Can You Extend Python\'s Syntax? A Deep Dive into Custom Statements

Custom Syntax Statements in Python

Can Python's syntax be extended to include new statements like "mystatement" or "new_if"? While the feasibility of such modifications is questionable, it is theoretically possible to extend Python's syntax.

Mechanism

To add a new statement, one must modify the following components:

1. Grammar:
Define the new statement syntax in Grammar/Grammar. Add an entry for the new statement in the compound statement list.

2. AST Generation Code:
Add a definition for the new AST node in Python/ast.c. Create a function to convert the parse tree node for the new statement into an AST node.

3. Bytecode Compilation:
In Python/compile.c, add a clause to the statement visitor function for the new statement. Implement a compilation function to convert the AST node for the new statement into bytecode.

Example: Until Statement

The provided example adds an "until" statement to Python, analogous to Ruby's until statement. Here's a brief explanation of the changes made:

1. Grammar Modification:

   • Added until_stmt to the list of compound statements in Grammar/Grammar.

2. AST Generation Code Modification:

   • Added Until node to the AST definition in Python/ast.c.
   • Implemented ast_for_until_stmt to convert the parse tree node for until into an AST node.

3. Bytecode Compilation Modification:

   • Added Until_kind to compiler_visit_stmt and implemented compiler_until to compile the Until AST node into bytecode.

Limitations and Implications

Adding new syntax statements requires a deep understanding of the Python compiler and may introduce subtle bugs. It is generally not recommended to modify the syntax of Python unless necessary.

The provided example does not handle the else clause for the until statement. One could extend the implementation to include support for the else clause using a similar approach.

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