String annotations [PEP 563] (#4390)

* Document `from __future__ import annotations`
* Provide plumbing and tests for `from __future__ import annotations`
* Implement unparsing the AST back to string form

This is required for PEP 563 and as such only implements a part of the
unparsing process that covers expressions.

1 files changed, 1163 insertions, 0 deletions

diff --git a/Python/ast_unparse.c b/Python/ast_unparse.c
new file mode 100644
index 0000000000..ef9e948dc8
--- /dev/null
+++ b/Python/ast_unparse.c
@@ -0,0 +1,1163 @@
+#include <stdbool.h>
+#include "Python.h"
+#include "Python-ast.h"
+
+static PyObject *_str_open_br;
+static PyObject *_str_dbl_open_br;
+static PyObject *_str_close_br;
+static PyObject *_str_dbl_close_br;
+
+/* Forward declarations for recursion via helper functions. */
+static PyObject *
+expr_as_unicode(expr_ty e, bool omit_parens);
+static int
+append_ast_expr(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens);
+static int
+append_joinedstr(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec);
+static int
+append_formattedvalue(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec);
+
+static int
+append_charp(_PyUnicodeWriter *writer, const char *charp)
+{
+        return _PyUnicodeWriter_WriteASCIIString(writer, charp, -1);
+}
+
+static int
+append_repr(_PyUnicodeWriter *writer, PyObject *obj)
+{
+    int ret;
+    PyObject *repr;
+    repr = PyObject_Repr(obj);
+    if (!repr) {
+        return -1;
+    }
+    ret = _PyUnicodeWriter_WriteStr(writer, repr);
+    Py_DECREF(repr);
+    return ret;
+}
+
+static int
+append_ast_boolop(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    Py_ssize_t i, value_count;
+    asdl_seq *values;
+
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    values = e->v.BoolOp.values;
+    value_count = asdl_seq_LEN(values) - 1;
+    assert(value_count >= 0);
+
+    if (-1 == append_ast_expr(writer,
+                              (expr_ty)asdl_seq_GET(values, 0),
+                              false)) {
+        return -1;
+    }
+
+    const char *op = (e->v.BoolOp.op == And) ? " and " : " or ";
+    for (i = 1; i <= value_count; ++i) {
+        if (-1 == append_charp(writer, op)) {
+            return -1;
+        }
+
+        if (-1 == append_ast_expr(writer,
+                                  (expr_ty)asdl_seq_GET(values, i),
+                                  false)) {
+            return -1;
+        }
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_binop(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    const char *op;
+
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.BinOp.left, false)) {
+        return -1;
+    }
+
+    switch(e->v.BinOp.op) {
+    case Add: op = " + "; break;
+    case Sub: op = " - "; break;
+    case Mult: op = " * "; break;
+    case MatMult: op = " @ "; break;
+    case Div: op = " / "; break;
+    case Mod: op = " % "; break;
+    case LShift: op = " << "; break;
+    case RShift: op = " >> "; break;
+    case BitOr: op = " | "; break;
+    case BitXor: op = " ^ "; break;
+    case BitAnd: op = " & "; break;
+    case FloorDiv: op = " // "; break;
+    case Pow: op = " ** "; break;
+    }
+
+    if (-1 == append_charp(writer, op)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.BinOp.right, false)) {
+        return -1;
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_unaryop(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    const char *op;
+
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    switch(e->v.UnaryOp.op) {
+    case Invert: op = "~"; break;
+    case Not: op = "not "; break;
+    case UAdd: op = "+"; break;
+    case USub: op = "-"; break;
+    }
+
+    if (-1 == append_charp(writer, op)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.UnaryOp.operand, false)) {
+        return -1;
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_arg(_PyUnicodeWriter *writer, arg_ty arg)
+{
+    if (-1 == _PyUnicodeWriter_WriteStr(writer, arg->arg)) {
+        return -1;
+    }
+    if (arg->annotation) {
+        if (-1 == append_charp(writer, ": ")) {
+            return -1;
+        }
