'_[1]' in .co_names using builtin compile() in Python 2.6

magnus.lycka at gmail.com magnus.lycka at gmail.com
Wed Nov 27 20:40:52 CET 2013

When I run e.g. compile('sin(5) * cos(6)', '<string>', 'eval').co_names, I get ('sin', 'cos'), which is just what I expected.

But when I have a list comprehension in the expression, I get a little surprise:
>>> compile('[x*x for x in y]',  '<string>', 'eval').co_names
('_[1]', 'y', 'x')

This happens in Python 2.6.6 on Red Hat Linux, but not when I run Python 2.7.3 in Windows. Unfortunately I'm stuck with 2.6.

* Are there more surprises similar to this one that I can expect from compile(...).co_names? Is this "behaviour" documented somewhere?

* Is there perhaps a better way to achieve what I'm trying to do?

What I'm really after, is to check that python expressions embedded in text files are:
- well behaved (no syntax errors etc)
- don't accidentally access anything it shouldn't
- I serve them with the values they need on execution

So, in the case of "a.b + x" I'm really just interested in a and x, not b. So the (almost) whole story is that I do:

    # Find names not starting with ".", i.e a & b in "a.c + b"
    abbr_expr = re.sub(r"\.\w+", "", expr)
    names = compile(abbr_expr, '<string>', 'eval').co_names
    # Python 2.6 returns '_[1]' in co_names for list comprehension. Bug?
    names = [name for name in names if re.match(r'\w+$', name)]

    for name in names:
        if name not in allowed_names:
            raise NameError('Name: %s not permitted in expression: %s' % (name, expr))

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