Running Brython

[dbohdan]

Finally, a way to sandbox Python in pure Python. :-)

I have tried running an old release of the Python 3 implementation Brython in micro-javascript and got a micro-javascript exception. I picked an old release to guarantee ES5.

Here are the steps to reproduce the exception:

$ wget https://github.com/brython-dev/brython/releases/download/2.2.1/Brython2.2.1-20140915-210107.zip
$ unzip Brython2.2.1-20140915-210107.zip
$ cd Brython2.2.1-20140915-210107/

$ # Fix incompatible JS syntax. Change to `-i.bak` for FreeBSD sed. 
$ sed -i 's|return *$|return;|;s|0\.)|0.0)|' brython.js

$ cat > test.py
from pathlib import Path

from microjs import Context

ctx = Context(memory_limit=1024*1024, time_limit=5.0)
result = ctx.eval(Path("brython.js").read_text())

print(result)

$ uv run --with micro-javascript python3 test.py
Traceback (most recent call last):
  File "/tmp/Brython2.2.1-20140915-210107/test.py", line 6, in <module>
    result = ctx.eval(Path("brython.js").read_text())
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/context.py", line 1208, in eval
    compiled = compiler.compile(ast)
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/compiler.py", line 122, in compile
    self._compile_statement(stmt)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/compiler.py", line 384, in _compile_statement
    self._compile_expression(node.expression)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/compiler.py", line 1494, in _compile_expression
    self._compile_expression(node.callee)
    ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/compiler.py", line 1507, in _compile_expression
    func = self._compile_function(
        name, node.params, node.body, is_expression=True
    )
  File "/home/dbohdan/.cache/uv/archive-v0/qRjrQTsy8XxabUpgxFIky/lib/python3.14/site-packages/microjs/compiler.py", line 1092, in _compile_function
    bytecode=bytes(self.bytecode),
             ~~~~~^^^^^^^^^^^^^^^
ValueError: bytes must be in range(0, 256)

The same thing happens with other large, minified JavaScript source files, like an old version of jQuery.