Issue #24965: Implement PEP 498 "Literal String Interpolation". Documentation is still needed, I'll open an issue for that.

1 files changed, 929 insertions, 56 deletions

diff --git a/Python/ast.c b/Python/ast.c
index 1f7ddfc..735424b 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -257,6 +257,14 @@ validate_expr(expr_ty exp, expr_context_ty ctx)
         }
         return 1;
     }
+    case JoinedStr_kind:
+        return validate_exprs(exp->v.JoinedStr.values, Load, 0);
+    case FormattedValue_kind:
+        if (validate_expr(exp->v.FormattedValue.value, Load) == 0)
+            return 0;
+        if (exp->v.FormattedValue.format_spec)
+            return validate_expr(exp->v.FormattedValue.format_spec, Load);
+        return 1;
     case Bytes_kind: {
         PyObject *b = exp->v.Bytes.s;
         if (!PyBytes_CheckExact(b)) {
@@ -535,9 +543,7 @@ static stmt_ty ast_for_for_stmt(struct compiling *, const node *, int);
 static expr_ty ast_for_call(struct compiling *, const node *, expr_ty);
 
 static PyObject *parsenumber(struct compiling *, const char *);
-static PyObject *parsestr(struct compiling *, const node *n, int *bytesmode);
-static PyObject *parsestrplus(struct compiling *, const node *n,
-                              int *bytesmode);
+static expr_ty parsestrplus(struct compiling *, const node *n);
 
 #define COMP_GENEXP   0
 #define COMP_LISTCOMP 1
@@ -986,6 +992,8 @@ set_context(struct compiling *c, expr_ty e, expr_context_ty ctx, const node *n)
         case Num_kind:
         case Str_kind:
         case Bytes_kind:
+        case JoinedStr_kind:
+        case FormattedValue_kind:
             expr_name = "literal";
             break;
         case NameConstant_kind:
@@ -2001,7 +2009,6 @@ ast_for_atom(struct compiling *c, const node *n)
        | '...' | 'None' | 'True' | 'False'
     */
     node *ch = CHILD(n, 0);
-    int bytesmode = 0;
 
     switch (TYPE(ch)) {
     case NAME: {
@@ -2023,7 +2030,7 @@ ast_for_atom(struct compiling *c, const node *n)
         return Name(name, Load, LINENO(n), n->n_col_offset, c->c_arena);
     }
     case STRING: {
-        PyObject *str = parsestrplus(c, n, &bytesmode);
+        expr_ty str = parsestrplus(c, n);
         if (!str) {
             const char *errtype = NULL;
             if (PyErr_ExceptionMatches(PyExc_UnicodeError))
@@ -2050,14 +2057,7 @@ ast_for_atom(struct compiling *c, const node *n)
             }
             return NULL;
         }
-        if (PyArena_AddPyObject(c->c_arena, str) < 0) {
-            Py_DECREF(str);
-            return NULL;
-        }
-        if (bytesmode)
-            return Bytes(str, LINENO(n), n->n_col_offset, c->c_arena);
-        else
-            return Str(str, LINENO(n), n->n_col_offset, c->c_arena);
+        return str;
     }
     case NUMBER: {
         PyObject *pynum = parsenumber(c, STR(ch));
@@ -4002,12 +4002,838 @@ decode_unicode(struct compiling *c, const char *s, size_t len, int rawmode, cons
     return v;
 }
 
-/* s is a Python string literal, including the bracketing quote characters,
- * and r &/or b prefixes (if any), and embedded escape sequences (if any).
- * parsestr parses it, and returns the decoded Python string object.
- */
+/* Compile this expression in to an expr_ty. We know that we can
+   temporarily modify the character before the start of this string
+   (it's '{'), and we know we can temporarily modify the character
+   after this string (it is a '}').  Leverage this to create a
+   sub-string with enough room for us to add parens around the
+   expression. This is to allow strings with embedded newlines, for
+   example. */
+static expr_ty
+fstring_expression_compile(PyObject *str, Py_ssize_t expr_start,
+                           Py_ssize_t expr_end, PyArena *arena)
+{
+    PyCompilerFlags cf;
+    mod_ty mod;
+    char *utf_expr;
+    Py_ssize_t i;
+    int all_whitespace;
+    PyObject *sub = NULL;
+
+    /* We only decref sub if we allocated it with a PyUnicode_Substring.