+        if (-1 == append_ast_expr(writer, arg->annotation, true)) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+static int
+append_ast_args(_PyUnicodeWriter *writer, arguments_ty args)
+{
+    bool first;
+    Py_ssize_t i, di, arg_count, default_count;
+    arg_ty arg;
+    expr_ty default_;
+
+    first = true;
+
+    /* positional arguments with defaults */
+    arg_count = asdl_seq_LEN(args->args);
+    default_count = asdl_seq_LEN(args->defaults);
+    for (i = 0; i < arg_count; i++) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        arg = (arg_ty)asdl_seq_GET(args->args, i);
+        if (-1 == append_ast_arg(writer, arg)) {
+            return -1;
+        }
+
+        di = i - arg_count + default_count;
+        if (di >= 0) {
+            if (-1 == append_charp(writer, "=")) {
+                return -1;
+            }
+            default_ = (expr_ty)asdl_seq_GET(args->defaults, di);
+            if (-1 == append_ast_expr(writer, default_, false)) {
+                return -1;
+            }
+        }
+    }
+
+    /* vararg, or bare '*' if no varargs but keyword-only arguments present */
+    if (args->vararg || args->kwonlyargs) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        if (-1 == append_charp(writer, "*")) {
+            return -1;
+        }
+
+        if (args->vararg) {
+            if (-1 == append_ast_arg(writer, args->vararg)) {
+                return -1;
+            }
+        }
+    }
+
+    /* keyword-only arguments */
+    arg_count = asdl_seq_LEN(args->kwonlyargs);
+    default_count = asdl_seq_LEN(args->kw_defaults);
+    for (i = 0; i < arg_count; i++) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        arg = (arg_ty)asdl_seq_GET(args->kwonlyargs, i);
+        if (-1 == append_ast_arg(writer, arg)) {
+            return -1;
+        }
+
+        di = i - arg_count + default_count;
+        if (di >= 0) {
+            if (-1 == append_charp(writer, "=")) {
+                return -1;
+            }
+            default_ = (expr_ty)asdl_seq_GET(args->kw_defaults, di);
+            if (-1 == append_ast_expr(writer, default_, false)) {
+                return -1;
+            }
+        }
+    }
+
+    /* **kwargs */
+    if (args->kwarg) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        if (-1 == append_charp(writer, "**")) {
+            return -1;
+        }
+
+        if (-1 == append_ast_arg(writer, args->kwarg)) {
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+static int
+append_ast_lambda(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, "lambda ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_args(writer, e->v.Lambda.args)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, ": ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.Lambda.body, true)) {
+        return -1;
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_ifexp(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.IfExp.body, false)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, " if ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.IfExp.test, false)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, " else ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.IfExp.orelse, false)) {
+        return -1;
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_dict(_PyUnicodeWriter *writer, expr_ty e)
+{
+    Py_ssize_t i, value_count;
+    expr_ty key_node, value_node;
+
+    if (-1 == append_charp(writer, "{")) {
+        return -1;
+    }
+
+    value_count = asdl_seq_LEN(e->v.Dict.values);
+
+    for (i = 0; i < value_count; i++) {
+        if (i > 0 && -1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+        key_node = (expr_ty)asdl_seq_GET(e->v.Dict.keys, i);
+        if (key_node != NULL) {
+            if (-1 == append_ast_expr(writer, key_node, false)) {
+                return -1;
+            }
+
+            if (-1 == append_charp(writer, ": ")) {
+                return -1;
+            }
+        }
+        else if (-1 == append_charp(writer, "**")) {
+            return -1;
+        }
+
+        value_node = (expr_ty)asdl_seq_GET(e->v.Dict.values, i);
