context manager methods (__enter__ and __exit__) raise a SyntaxError

[germanptr]

BUG/PROBLEM REPORT (OR OTHER COMMON ISSUE)

If this ticket is accepted as a bug or feature request, I volunteer to work on it (once contributor agreement is approved).

What I did:

I tried to create a context manager in restricted python, here is the code:

from RestrictedPython import compile_restricted
source_code = """
class example:
    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, exc_traceback):
        pass
"""
compile_restricted(source_code, '<inline>', 'exec')

What I expect to happen:

I expected to have a context manager that I could use in a with statement

What actually happened:

I get the following error:

SyntaxError: ('Line 3: "__enter__" is an invalid variable name because it starts with "_"', 'Line 6: "__exit__" is an invalid variable name because it starts with "_"')

These methods (__enter__ and __exit__) are missing from transformer.ALLOWED_FUNC_NAMES

What version of Python and Zope/Addons I am using:

Python 3.8.2
RestrictedPython 5.2
macOS Monterey 12.5.1

[icemac]

Currently you are not able to define a context manager inside restricted Python because the needed method names start with _.
But I see the value in being able to do so.

A pull request is welcome adding __enter__ and __exit__ to ALLOWED_FUNC_NAMES including a test showing that context managers can be defined and used afterwards in restricted Python.