+       decref_sub records that. */
+    int decref_sub = 0;
+
+    assert(str);
+
+    /* If the substring is all whitespace, it's an error. We need to
+        catch this here, and not when we call PyParser_ASTFromString,
+        because turning the expression '' in to '()' would go from
+        being invalid to valid. */
+    /* Note that this code says an empty string is all
+        whitespace. That's important. There's a test for it: f'{}'. */
+    all_whitespace = 1;
+    for (i = expr_start; i < expr_end; i++) {
+        if (!Py_UNICODE_ISSPACE(PyUnicode_READ_CHAR(str, i))) {
+            all_whitespace = 0;
+            break;
+        }
+    }
+    if (all_whitespace) {
+        PyErr_SetString(PyExc_SyntaxError, "f-string: empty expression "
+                                           "not allowed");
+        goto error;
+    }
+
+    /* If the substring will be the entire source string, we can't use
+        PyUnicode_Substring, since it will return another reference to
+        our original string. Because we're modifying the string in
+        place, that's a no-no. So, detect that case and just use our
+        string directly. */
+
+    if (expr_start-1 == 0 && expr_end+1 == PyUnicode_GET_LENGTH(str)) {
+        /* No need to actually remember these characters, because we
+           know they must be braces. */
+        assert(PyUnicode_ReadChar(str, 0) == '{');
+        assert(PyUnicode_ReadChar(str, expr_end-expr_start+1) == '}');
+        sub = str;
+    } else {
+        /* Create a substring object. It must be a new object, with
+           refcount==1, so that we can modify it. */
+        sub = PyUnicode_Substring(str, expr_start-1, expr_end+1);
+        if (!sub)
+            goto error;
+        assert(sub != str);  /* Make sure it's a new string. */
+        decref_sub = 1;      /* Remember to deallocate it on error. */
+    }
+
+    if (PyUnicode_WriteChar(sub, 0, '(') < 0 ||
+        PyUnicode_WriteChar(sub, expr_end-expr_start+1, ')') < 0)
+        goto error;
+
+    cf.cf_flags = PyCF_ONLY_AST;
+
+    /* No need to free the memory returned here: it's managed by the
+       string. */
+    utf_expr = PyUnicode_AsUTF8(sub);
+    if (!utf_expr)
+        goto error;
+    mod = PyParser_ASTFromString(utf_expr, "<fstring>",
+                                 Py_eval_input, &cf, arena);
+    if (!mod)
+        goto error;
+    if (sub != str)
+        /* Clear instead of decref in case we ever modify this code to change
+           the error handling: this is safest because the XDECREF won't try
+           and decref it when it's NULL. */
+        /* No need to restore the chars in sub, since we know it's getting
+           ready to get deleted (refcount must be 1, since we got a new string
+           in PyUnicode_Substring). */
+        Py_CLEAR(sub);
+    else {
+        assert(!decref_sub);
+        /* Restore str, which we earlier modified directly. */
+        if (PyUnicode_WriteChar(str, 0, '{') < 0 ||
+            PyUnicode_WriteChar(str, expr_end-expr_start+1, '}') < 0)
+            goto error;
+    }
+    return mod->v.Expression.body;
+
+error:
+    /* Only decref sub if it was the result of a call to SubString. */
+    if (decref_sub)
+        Py_XDECREF(sub);
+    return NULL;
+}
+
+/* Return -1 on error.
+
+   Return 0 if we reached the end of the literal.
+
+   Return 1 if we haven't reached the end of the literal, but we want
+   the caller to process the literal up to this point. Used for
+   doubled braces.
+*/
+static int
+fstring_find_literal(PyObject *str, Py_ssize_t *ofs, PyObject **literal,
+                     int recurse_lvl, struct compiling *c, const node *n)
+{
+    /* Get any literal string. It ends when we hit an un-doubled brace, or the
+       end of the string. */
+
+    Py_ssize_t literal_start, literal_end;
+    int result = 0;
+
+    enum PyUnicode_Kind kind = PyUnicode_KIND(str);
+    void *data = PyUnicode_DATA(str);
+
+    assert(*literal == NULL);
+
+    literal_start = *ofs;
+    for (; *ofs < PyUnicode_GET_LENGTH(str); *ofs += 1) {
+        Py_UCS4 ch = PyUnicode_READ(kind, data, *ofs);
+        if (ch == '{' || ch == '}') {
+            /* Check for doubled braces, but only at the top level. If
+               we checked at every level, then f'{0:{3}}' would fail
+               with the two closing braces. */
+            if (recurse_lvl == 0) {
+                if (*ofs + 1 < PyUnicode_GET_LENGTH(str) &&
+                    PyUnicode_READ(kind, data, *ofs + 1) == ch) {
+                    /* We're going to tell the caller that the literal ends
+                       here, but that they should continue scanning. But also
+                       skip over the second brace when we resume scanning. */
+                    literal_end = *ofs + 1;
+                    *ofs += 2;
+                    result = 1;
+                    goto done;
+                }
+
+                /* Where a single '{' is the start of a new expression, a
+                   single '}' is not allowed. */
+                if (ch == '}') {
+                    ast_error(c, n, "f-string: single '}' is not allowed");
+                    return -1;
+                }
+            }
+
+            /* We're either at a '{', which means we're starting another
+               expression; or a '}', which means we're at the end of this
+               f-string (for a nested format_spec). */
+            break;
+        }
+    }
+    literal_end = *ofs;
+
+    assert(*ofs == PyUnicode_GET_LENGTH(str) ||
+           PyUnicode_READ(kind, data, *ofs) == '{' ||
+           PyUnicode_READ(kind, data, *ofs) == '}');
+done:
+    if (literal_start != literal_end) {
+        *literal = PyUnicode_Substring(str, literal_start, literal_end);
+        if (!*literal)
+            return -1;
+    }
+
+    return result;
+}
+
+/* Forward declaration because parsing is recursive. */
+static expr_ty
+fstring_parse(PyObject *str, Py_ssize_t *ofs, int recurse_lvl,
+              struct compiling *c, const node *n);
+
+/* Parse the f-string str, starting at ofs. We know *ofs starts an
+   expression (so it must be a '{'). Returns the FormattedValue node,
+   which includes the expression, conversion character, and
+   format_spec expression.
