How can I add new statements to the syntax of Python?

Is it possible to add new statements to Python's syntax?

Yes, it is possible to add new statements to Python's syntax. However, this requires modifying the Python interpreter's code.

How can new statements be added to Python's syntax?

You can modify the grammar file (Grammar/Grammar) to add a definition for the new statement and modify the AST generation code (Python/ast.c) to convert the new parse tree node into an AST node. Then, modify the bytecode compilation code (Python/compile.c) to compile the new statement into bytecode. Finally, modify the symbol table generation code (Python/symtable.c) to handle the new statement.

Example:

To add an "until" statement that is the complement of "while":

1. Add a definition for the "until" statement to Grammar/Grammar:

<code class="text">compound_stmt: if_stmt | while_stmt | until_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated
until_stmt: 'until' test ':' suite</code>

2. Add an AST node for the "until" statement in Python/ast.c:

<code class="c">| Until(expr test, stmt* body)</code>

3. Implement the ast_for_until_stmt() function to create an AST node for the "until" statement:

<code class="c">static stmt_ty
ast_for_until_stmt(struct compiling *c, const node *n)
{
    /* until_stmt: 'until' test ':' suite */
    REQ(n, until_stmt);

    if (NCH(n) == 4) {
        expr_ty expression;
        asdl_seq *suite_seq;

        expression = ast_for_expr(c, CHILD(n, 1));
        if (!expression)
            return NULL;
        suite_seq = ast_for_suite(c, CHILD(n, 3));
        if (!suite_seq)
            return NULL;
        return Until(expression, suite_seq, LINENO(n), n->n_col_offset, c->c_arena);
    }

    PyErr_Format(PyExc_SystemError,
                 "wrong number of tokens for 'until' statement: %d",
                 NCH(n));
    return NULL;
}</code>

4. Implement the compiler_until() function to compile the "until" statement into bytecode:

<code class="c">static int
compiler_until(struct compiler *c, stmt_ty s)
{
    basicblock *loop, *end, *anchor = NULL;
    int constant = expr_constant(s->v.Until.test);

    if (constant == 1) {
        return 1;
    }
    loop = compiler_new_block(c);
    end = compiler_new_block(c);
    if (constant == -1) {
        anchor = compiler_new_block(c);
        if (anchor == NULL)
            return 0;
    }
    if (loop == NULL || end == NULL)
        return 0;

    ADDOP_JREL(c, SETUP_LOOP, end);
    compiler_use_next_block(c, loop);
    if (!compiler_push_fblock(c, LOOP, loop))
        return 0;
    if (constant == -1) {
        VISIT(c, expr, s->v.Until.test);
        ADDOP_JABS(c, POP_JUMP_IF_TRUE, anchor);
    }
    VISIT_SEQ(c, stmt, s->v.Until.body);
    ADDOP_JABS(c, JUMP_ABSOLUTE, loop);

    if (constant == -1) {
        compiler_use_next_block(c, anchor);
        ADDOP(c, POP_BLOCK);
    }
    compiler_pop_fblock(c, LOOP, loop);
    compiler_use_next_block(c, end);

    return 1;
}</code>

5. Modify the symtable_visit_stmt() function in Python/symtable.c to handle "until" statements:

<code class="c">case While_kind:
    VISIT(st, expr, s->v.While.test);
    VISIT_SEQ(st, stmt, s->v.While.body);
    if (s->v.While.orelse)
        VISIT_SEQ(st, stmt, s->v.While.orelse);
    break;
case Until_kind:
    VISIT(st, expr, s->v.Until.test);
    VISIT_SEQ(st, stmt, s->v.Until.body);
    break;</code>

Note: This is a high-level overview. Refer to the quoted article for more detailed steps and explanations.

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