+        if (-1 == append_ast_expr(writer, value_node, false)) {
+            return -1;
+        }
+    }
+
+    return append_charp(writer, "}");
+}
+
+static int
+append_ast_set(_PyUnicodeWriter *writer, expr_ty e)
+{
+    Py_ssize_t i, elem_count;
+    expr_ty elem_node;
+
+    if (-1 == append_charp(writer, "{")) {
+        return -1;
+    }
+
+    elem_count = asdl_seq_LEN(e->v.Set.elts);
+    for (i = 0; i < elem_count; i++) {
+        if (i > 0 && -1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        elem_node = (expr_ty)asdl_seq_GET(e->v.Set.elts, i);
+        if (-1 == append_ast_expr(writer, elem_node, false)) {
+            return -1;
+        }
+    }
+
+    return append_charp(writer, "}");
+}
+
+static int
+append_ast_list(_PyUnicodeWriter *writer, expr_ty e)
+{
+    Py_ssize_t i, elem_count;
+    expr_ty elem_node;
+
+    if (-1 == append_charp(writer, "[")) {
+        return -1;
+    }
+
+    elem_count = asdl_seq_LEN(e->v.List.elts);
+    for (i = 0; i < elem_count; i++) {
+        if (i > 0 && -1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+        elem_node = (expr_ty)asdl_seq_GET(e->v.List.elts, i);
+        if (-1 == append_ast_expr(writer, elem_node, false)) {
+            return -1;
+        }
+    }
+
+    return append_charp(writer, "]");
+}
+
+static int
+append_ast_tuple(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    Py_ssize_t i, elem_count;
+    expr_ty elem_node;
+
+    elem_count = asdl_seq_LEN(e->v.Tuple.elts);
+
+    if (!omit_parens || elem_count < 2) {
+        if (-1 == append_charp(writer, "(")) {
+            return -1;
+        }
+    }
+
+    for (i = 0; i < elem_count; i++) {
+        if ((i > 0 || elem_count == 1) && -1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+        elem_node = (expr_ty)asdl_seq_GET(e->v.Tuple.elts, i);
+        if (-1 == append_ast_expr(writer, elem_node, false)) {
+            return -1;
+        }
+    }
+
+    if (!omit_parens || elem_count < 2) {
+        return append_charp(writer, ")");
+    }
+
+    return 0;
+}
+
+static int
+append_ast_comprehension(_PyUnicodeWriter *writer, comprehension_ty gen)
+{
+    Py_ssize_t i, if_count;
+
+    if (-1 == append_charp(writer, gen->is_async ? " async for " : " for ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, gen->target, true)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, " in ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, gen->iter, false)) {
+        return -1;
+    }
+
+    if_count = asdl_seq_LEN(gen->ifs);
+    for (i = 0; i < if_count; i++) {
+        if (-1 == append_charp(writer, " if ")) {
+            return -1;
+        }
+
+        if (-1 == append_ast_expr(writer,
+                                  (expr_ty)asdl_seq_GET(gen->ifs, i),
+                                  false)) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+static int
+append_ast_comprehensions(_PyUnicodeWriter *writer, asdl_seq *comprehensions)
+{
+    Py_ssize_t i, gen_count;
+    comprehension_ty comp_node;
+    gen_count = asdl_seq_LEN(comprehensions);
+
+    for (i = 0; i < gen_count; i++) {
+        comp_node = (comprehension_ty)asdl_seq_GET(comprehensions, i);
+        if (-1 == append_ast_comprehension(writer, comp_node)) {
+            return -1;
+        }
+    }
+
+    return 0;
+}
+
+static int
+append_ast_genexp(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.GeneratorExp.elt, false)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_comprehensions(writer, e->v.GeneratorExp.generators)) {
+        return -1;
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_listcomp(_PyUnicodeWriter *writer, expr_ty e)
+{
+    if (-1 == append_charp(writer, "[")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.ListComp.elt, false)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_comprehensions(writer, e->v.ListComp.generators)) {
+        return -1;
+    }
+
+    return append_charp(writer, "]");
+}
+
+static int
+append_ast_setcomp(_PyUnicodeWriter *writer, expr_ty e)
+{