+
+   Note that I don't do a perfect job here: I don't make sure that a
+   closing brace doesn't match an opening paren, for example. It
+   doesn't need to error on all invalid expressions, just correctly
+   find the end of all valid ones. Any errors inside the expression
+   will be caught when we parse it later. */
+static int
+fstring_find_expr(PyObject *str, Py_ssize_t *ofs, int recurse_lvl,
+                  expr_ty *expression, struct compiling *c, const node *n)
+{
+    /* Return -1 on error, else 0. */
+
+    Py_ssize_t expr_start;
+    Py_ssize_t expr_end;
+    expr_ty simple_expression;
+    expr_ty format_spec = NULL; /* Optional format specifier. */
+    Py_UCS4 conversion = -1; /* The conversion char. -1 if not specified. */
+
+    enum PyUnicode_Kind kind = PyUnicode_KIND(str);
+    void *data = PyUnicode_DATA(str);
+
+    /* 0 if we're not in a string, else the quote char we're trying to
+       match (single or double quote). */
+    Py_UCS4 quote_char = 0;
+
+    /* If we're inside a string, 1=normal, 3=triple-quoted. */
+    int string_type = 0;
+
+    /* Keep track of nesting level for braces/parens/brackets in
+       expressions. */
+    Py_ssize_t nested_depth = 0;
+
+    /* Can only nest one level deep. */
+    if (recurse_lvl >= 2) {
+        ast_error(c, n, "f-string: expressions nested too deeply");
+        return -1;
+    }
+
+    /* The first char must be a left brace, or we wouldn't have gotten
+       here. Skip over it. */
+    assert(PyUnicode_READ(kind, data, *ofs) == '{');
+    *ofs += 1;
+
+    expr_start = *ofs;
+    for (; *ofs < PyUnicode_GET_LENGTH(str); *ofs += 1) {
+        Py_UCS4 ch;
+
+        /* Loop invariants. */
+        assert(nested_depth >= 0);
+        assert(*ofs >= expr_start);
+        if (quote_char)
+            assert(string_type == 1 || string_type == 3);
+        else
+            assert(string_type == 0);
+
+        ch = PyUnicode_READ(kind, data, *ofs);
+        if (quote_char) {
+            /* We're inside a string. See if we're at the end. */
+            /* This code needs to implement the same non-error logic
+               as tok_get from tokenizer.c, at the letter_quote
+               label. To actually share that code would be a
+               nightmare. But, it's unlikely to change and is small,
+               so duplicate it here. Note we don't need to catch all
+               of the errors, since they'll be caught when parsing the
+               expression. We just need to match the non-error
+               cases. Thus we can ignore \n in single-quoted strings,
+               for example. Or non-terminated strings. */
+            if (ch == quote_char) {
+                /* Does this match the string_type (single or triple
+                   quoted)? */
+                if (string_type == 3) {
+                    if (*ofs+2 < PyUnicode_GET_LENGTH(str) &&
+                        PyUnicode_READ(kind, data, *ofs+1) == ch &&
+                        PyUnicode_READ(kind, data, *ofs+2) == ch) {
+                        /* We're at the end of a triple quoted string. */
+                        *ofs += 2;
+                        string_type = 0;
+                        quote_char = 0;
+                        continue;
+                    }
+                } else {
+                    /* We're at the end of a normal string. */
+                    quote_char = 0;
+                    string_type = 0;
+                    continue;
+                }
+            }
+            /* We're inside a string, and not finished with the
+               string. If this is a backslash, skip the next char (it
+               might be an end quote that needs skipping). Otherwise,
+               just consume this character normally. */
+            if (ch == '\\' && *ofs+1 < PyUnicode_GET_LENGTH(str)) {
+                /* Just skip the next char, whatever it is. */
+                *ofs += 1;
+            }
+        } else if (ch == '\'' || ch == '"') {
+            /* Is this a triple quoted string? */
+            if (*ofs+2 < PyUnicode_GET_LENGTH(str) &&
+                PyUnicode_READ(kind, data, *ofs+1) == ch &&
+                PyUnicode_READ(kind, data, *ofs+2) == ch) {
+                string_type = 3;
+                *ofs += 2;
+            } else {
+                /* Start of a normal string. */
+                string_type = 1;
+            }
+            /* Start looking for the end of the string. */
+            quote_char = ch;
+        } else if (ch == '[' || ch == '{' || ch == '(') {
