)
-        self.leave()
-
-    # expr
-    def _JoinedStr(self, t):
-        self.write("f")
-        string = io.StringIO()
-        self._fstring_JoinedStr(t, string.write)
-        self.write(repr(string.getvalue()))
-
-    def _FormattedValue(self, t):
-        self.write("f")
-        string = io.StringIO()
-        self._fstring_FormattedValue(t, string.write)
-        self.write(repr(string.getvalue()))
-
-    def _fstring_JoinedStr(self, t, write):
-        for value in t.values:
-            meth = getattr(self, "_fstring_" + type(value).__name__)
-            meth(value, write)
-
-    def _fstring_Constant(self, t, write):
-        assert isinstance(t.value, str)
-        value = t.value.replace("{", "{{").replace("}", "}}")
-        write(value)
-
-    def _fstring_FormattedValue(self, t, write):
-        write("{")
-        expr = io.StringIO()
-        Unparser(t.value, expr)
-        expr = expr.getvalue().rstrip("\n")
-        if expr.startswith("{"):
-            write(" ")  # Separate pair of opening brackets as "{ {"
-        write(expr)
-        if t.conversion != -1:
-            conversion = chr(t.conversion)
-            assert conversion in "sra"
-            write(f"!{conversion}")
-        if t.format_spec:
-            write(":")
-            meth = getattr(self, "_fstring_" + type(t.format_spec).__name__)
-            meth(t.format_spec, write)
-        write("}")
-
-    def _Name(self, t):
-        self.write(t.id)
-
-    def _write_constant(self, value):
-        if isinstance(value, (float, complex)):
-            # Substitute overflowing decimal literal for AST infinities.
-            self.write(repr(value).replace("inf", INFSTR))
-        else:
-            self.write(repr(value))
-
-    def _Constant(self, t):
-        value = t.value
-        if isinstance(value, tuple):
-            self.write("(")
-            if len(value) == 1:
-                self._write_constant(value[0])
-                self.write(",")
-            else:
-                interleave(lambda: self.write(", "), self._write_constant, value)
-            self.write(")")
-        elif value is ...:
-            self.write("...")
-        else:
-            if t.kind == "u":
-                self.write("u")
-            self._write_constant(t.value)
-
-    def _List(self, t):
-        self.write("[")
-        interleave(lambda: self.write(", "), self.dispatch, t.elts)
-        self.write("]")
-
-    def _ListComp(self, t):
-        self.write("[")
-        self.dispatch(t.elt)
-        for gen in t.generators:
-            self.dispatch(gen)
-        self.write("]")
-
-    def _GeneratorExp(self, t):
-        self.write("(")
-        self.dispatch(t.elt)
-        for gen in t.generators:
-            self.dispatch(gen)
-        self.write(")")
-
-    def _SetComp(self, t):
-        self.write("{")
-        self.dispatch(t.elt)
-        for gen in t.generators:
-            self.dispatch(gen)
-        self.write("}")
-
-    def _DictComp(self, t):
-        self.write("{")
-        self.dispatch(t.key)
-        self.write(": ")
-        self.dispatch(t.value)
-        for gen in t.generators:
-            self.dispatch(gen)
-        self.write("}")
-
-    def _comprehension(self, t):
-        if t.is_async:
-            self.write(" async for ")
-        else:
-            self.write(" for ")
-        self.dispatch(t.target)
-        self.write(" in ")
-        self.dispatch(t.iter)
-        for if_clause in t.ifs:
-            self.write(" if ")
-            self.dispatch(if_clause)
-
-    def _IfExp(self, t):
-        self.write("(")
-        self.dispatch(t.body)
-        self.write(" if ")
-        self.dispatch(t.test)
