Transpilers Research¶

What is a transpiler?¶

Introduces a transpiler and where it fits in Transpilers.

A source-to-source compiler translates code between languages at the same abstraction level (unlike a compiler, which lowers to machine code).

Python source → tokenize → parse (AST) → transform → generate target source

Existing Python transpilers¶

Cython, mypyc, Transcrypt, and Codon as real-world examples.

Reading the AST¶

Parse source into a tree — the front half of any transpiler.

The standard library parses Python into an AST you can walk — the front half of any transpiler:

import ast

tree = ast.parse("x = 1 + 2 * 3")
print(ast.dump(tree, indent=None)[:60])
# Module(body=[Assign(targets=[Name(id='x', ctx=Store())

ast.unparse turns an AST back into source — a trivial Python→Python transpiler:

import ast

tree = ast.parse("a=1+2")
print(ast.unparse(tree))   # a = 1 + 2

A working Python-to-C expression transpiler¶

A working Python-to-C expression transpiler — a key concept in Transpilers.

This walks an arithmetic expression's AST and emits equivalent C. It handles numbers, names, and the four basic operators:

import ast

def to_c(node):
    if isinstance(node, ast.Expression):
        return to_c(node.body)
    if isinstance(node, ast.Constant):
        return str(node.value)
    if isinstance(node, ast.Name):
        return node.id
    if isinstance(node, ast.BinOp):
        op = {ast.Add: "+", ast.Sub: "-", ast.Mult: "*", ast.Div: "/"}[type(node.op)]
        return f"({to_c(node.left)} {op} {to_c(node.right)})"
    raise NotImplementedError(type(node).__name__)

tree = ast.parse("a + 2 * b", mode="eval")
print(to_c(tree))   # (a + (2 * b))

The output is valid C for the same expression. A real transpiler extends this to statements, control flow, and type inference — but the shape is always parse → walk → emit.

The hard parts¶

Dynamic typing, runtime semantics, and the standard library.

• Dynamic typing — the target may need inferred or declared types.
• Runtime semantics — duck typing, exceptions, and the GC rarely map 1:1.
• Standard library — every used function must have a target equivalent.

Practice exercises¶

1. Extend to_c to support unary minus (ast.UnaryOp).
2. Add comparison operators (<, >, ==) to the transpiler.
3. Write a transpiler that emits JavaScript instead of C from the same AST.
4. Use ast.NodeVisitor to collect every variable name referenced in an expression.