+            nested_depth++;
+        } else if (nested_depth != 0 &&
+                   (ch == ']' || ch == '}' || ch == ')')) {
+            nested_depth--;
+        } else if (ch == '#') {
+            /* Error: can't include a comment character, inside parens
+               or not. */
+            ast_error(c, n, "f-string cannot include '#'");
+            return -1;
+        } else if (nested_depth == 0 &&
+                   (ch == '!' || ch == ':' || ch == '}')) {
+            /* First, test for the special case of "!=". Since '=' is
+               not an allowed conversion character, nothing is lost in
+               this test. */
+            if (ch == '!' && *ofs+1 < PyUnicode_GET_LENGTH(str) &&
+                  PyUnicode_READ(kind, data, *ofs+1) == '=')
+                /* This isn't a conversion character, just continue. */
+                continue;
+
+            /* Normal way out of this loop. */
+            break;
+        } else {
+            /* Just consume this char and loop around. */
+        }
+    }
+    expr_end = *ofs;
+    /* If we leave this loop in a string or with mismatched parens, we
+       don't care. We'll get a syntax error when compiling the
+       expression. But, we can produce a better error message, so
+       let's just do that.*/
+    if (quote_char) {
+        ast_error(c, n, "f-string: unterminated string");
+        return -1;
+    }
+    if (nested_depth) {
+        ast_error(c, n, "f-string: mismatched '(', '{', or '['");
+        return -1;
+    }
+
+    /* Check for a conversion char, if present. */
+    if (*ofs >= PyUnicode_GET_LENGTH(str))
+        goto unexpected_end_of_string;
+    if (PyUnicode_READ(kind, data, *ofs) == '!') {
+        *ofs += 1;
+        if (*ofs >= PyUnicode_GET_LENGTH(str))
+            goto unexpected_end_of_string;
+
+        conversion = PyUnicode_READ(kind, data, *ofs);
+        *ofs += 1;
+
+        /* Validate the conversion. */
+        if (!(conversion == 's' || conversion == 'r'
+              || conversion == 'a')) {
+            ast_error(c, n, "f-string: invalid conversion character: "
+                            "expected 's', 'r', or 'a'");
+            return -1;
+        }
+    }
+
+    /* Check for the format spec, if present. */
+    if (*ofs >= PyUnicode_GET_LENGTH(str))
+        goto unexpected_end_of_string;
+    if (PyUnicode_READ(kind, data, *ofs) == ':') {
+        *ofs += 1;
+        if (*ofs >= PyUnicode_GET_LENGTH(str))
+            goto unexpected_end_of_string;
+
+        /* Parse the format spec. */
+        format_spec = fstring_parse(str, ofs, recurse_lvl+1, c, n);
+        if (!format_spec)
+            return -1;
+    }
+
+    if (*ofs >= PyUnicode_GET_LENGTH(str) ||
+          PyUnicode_READ(kind, data, *ofs) != '}')
+        goto unexpected_end_of_string;
+
+    /* We're at a right brace. Consume it. */
+    assert(*ofs < PyUnicode_GET_LENGTH(str));
+    assert(PyUnicode_READ(kind, data, *ofs) == '}');
+    *ofs += 1;
+
+    /* Compile the expression. */
+    simple_expression = fstring_expression_compile(str, expr_start, expr_end,
+                                                   c->c_arena);
+    if (!simple_expression)
+        return -1;
+
+    /* And now create the FormattedValue node that represents this entire
+       expression with the conversion and format spec. */
+    *expression = FormattedValue(simple_expression, (int)conversion,
+                                 format_spec, LINENO(n), n->n_col_offset,
+                                 c->c_arena);
+    if (!*expression)
+        return -1;
+
+    return 0;
+
+unexpected_end_of_string:
+    ast_error(c, n, "f-string: expecting '}'");
+    return -1;
+}
+
+/* Return -1 on error.
+
+   Return 0 if we have a literal (possible zero length) and an
+   expression (zero length if at the end of the string.
+
+   Return 1 if we have a literal, but no expression, and we want the
+   caller to call us again. This is used to deal with doubled
+   braces.
+
+   When called multiple times on the string 'a{{b{0}c', this function
+   will return:
+
+   1. the literal 'a{' with no expression, and a return value
+      of 1. Despite the fact that there's no expression, the return
+      value of 1 means we're not finished yet.
+
+   2. the literal 'b' and the expression '0', with a return value of
+      0. The fact that there's an expression means we're not finished.
+
+   3. literal 'c' with no expression and a return value of 0. The
+      combination of the return value of 0 with no expression means
+      we're finished.