-        self.write(" else ")
-        self.dispatch(t.orelse)
-        self.write(")")
-
-    def _Set(self, t):
-        assert(t.elts) # should be at least one element
-        self.write("{")
-        interleave(lambda: self.write(", "), self.dispatch, t.elts)
-        self.write("}")
-
-    def _Dict(self, t):
-        self.write("{")
-        def write_key_value_pair(k, v):
-            self.dispatch(k)
-            self.write(": ")
-            self.dispatch(v)
-
-        def write_item(item):
-            k, v = item
-            if k is None:
-                # for dictionary unpacking operator in dicts {**{'y': 2}}
-                # see PEP 448 for details
-                self.write("**")
-                self.dispatch(v)
-            else:
-                write_key_value_pair(k, v)
-        interleave(lambda: self.write(", "), write_item, zip(t.keys, t.values))
-        self.write("}")
-
-    def _Tuple(self, t):
-        self.write("(")
-        if len(t.elts) == 1:
-            elt = t.elts[0]
-            self.dispatch(elt)
-            self.write(",")
-        else:
-            interleave(lambda: self.write(", "), self.dispatch, t.elts)
-        self.write(")")
-
-    unop = {"Invert":"~", "Not": "not", "UAdd":"+", "USub":"-"}
-    def _UnaryOp(self, t):
-        self.write("(")
-        self.write(self.unop[t.op.__class__.__name__])
-        self.write(" ")
-        self.dispatch(t.operand)
-        self.write(")")
-
-    binop = { "Add":"+", "Sub":"-", "Mult":"*", "MatMult":"@", "Div":"/", "Mod":"%",
-                    "LShift":"<<", "RShift":">>", "BitOr":"|", "BitXor":"^", "BitAnd":"&",
-                    "FloorDiv":"//", "Pow": "**"}
-    def _BinOp(self, t):
-        self.write("(")
-        self.dispatch(t.left)
-        self.write(" " + self.binop[t.op.__class__.__name__] + " ")
-        self.dispatch(t.right)
-        self.write(")")
-
-    cmpops = {"Eq":"==", "NotEq":"!=", "Lt":"<", "LtE":"<=", "Gt":">", "GtE":">=",
-                        "Is":"is", "IsNot":"is not", "In":"in", "NotIn":"not in"}
-    def _Compare(self, t):
-        self.write("(")
-        self.dispatch(t.left)
-        for o, e in zip(t.ops, t.comparators):
-            self.write(" " + self.cmpops[o.__class__.__name__] + " ")
-            self.dispatch(e)
-        self.write(")")
-
-    boolops = {ast.And: 'and', ast.Or: 'or'}
-    def _BoolOp(self, t):
-        self.write("(")
-        s = " %s " % self.boolops[t.op.__class__]
-        interleave(lambda: self.write(s), self.dispatch, t.values)
-        self.write(")")
-
-    def _Attribute(self,t):
-        self.dispatch(t.value)
-        # Special case: 3.__abs__() is a syntax error, so if t.value
-        # is an integer literal then we need to either parenthesize
-        # it or add an extra space to get 3 .__abs__().
-        if isinstance(t.value, ast.Constant) and isinstance(t.value.value, int):
-            self.write(" ")
-        self.write(".")
-        self.write(t.attr)
-
-    def _Call(self, t):
-        self.dispatch(t.func)
-        self.write("(")
-        comma = False
-        for e in t.args:
-            if comma: self.write(", ")
-            else: comma = True
-            self.dispatch(e)
-        for e in t.keywords:
-            if comma: self.write(", ")
-            else: comma = True
-            self.dispatch(e)
-        self.write(")")
-
-    def _Subscript(self, t):
-        self.dispatch(t.value)
-        self.write("[")
-        self.dispatch(t.slice)
-        self.write("]")
-
-    def _Starred(self, t):
-        self.write("*")
-        self.dispatch(t.value)
-
-    # slice
-    def _Ellipsis(self, t):
-        self.write("...")