+    if (-1 == append_charp(writer, "{")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.SetComp.elt, false)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_comprehensions(writer, e->v.SetComp.generators)) {
+        return -1;
+    }
+
+    return append_charp(writer, "}");
+}
+
+static int
+append_ast_dictcomp(_PyUnicodeWriter *writer, expr_ty e)
+{
+    if (-1 == append_charp(writer, "{")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.DictComp.key, false)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, ": ")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_expr(writer, e->v.DictComp.value, false)) {
+        return -1;
+    }
+
+    if (-1 == append_ast_comprehensions(writer, e->v.DictComp.generators)) {
+        return -1;
+    }
+
+    return append_charp(writer, "}");
+}
+
+static int
+append_ast_compare(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    const char *op;
+    Py_ssize_t i, comparator_count;
+    asdl_seq *comparators;
+    asdl_int_seq *ops;
+
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    comparators = e->v.Compare.comparators;
+    ops = e->v.Compare.ops;
+    comparator_count = asdl_seq_LEN(comparators);
+    assert(comparator_count > 0);
+    assert(comparator_count == asdl_seq_LEN(ops));
+
+    if (-1 == append_ast_expr(writer, e->v.Compare.left, false)) {
+        return -1;
+    }
+
+    for (i = 0; i < comparator_count; i++) {
+        switch ((cmpop_ty)asdl_seq_GET(ops, i)) {
+        case Eq:
+            op = " == ";
+            break;
+        case NotEq:
+            op = " != ";
+            break;
+        case Lt:
+            op = " < ";
+            break;
+        case LtE:
+            op = " <= ";
+            break;
+        case Gt:
+            op = " > ";
+            break;
+        case GtE:
+            op = " >= ";
+            break;
+        case Is:
+            op = " is ";
+            break;
+        case IsNot:
+            op = " is not ";
+            break;
+        case In:
+            op = " in ";
+            break;
+        case NotIn:
+            op = " not in ";
+            break;
+        default:
+            PyErr_SetString(PyExc_SystemError,
+                            "unexpected comparison kind");
+            return -1;
+        }
+
+        if (-1 == append_charp(writer, op)) {
+            return -1;
+        }
+
+        if (-1 == append_ast_expr(writer,
+                                  (expr_ty)asdl_seq_GET(comparators, i),
+                                  false)) {
+            return -1;
+        }
+    }
+
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_keyword(_PyUnicodeWriter *writer, keyword_ty kw)
+{
+    if (kw->arg == NULL) {
+        if (-1 == append_charp(writer, "**")) {
+            return -1;
+        }
+    }
+    else {
+        if (-1 == _PyUnicodeWriter_WriteStr(writer, kw->arg)) {
+            return -1;
+        }
+
+        if (-1 == append_charp(writer, "=")) {
+            return -1;
+        }
+    }
+
+    return append_ast_expr(writer, kw->value, false);
+}
+
+static int
+append_ast_call(_PyUnicodeWriter *writer, expr_ty e)
+{
+    bool first;
+    Py_ssize_t i, arg_count, kw_count;
+    expr_ty expr;
+    keyword_ty kw;
+
+    if (-1 == append_ast_expr(writer, e->v.Call.func, false)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    first = true;
+    arg_count = asdl_seq_LEN(e->v.Call.args);
+    for (i = 0; i < arg_count; i++) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        expr = (expr_ty)asdl_seq_GET(e->v.Call.args, i);
+        if (-1 == append_ast_expr(writer, expr, false)) {
+            return -1;
+        }
+    }
+
+    kw_count = asdl_seq_LEN(e->v.Call.keywords);
+    for (i = 0; i < kw_count; i++) {
+        if (first) {
+            first = false;
+        }
+        else if (-1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+
+        kw = (keyword_ty)asdl_seq_GET(e->v.Call.keywords, i);
+        if (-1 == append_ast_keyword(writer, kw)) {
+            return -1;
+        }
+    }
+
+    return append_charp(writer, ")");
+}
+
+static PyObject *
+escape_braces(PyObject *orig)
+{
+    PyObject *temp;
+    PyObject *result;
+    temp = PyUnicode_Replace(orig, _str_open_br, _str_dbl_open_br, -1);
