Adding New Statements to Python Syntax

Python's syntax allows for statement definitions such as print, raise, and with. While these statements provide a wide range of functionality, it is possible to extend this syntax to accommodate custom statements.

Creating Custom Statements

There are two main steps involved in creating a custom statement:

1. Modify the Grammar: You need to update Python's grammar to include the definition of the new statement. This involves modifying the Grammar/Grammar file.
2. Implement the AST Generation and Bytecode Compilation: After defining the new statement in the grammar, you must implement the necessary code to convert the statement into an Abstract Syntax Tree (AST) and then compile the AST into Python bytecode. This involves modifying files such as Python/ast.c and Python/compile.c.

Example: Creating the "Until" Statement

As an illustration, let's create an "until" statement that functions like the complement of the "while" statement. It will execute the body of the "until" statement until a specified condition becomes true.

1. Modify the Grammar: Add the "until" statement definition to the Grammar/Grammar file:

compound_stmt: if_stmt | while_stmt | until_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated
...
until_stmt: 'until' test ':' suite

2. Implement the AST Generation and Bytecode Compilation:

   • In Parser/Python.asdl, create an AST node for the "until" statement:

   | While(expr test, stmt* body, stmt* orelse)
   | Until(expr test, stmt* body)

   • Implement the ast_for_until_stmt function in Python/ast.c to convert the parse-tree node for the "until" statement into an AST node.
   • Implement the compiler_until function in Python/compile.c to compile the AST node for the "until" statement into Python bytecode.

Cautions:

While it is technically possible to add new statements to Python's syntax, it's important to approach this with caution. Adding custom statements can impact the language's maintainability and compatibility. Additionally, it's essential to consider the potential implications on code readability and debugging.