-
-    def _Index(self, t):
-        self.dispatch(t.value)
-
-    def _Slice(self, t):
-        if t.lower:
-            self.dispatch(t.lower)
-        self.write(":")
-        if t.upper:
-            self.dispatch(t.upper)
-        if t.step:
-            self.write(":")
-            self.dispatch(t.step)
-
-    def _ExtSlice(self, t):
-        interleave(lambda: self.write(', '), self.dispatch, t.dims)
-
-    # argument
-    def _arg(self, t):
-        self.write(t.arg)
-        if t.annotation:
-            self.write(": ")
-            self.dispatch(t.annotation)
-
-    # others
-    def _arguments(self, t):
-        first = True
-        # normal arguments
-        all_args = t.posonlyargs + t.args
-        defaults = [None] * (len(all_args) - len(t.defaults)) + t.defaults
-        for index, elements in enumerate(zip(all_args, defaults), 1):
-            a, d = elements
-            if first:first = False
-            else: self.write(", ")
-            self.dispatch(a)
-            if d:
-                self.write("=")
-                self.dispatch(d)
-            if index == len(t.posonlyargs):
-                self.write(", /")
-
-        # varargs, or bare '*' if no varargs but keyword-only arguments present
-        if t.vararg or t.kwonlyargs:
-            if first:first = False
-            else: self.write(", ")
-            self.write("*")
-            if t.vararg:
-                self.write(t.vararg.arg)
-                if t.vararg.annotation:
-                    self.write(": ")
-                    self.dispatch(t.vararg.annotation)
-
-        # keyword-only arguments
-        if t.kwonlyargs:
-            for a, d in zip(t.kwonlyargs, t.kw_defaults):
-                if first:first = False
-                else: self.write(", ")
-                self.dispatch(a),
-                if d:
-                    self.write("=")
-                    self.dispatch(d)
-
-        # kwargs
-        if t.kwarg:
-            if first:first = False
-            else: self.write(", ")
-            self.write("**"+t.kwarg.arg)
-            if t.kwarg.annotation:
-                self.write(": ")
-                self.dispatch(t.kwarg.annotation)
-
-    def _keyword(self, t):
-        if t.arg is None:
-            self.write("**")
-        else:
-            self.write(t.arg)
-            self.write("=")
-        self.dispatch(t.value)
-
-    def _Lambda(self, t):
-        self.write("(")
-        self.write("lambda ")
-        self.dispatch(t.args)
-        self.write(": ")
-        self.dispatch(t.body)
-        self.write(")")
-
-    def _alias(self, t):
-        self.write(t.name)
-        if t.asname:
-            self.write(" as "+t.asname)
-
-    def _withitem(self, t):
-        self.dispatch(t.context_expr)
-        if t.optional_vars:
-            self.write(" as ")
-            self.dispatch(t.optional_vars)
-
-def roundtrip(filename, output=sys.stdout):
-    with open(filename, "rb") as pyfile:
-        encoding = tokenize.detect_encoding(pyfile.readline)[0]
-    with open(filename, "r", encoding=encoding) as pyfile:
-        source = pyfile.read()
-    tree = compile(source, filename, "exec", ast.PyCF_ONLY_AST)
-    Unparser(tree, output)
-
-
-
-def testdir(a):
-    try:
-        names = [n for n in os.listdir(a) if n.endswith('.py')]
-    except OSError:
-        print("Directory not readable: %s" % a, file=sys.stderr)
-    else:
-        for n in names:
-            fullname = os.path.join(a, n)
-            if os.path.isfile(fullname):
-                output = io.StringIO()
-                print('Testing %s' % fullname)
-                try:
-                    roundtrip(fullname, output)
-                except Exception as e:
-                    print('  Failed to compile, exception is %s' % repr(e))
-            elif os.path.isdir(fullname):
-                testdir(fullname)
-
-def main(args):
-    if args[0] == '--testdir':
-        for a in args[1:]:
-            testdir(a)
-    else:
-        for a in args:
-            roundtrip(a)
-
-if __name__=='__main__':
-    main(sys.argv[1:])