+    if (!temp) {
+        return NULL;
+    }
+    result = PyUnicode_Replace(temp, _str_close_br, _str_dbl_close_br, -1);
+    Py_DECREF(temp);
+    return result;
+}
+
+static int
+append_fstring_unicode(_PyUnicodeWriter *writer, PyObject *unicode)
+{
+    PyObject *escaped;
+    int result = -1;
+    escaped = escape_braces(unicode);
+    if (escaped) {
+        result = _PyUnicodeWriter_WriteStr(writer, escaped);
+        Py_DECREF(escaped);
+    }
+    return result;
+}
+
+static int
+append_fstring_element(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec)
+{
+    switch (e->kind) {
+    case Constant_kind:
+        return append_fstring_unicode(writer, e->v.Constant.value);
+    case Str_kind:
+        return append_fstring_unicode(writer, e->v.Str.s);
+    case JoinedStr_kind:
+        return append_joinedstr(writer, e, is_format_spec);
+    case FormattedValue_kind:
+        return append_formattedvalue(writer, e, is_format_spec);
+    default:
+        PyErr_SetString(PyExc_SystemError,
+                        "unknown expression kind inside f-string");
+        return -1;
+    }
+}
+
+/* Build body separately to enable wrapping the entire stream of Strs,
+   Constants and FormattedValues in one opening and one closing quote. */
+static PyObject *
+build_fstring_body(asdl_seq *values, bool is_format_spec)
+{
+    Py_ssize_t i, value_count;
+    _PyUnicodeWriter body_writer;
+    _PyUnicodeWriter_Init(&body_writer);
+    body_writer.min_length = 256;
+    body_writer.overallocate = 1;
+
+    value_count = asdl_seq_LEN(values) - 1;
+    assert(value_count >= 0);
+    for (i = 0; i <= value_count; ++i) {
+        if (-1 == append_fstring_element(&body_writer,
+                                         (expr_ty)asdl_seq_GET(values, i),
+                                         is_format_spec
+                                         )) {
+            _PyUnicodeWriter_Dealloc(&body_writer);
+            return NULL;
+        }
+    }
+
+    return _PyUnicodeWriter_Finish(&body_writer);
+}
+
+static int
+append_joinedstr(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec)
+{
+    int result = -1;
+    PyObject *body = build_fstring_body(e->v.JoinedStr.values, is_format_spec);
+    if (!body) {
+        return -1;
+    }
+
+    if (!is_format_spec) {
+        if (-1 != append_charp(writer, "f") &&
+            -1 != append_repr(writer, body))
+        {
+            result = 0;
+        }
+    }
+    else {
+        result = _PyUnicodeWriter_WriteStr(writer, body);
+    }
+    Py_DECREF(body);
+    return result;
+}
+
+static int
+append_formattedvalue(_PyUnicodeWriter *writer, expr_ty e, bool is_format_spec)
+{
+    char *conversion;
+    char *outer_brace = "{";
+    PyObject *temp_fv_str = expr_as_unicode(e->v.FormattedValue.value, true);
+    if (!temp_fv_str) {
+        return -1;
+    }
+    if (PyUnicode_Find(temp_fv_str, _str_open_br, 0, 1, 1) == 0) {
+        /* Expression starts with a brace, split it with a space from the outer
+           one. */
+        outer_brace = "{ ";
+    }
+    if (-1 == append_charp(writer, outer_brace)) {
+        Py_DECREF(temp_fv_str);
+        return -1;
+    }
+    if (-1 == _PyUnicodeWriter_WriteStr(writer, temp_fv_str)) {
+        Py_DECREF(temp_fv_str);
+        return -1;
+    }
+    Py_DECREF(temp_fv_str);
+
+    if (e->v.FormattedValue.conversion > 0) {
+        switch (e->v.FormattedValue.conversion) {
+        case 97:
+            conversion = "!a";
+            break;
+        case 114:
+            conversion = "!r";
+            break;
+        case 115:
+            conversion = "!s";
+            break;
+        default:
+            PyErr_SetString(PyExc_SystemError,
+                            "unknown f-value conversion kind");
+            return -1;
+        }
+        if (-1 == append_charp(writer, conversion)) {
+            return -1;
+        }
+    }
+    if (e->v.FormattedValue.format_spec > 0) {
+        if (-1 == _PyUnicodeWriter_WriteASCIIString(writer, ":", 1) ||
+            -1 == append_fstring_element(writer,