+*/
+static int
+fstring_find_literal_and_expr(PyObject *str, Py_ssize_t *ofs, int recurse_lvl,
+                              PyObject **literal, expr_ty *expression,
+                              struct compiling *c, const node *n)
+{
+    int result;
+
+    assert(*literal == NULL && *expression == NULL);
+
+    /* Get any literal string. */
+    result = fstring_find_literal(str, ofs, literal, recurse_lvl, c, n);
+    if (result < 0)
+        goto error;
+
+    assert(result == 0 || result == 1);
+
+    if (result == 1)
+        /* We have a literal, but don't look at the expression. */
+        return 1;
+
+    assert(*ofs <= PyUnicode_GET_LENGTH(str));
+
+    if (*ofs >= PyUnicode_GET_LENGTH(str) ||
+        PyUnicode_READ_CHAR(str, *ofs) == '}')
+        /* We're at the end of the string or the end of a nested
+           f-string: no expression. The top-level error case where we
+           expect to be at the end of the string but we're at a '}' is
+           handled later. */
+        return 0;
+
+    /* We must now be the start of an expression, on a '{'. */
+    assert(*ofs < PyUnicode_GET_LENGTH(str) &&
+           PyUnicode_READ_CHAR(str, *ofs) == '{');
+
+    if (fstring_find_expr(str, ofs, recurse_lvl, expression, c, n) < 0)
+        goto error;
+
+    return 0;
+
+error:
+    Py_XDECREF(*literal);
+    *literal = NULL;
+    return -1;
+}
+
+#define EXPRLIST_N_CACHED  64
+
+typedef struct {
+    /* Incrementally build an array of expr_ty, so be used in an
+       asdl_seq. Cache some small but reasonably sized number of
+       expr_ty's, and then after that start dynamically allocating,
+       doubling the number allocated each time. Note that the f-string
+       f'{0}a{1}' contains 3 expr_ty's: 2 FormattedValue's, and one
+       Str for the literal 'a'. So you add expr_ty's about twice as
+       fast as you add exressions in an f-string. */
+
+    Py_ssize_t allocated;  /* Number we've allocated. */
+    Py_ssize_t size;       /* Number we've used. */
+    expr_ty    *p;         /* Pointer to the memory we're actually
+                              using. Will point to 'data' until we
+                              start dynamically allocating. */
+    expr_ty    data[EXPRLIST_N_CACHED];
+} ExprList;
+
+#ifdef NDEBUG
+#define ExprList_check_invariants(l)
+#else
+static void
+ExprList_check_invariants(ExprList *l)
+{
+    /* Check our invariants. Make sure this object is "live", and
+       hasn't been deallocated. */
+    assert(l->size >= 0);
+    assert(l->p != NULL);
+    if (l->size <= EXPRLIST_N_CACHED)
+        assert(l->data == l->p);
+}
+#endif
+
+static void
+ExprList_Init(ExprList *l)
+{
+    l->allocated = EXPRLIST_N_CACHED;
+    l->size = 0;
+
+    /* Until we start allocating dynamically, p points to data. */
+    l->p = l->data;
+
+    ExprList_check_invariants(l);
+}
+
+static int
+ExprList_Append(ExprList *l, expr_ty exp)
+{
+    ExprList_check_invariants(l);
+    if (l->size >= l->allocated) {
+        /* We need to alloc (or realloc) the memory. */
+        Py_ssize_t new_size = l->allocated * 2;
+
+        /* See if we've ever allocated anything dynamically. */
+        if (l->p == l->data) {
+            Py_ssize_t i;
+            /* We're still using the cached data. Switch to
+               alloc-ing. */
+            l->p = PyMem_RawMalloc(sizeof(expr_ty) * new_size);
+            if (!l->p)
+                return -1;
+            /* Copy the cached data into the new buffer. */
+            for (i = 0; i < l->size; i++)
+                l->p[i] = l->data[i];
+        } else {
+            /* Just realloc. */
+            expr_ty *tmp = PyMem_RawRealloc(l->p, sizeof(expr_ty) * new_size);
+            if (!tmp) {
+                PyMem_RawFree(l->p);
+                l->p = NULL;
+                return -1;
+            }
+            l->p = tmp;
+        }
+
+        l->allocated = new_size;
+        assert(l->allocated == 2 * l->size);
+    }
+
+    l->p[l->size++] = exp;
+
+    ExprList_check_invariants(l);
+    return 0;
+}
+
+static void
+ExprList_Dealloc(ExprList *l)
+{
+    ExprList_check_invariants(l);
+
+    /* If there's been an error, or we've never dynamically allocated,
+       do nothing. */
+    if (!l->p || l->p == l->data) {
+        /* Do nothing. */
+    } else {
+        /* We have dynamically allocated. Free the memory. */
+        PyMem_RawFree(l->p);
+    }
+    l->p = NULL;
+    l->size = -1;
+}
+
+static asdl_seq *
+ExprList_Finish(ExprList *l, PyArena *arena)
+{
+    asdl_seq *seq;
+
+    ExprList_check_invariants(l);
+
+    /* Allocate the asdl_seq and copy the expressions in to it. */
+    seq = _Py_asdl_seq_new(l->size, arena);
+    if (seq) {
+        Py_ssize_t i;
+        for (i = 0; i < l->size; i++)
+            asdl_seq_SET(seq, i, l->p[i]);
+    }
+    ExprList_Dealloc(l);
+    return seq;
+}
+
+/* The FstringParser is designed to add a mix of strings and
+   f-strings, and concat them together as needed. Ultimately, it