+                                         e->v.FormattedValue.format_spec,
+                                         true
+                                        ))
+        {
+            return -1;
+        }
+    }
+    return append_charp(writer, "}");
+}
+
+static int
+append_ast_attribute(_PyUnicodeWriter *writer, expr_ty e)
+{
+    const char *period;
+    if (-1 == append_ast_expr(writer, e->v.Attribute.value, false)) {
+        return -1;
+    }
+
+    /* Special case: integers require a space for attribute access to be
+       unambiguous.  Floats and complex numbers don't but work with it, too. */
+    if (e->v.Attribute.value->kind == Num_kind ||
+        e->v.Attribute.value->kind == Constant_kind)
+    {
+        period = " .";
+    }
+    else {
+        period = ".";
+    }
+    if (-1 == append_charp(writer, period)) {
+        return -1;
+    }
+
+    return _PyUnicodeWriter_WriteStr(writer, e->v.Attribute.attr);
+}
+
+static int
+append_ast_simple_slice(_PyUnicodeWriter *writer, slice_ty slice)
+{
+    if (slice->v.Slice.lower) {
+        if (-1 == append_ast_expr(writer, slice->v.Slice.lower, false)) {
+            return -1;
+        }
+    }
+
+    if (-1 == append_charp(writer, ":")) {
+        return -1;
+    }
+
+    if (slice->v.Slice.upper) {
+        if (-1 == append_ast_expr(writer, slice->v.Slice.upper, false)) {
+            return -1;
+        }
+    }
+
+    if (slice->v.Slice.step) {
+        if (-1 == append_charp(writer, ":")) {
+            return -1;
+        }
+        if (-1 == append_ast_expr(writer, slice->v.Slice.step, false)) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+static int
+append_ast_ext_slice(_PyUnicodeWriter *writer, slice_ty slice)
+{
+    Py_ssize_t i, dims_count;
+    dims_count = asdl_seq_LEN(slice->v.ExtSlice.dims);
+    for (i = 0; i < dims_count; i++) {
+        if (i > 0 && -1 == append_charp(writer, ", ")) {
+            return -1;
+        }
+        if (-1 == append_ast_expr(writer,
+                                  (expr_ty)asdl_seq_GET(slice->v.ExtSlice.dims, i),
+                                  false)) {
+            return -1;
+        }
+    }
+    return 0;
+}
+
+static int
+append_ast_slice(_PyUnicodeWriter *writer, slice_ty slice, bool omit_parens)
+{
+    switch(slice->kind) {
+    case Slice_kind:
+        return append_ast_simple_slice(writer, slice);
+    case ExtSlice_kind:
+        return append_ast_ext_slice(writer, slice);
+    case Index_kind:
+        return append_ast_expr(writer, slice->v.Index.value, omit_parens);
+    default:
+        PyErr_SetString(PyExc_SystemError,
+                        "unexpected slice kind");
+        return -1;
+    }
+}
+
+static int
+append_ast_subscript(_PyUnicodeWriter *writer, expr_ty e)
+{
+    if (-1 == append_ast_expr(writer, e->v.Subscript.value, false)) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, "[")) {
+        return -1;
+    }
+
+    if (-1 == append_ast_slice(writer, e->v.Subscript.slice, true)) {
+        return -1;
+    }
+
+    return append_charp(writer, "]");
+}
+
+static int
+append_ast_starred(_PyUnicodeWriter *writer, expr_ty e)
+{
+    if (-1 == append_charp(writer, "*")) {
+        return -1;
+    }
+
+    return append_ast_expr(writer, e->v.Starred.value, false);
+}
+
+static int
+append_ast_yield(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, e->v.Yield.value ? "yield " : "yield")) {
+        return -1;
+    }
+
+    if (e->v.Yield.value) {
+        if (-1 == append_ast_expr(writer, e->v.Yield.value, false)) {
+            return -1;
+        }
+    }
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_yield_from(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer,
+                           e->v.YieldFrom.value ? "yield from " : "yield from")) {
+        return -1;
+    }
+
+    if (e->v.YieldFrom.value) {
+        if (-1 == append_ast_expr(writer, e->v.YieldFrom.value, false)) {
+            return -1;
+        }
+    }
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_await(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    if (!omit_parens && -1 == append_charp(writer, "(")) {