+   generates an expr_ty. */
+typedef struct {
+    PyObject *last_str;
+    ExprList expr_list;
+} FstringParser;
+
+#ifdef NDEBUG
+#define FstringParser_check_invariants(state)
+#else
+static void
+FstringParser_check_invariants(FstringParser *state)
+{
+    if (state->last_str)
+        assert(PyUnicode_CheckExact(state->last_str));
+    ExprList_check_invariants(&state->expr_list);
+}
+#endif
+
+static void
+FstringParser_Init(FstringParser *state)
+{
+    state->last_str = NULL;
+    ExprList_Init(&state->expr_list);
+    FstringParser_check_invariants(state);
+}
+
+static void
+FstringParser_Dealloc(FstringParser *state)
+{
+    FstringParser_check_invariants(state);
+
+    Py_XDECREF(state->last_str);
+    ExprList_Dealloc(&state->expr_list);
+}
+
+/* Make a Str node, but decref the PyUnicode object being added. */
+static expr_ty
+make_str_node_and_del(PyObject **str, struct compiling *c, const node* n)
+{
+    PyObject *s = *str;
+    *str = NULL;
+    assert(PyUnicode_CheckExact(s));
+    if (PyArena_AddPyObject(c->c_arena, s) < 0) {
+        Py_DECREF(s);
+        return NULL;
+    }
+    return Str(s, LINENO(n), n->n_col_offset, c->c_arena);
+}
+
+/* Add a non-f-string (that is, a regular literal string). str is
+   decref'd. */
+static int
+FstringParser_ConcatAndDel(FstringParser *state, PyObject *str)
+{
+    FstringParser_check_invariants(state);
+
+    assert(PyUnicode_CheckExact(str));
+
+    if (PyUnicode_GET_LENGTH(str) == 0) {
+        Py_DECREF(str);
+        return 0;
+    }
+
+    if (!state->last_str) {
+        /* We didn't have a string before, so just remember this one. */
+        state->last_str = str;
+    } else {
+        /* Concatenate this with the previous string. */
+        PyObject *temp = PyUnicode_Concat(state->last_str, str);
+        Py_DECREF(state->last_str);
+        Py_DECREF(str);
+        state->last_str = temp;
+        if (!temp)
+            return -1;
+    }
+    FstringParser_check_invariants(state);
+    return 0;
+}
+
+/* Parse an f-string. The f-string is in str, starting at ofs, with no 'f'
+   or quotes. str is not decref'd, since we don't know if it's used elsewhere.
+   And if we're only looking at a part of a string, then decref'ing is
+   definitely not the right thing to do! */
+static int
+FstringParser_ConcatFstring(FstringParser *state, PyObject *str,
+                            Py_ssize_t *ofs, int recurse_lvl,
+                            struct compiling *c, const node *n)
+{
+    FstringParser_check_invariants(state);
+
+    /* Parse the f-string. */
+    while (1) {
+        PyObject *literal = NULL;
+        expr_ty expression = NULL;
+
+        /* If there's a zero length literal in front of the
+           expression, literal will be NULL. If we're at the end of
+           the f-string, expression will be NULL (unless result == 1,
+           see below). */
+        int result = fstring_find_literal_and_expr(str, ofs, recurse_lvl,
+                                                   &literal, &expression,
+                                                   c, n);
+        if (result < 0)
+            return -1;
+
+        /* Add the literal, if any. */
+        if (!literal) {
+            /* Do nothing. Just leave last_str alone (and possibly
+               NULL). */
+        } else if (!state->last_str) {
+            state->last_str = literal;
+            literal = NULL;
+        } else {
+            /* We have a literal, concatenate it. */
+            assert(PyUnicode_GET_LENGTH(literal) != 0);
+            if (FstringParser_ConcatAndDel(state, literal) < 0)
+                return -1;
+            literal = NULL;
+        }
+        assert(!state->last_str ||
+               PyUnicode_GET_LENGTH(state->last_str) != 0);
+
+        /* We've dealt with the literal now. It can't be leaked on further
+           errors. */
+        assert(literal == NULL);
+
+        /* See if we should just loop around to get the next literal
+           and expression, while ignoring the expression this
+           time. This is used for un-doubling braces, as an
+           optimization. */
+        if (result == 1)
+            continue;
+
+        if (!expression)
+            /* We're done with this f-string. */
+            break;
+
+        /* We know we have an expression. Convert any existing string
+           to a Str node. */
+        if (!state->last_str) {
+            /* Do nothing. No previous literal. */
+        } else {
+            /* Convert the existing last_str literal to a Str node. */
+            expr_ty str = make_str_node_and_del(&state->last_str, c, n);
+            if (!str || ExprList_Append(&state->expr_list, str) < 0)
+                return -1;
+        }
+
+        if (ExprList_Append(&state->expr_list, expression) < 0)
+            return -1;
+    }
+
+    assert(*ofs <= PyUnicode_GET_LENGTH(str));
+
+    /* If recurse_lvl is zero, then we must be at the end of the
+       string. Otherwise, we must be at a right brace. */
+
+    if (recurse_lvl == 0 && *ofs < PyUnicode_GET_LENGTH(str)) {
+        ast_error(c, n, "f-string: unexpected end of string");
+        return -1;
+    }
+    if (recurse_lvl != 0 && PyUnicode_READ_CHAR(str, *ofs) != '}') {
+        ast_error(c, n, "f-string: expecting '}'");
+        return -1;
+    }
+
+    FstringParser_check_invariants(state);
+    return 0;
+}
+
+/* Convert the partial state reflected in last_str and expr_list to an
+   expr_ty. The expr_ty can be a Str, or a JoinedStr. */
+static expr_ty
+FstringParser_Finish(FstringParser *state, struct compiling *c,
+                     const node *n)
+{
+    asdl_seq *seq;
+
+    FstringParser_check_invariants(state);
+
+    /* If we're just a constant string with no expressions, return
+       that. */
+    if(state->expr_list.size == 0) {
+        if (!state->last_str) {
+            /* Create a zero length string. */
+            state->last_str = PyUnicode_FromStringAndSize(NULL, 0);
+            if (!state->last_str)
+                goto error;
+        }
+        return make_str_node_and_del(&state->last_str, c, n);
+    }
+
+    /* Create a Str node out of last_str, if needed. It will be the
+       last node in our expression list. */
+    if (state->last_str) {
+        expr_ty str = make_str_node_and_del(&state->last_str, c, n);
+        if (!str || ExprList_Append(&state->expr_list, str) < 0)
+            goto error;
+    }
+    /* This has already been freed. */
+    assert(state->last_str == NULL);
+
+    seq = ExprList_Finish(&state->expr_list, c->c_arena);
+    if (!seq)
+        goto error;
+
+    /* If there's only one expression, return it. Otherwise, we need
+       to join them together. */
+    if (seq->size == 1)
+        return seq->elements[0];
+
+    return JoinedStr(seq, LINENO(n), n->n_col_offset, c->c_arena);
+
+error:
+    FstringParser_Dealloc(state);
+    return NULL;
+}
+
+/* Given an f-string (with no 'f' or quotes) that's in str starting at
+   ofs, parse it into an expr_ty. Return NULL on error. Does not
+   decref str. */
+static expr_ty
+fstring_parse(PyObject *str, Py_ssize_t *ofs, int recurse_lvl,
+              struct compiling *c, const node *n)
+{
+    FstringParser state;
+
+    FstringParser_Init(&state);
+    if (FstringParser_ConcatFstring(&state, str, ofs, recurse_lvl,
+                                    c, n) < 0) {
+        FstringParser_Dealloc(&state);
+        return NULL;
+    }
+
+    return FstringParser_Finish(&state, c, n);
+}
+
+/* n is a Python string literal, including the bracketing quote
+   characters, and r, b, u, &/or f prefixes (if any), and embedded
+   escape sequences (if any). parsestr parses it, and returns the
+   decoded Python string object.  If the string is an f-string, set
+   *fmode and return the unparsed string object.
+*/
 static PyObject *
-parsestr(struct compiling *c, const node *n, int *bytesmode)
+parsestr(struct compiling *c, const node *n, int *bytesmode, int *fmode)
 {
     size_t len;
     const char *s = STR(n);
@@ -4027,15 +4853,24 @@ parsestr(struct compiling *c, const node *n, int *bytesmode)
                 quote = *++s;
                 rawmode = 1;
             }
+            else if (quote == 'f' || quote == 'F') {
+                quote = *++s;
+                *fmode = 1;
+            }
             else {
                 break;
             }
         }
     }
+    if (*fmode && *bytesmode) {
+        PyErr_BadInternalCall();
+        return NULL;
+    }
     if (quote != '\'' && quote != '\"') {
         PyErr_BadInternalCall();
         return NULL;
     }
+    /* Skip the leading quote char. */
     s++;
     len = strlen(s);
     if (len > INT_MAX) {
@@ -4044,12 +4879,17 @@ parsestr(struct compiling *c, const node *n, int *bytesmode)
         return NULL;
     }
     if (s[--len] != quote) {
+        /* Last quote char must match the first. */
         PyErr_BadInternalCall();
         return NULL;
     }
     if (len >= 4 && s[0] == quote && s[1] == quote) {
+        /* A triple quoted string. We've already skipped one quote at
+           the start and one at the end of the string. Now skip the
+           two at the start. */
         s += 2;
         len -= 2;
+        /* And check that the last two match. */
         if (s[--len] != quote || s[--len] != quote) {
             PyErr_BadInternalCall();
             return NULL;
@@ -4088,51 +4928,84 @@ parsestr(struct compiling *c, const node *n, int *bytesmode)
         }
     }
     return PyBytes_DecodeEscape(s, len, NULL, 1,
-                                 need_encoding ? c->c_encoding : NULL);
+                                need_encoding ? c->c_encoding : NULL);
 }
 
-/* Build a Python string object out of a STRING+ atom.  This takes care of
- * compile-time literal catenation, calling parsestr() on each piece, and
- * pasting the intermediate results together.