+        return -1;
+    }
+
+    if (-1 == append_charp(writer, e->v.Await.value ? "await " : "await")) {
+        return -1;
+    }
+
+    if (e->v.Await.value) {
+        if (-1 == append_ast_expr(writer, e->v.Await.value, false)) {
+            return -1;
+        }
+    }
+    return omit_parens ? 0 : append_charp(writer, ")");
+}
+
+static int
+append_ast_expr(_PyUnicodeWriter *writer, expr_ty e, bool omit_parens)
+{
+    switch (e->kind) {
+    case BoolOp_kind:
+        return append_ast_boolop(writer, e, omit_parens);
+    case BinOp_kind:
+        return append_ast_binop(writer, e, omit_parens);
+    case UnaryOp_kind:
+        return append_ast_unaryop(writer, e, omit_parens);
+    case Lambda_kind:
+        return append_ast_lambda(writer, e, omit_parens);
+    case IfExp_kind:
+        return append_ast_ifexp(writer, e, omit_parens);
+    case Dict_kind:
+        return append_ast_dict(writer, e);
+    case Set_kind:
+        return append_ast_set(writer, e);
+    case GeneratorExp_kind:
+        return append_ast_genexp(writer, e, omit_parens);
+    case ListComp_kind:
+        return append_ast_listcomp(writer, e);
+    case SetComp_kind:
+        return append_ast_setcomp(writer, e);
+    case DictComp_kind:
+        return append_ast_dictcomp(writer, e);
+    case Yield_kind:
+        return append_ast_yield(writer, e, omit_parens);
+    case YieldFrom_kind:
+        return append_ast_yield_from(writer, e, omit_parens);
+    case Await_kind:
+        return append_ast_await(writer, e, omit_parens);
+    case Compare_kind:
+        return append_ast_compare(writer, e, omit_parens);
+    case Call_kind:
+        return append_ast_call(writer, e);
+    case Constant_kind:
+        return append_repr(writer, e->v.Constant.value);
+    case Num_kind:
+        return append_repr(writer, e->v.Num.n);
+    case Str_kind:
+        return append_repr(writer, e->v.Str.s);
+    case JoinedStr_kind:
+        return append_joinedstr(writer, e, false);
+    case FormattedValue_kind:
+        return append_formattedvalue(writer, e, false);
+    case Bytes_kind:
+        return append_repr(writer, e->v.Bytes.s);
+    case Ellipsis_kind:
+        return append_charp(writer, "...");
+    case NameConstant_kind:
+        return append_repr(writer, e->v.NameConstant.value);
+    /* The following exprs can be assignment targets. */
+    case Attribute_kind:
+        return append_ast_attribute(writer, e);
+    case Subscript_kind:
+        return append_ast_subscript(writer, e);
+    case Starred_kind:
+        return append_ast_starred(writer, e);
+    case Name_kind:
+        return _PyUnicodeWriter_WriteStr(writer, e->v.Name.id);
+    /* child nodes of List and Tuple will have expr_context set */
+    case List_kind:
+        return append_ast_list(writer, e);
+    case Tuple_kind:
+        return append_ast_tuple(writer, e, omit_parens);
+    default:
+        PyErr_SetString(PyExc_SystemError,
+                        "unknown expression kind");
+        return -1;
+    }
+}
+
+static int
+maybe_init_static_strings()
+{
+    if (!_str_open_br &&
+        !(_str_open_br = PyUnicode_InternFromString("{"))) {
+        return -1;
+    }
+    if (!_str_dbl_open_br &&
+        !(_str_dbl_open_br = PyUnicode_InternFromString("{{"))) {
+        return -1;
+    }
+    if (!_str_close_br &&
+        !(_str_close_br = PyUnicode_InternFromString("}"))) {
+        return -1;
+    }
+    if (!_str_dbl_close_br &&
+        !(_str_dbl_close_br = PyUnicode_InternFromString("}}"))) {
+        return -1;
+    }
+    return 0;
+}
+
+static PyObject *
+expr_as_unicode(expr_ty e, bool omit_parens)
+{
+    _PyUnicodeWriter writer;
+    _PyUnicodeWriter_Init(&writer);
+    writer.min_length = 256;
+    writer.overallocate = 1;
+    if (-1 == maybe_init_static_strings() ||
+        -1 == append_ast_expr(&writer, e, omit_parens))
+    {
+        _PyUnicodeWriter_Dealloc(&writer);
+        return NULL;
+    }
+    return _PyUnicodeWriter_Finish(&writer);
+}
+
+PyObject *
+_PyAST_ExprAsUnicode(expr_ty e, bool omit_parens)
+{
+    return expr_as_unicode(e, omit_parens);
+}