- */
-static PyObject *
-parsestrplus(struct compiling *c, const node *n, int *bytesmode)
+/* Accepts a STRING+ atom, and produces an expr_ty node. Run through
+   each STRING atom, and process it as needed. For bytes, just
+   concatenate them together, and the result will be a Bytes node. For
+   normal strings and f-strings, concatenate them together. The result
+   will be a Str node if there were no f-strings; a FormattedValue
+   node if there's just an f-string (with no leading or trailing
+   literals), or a JoinedStr node if there are multiple f-strings or
+   any literals involved. */
+static expr_ty
+parsestrplus(struct compiling *c, const node *n)
 {
-    PyObject *v;
+    int bytesmode = 0;
+    PyObject *bytes_str = NULL;
     int i;
-    REQ(CHILD(n, 0), STRING);
-    v = parsestr(c, CHILD(n, 0), bytesmode);
-    if (v != NULL) {
-        /* String literal concatenation */
-        for (i = 1; i < NCH(n); i++) {
-            PyObject *s;
-            int subbm = 0;
-            s = parsestr(c, CHILD(n, i), &subbm);
-            if (s == NULL)
-                goto onError;
-            if (*bytesmode != subbm) {
-                ast_error(c, n, "cannot mix bytes and nonbytes literals");
-                Py_DECREF(s);
-                goto onError;
-            }
-            if (PyBytes_Check(v) && PyBytes_Check(s)) {
-                PyBytes_ConcatAndDel(&v, s);
-                if (v == NULL)
-                    goto onError;
-            }
-            else {
-                PyObject *temp = PyUnicode_Concat(v, s);
-                Py_DECREF(s);
-                Py_DECREF(v);
-                v = temp;
-                if (v == NULL)
-                    goto onError;
+
+    FstringParser state;
+    FstringParser_Init(&state);
+
+    for (i = 0; i < NCH(n); i++) {
+        int this_bytesmode = 0;
+        int this_fmode = 0;
+        PyObject *s;
+
+        REQ(CHILD(n, i), STRING);
+        s = parsestr(c, CHILD(n, i), &this_bytesmode, &this_fmode);
+        if (!s)
+            goto error;
+
+        /* Check that we're not mixing bytes with unicode. */
+        if (i != 0 && bytesmode != this_bytesmode) {
+            ast_error(c, n, "cannot mix bytes and nonbytes literals");
+            Py_DECREF(s);
+            goto error;
+        }
+        bytesmode = this_bytesmode;
+
+        assert(bytesmode ? PyBytes_CheckExact(s) : PyUnicode_CheckExact(s));
+
+        if (bytesmode) {
+            /* For bytes, concat as we go. */
+            if (i == 0) {
+                /* First time, just remember this value. */
+                bytes_str = s;
+            } else {
+                PyBytes_ConcatAndDel(&bytes_str, s);
+                if (!bytes_str)
+                    goto error;
             }
+        } else if (this_fmode) {
+            /* This is an f-string. Concatenate and decref it. */
+            Py_ssize_t ofs = 0;
+            int result = FstringParser_ConcatFstring(&state, s, &ofs, 0, c, n);
+            Py_DECREF(s);
+            if (result < 0)
+                goto error;
+        } else {
+            /* This is a regular string. Concatenate it. */
+            if (FstringParser_ConcatAndDel(&state, s) < 0)
+                goto error;
         }
     }
-    return v;
+    if (bytesmode) {
+        /* Just return the bytes object and we're done. */
+        if (PyArena_AddPyObject(c->c_arena, bytes_str) < 0)
+            goto error;
+        return Bytes(bytes_str, LINENO(n), n->n_col_offset, c->c_arena);
+    }
+
+    /* We're not a bytes string, bytes_str should never have been set. */
+    assert(bytes_str == NULL);
+
+    return FstringParser_Finish(&state, c, n);
 
-  onError:
-    Py_XDECREF(v);
+error:
+    Py_XDECREF(bytes_str);
+    FstringParser_Dealloc(&state);
     return NULL;
 }