Why Python don't accept 03 as a number?

Ian Kelly ian.g.kelly at gmail.com
Sat Dec 8 22:57:10 EST 2018

On Sat, Dec 8, 2018 at 7:57 PM <jfong at ms4.hinet.net> wrote:
> Grant Edwards於 2018年12月9日星期日 UTC+8上午12時52分04秒寫道:
> > Just to be clear: you do _not_ want to use eval on the string.
> >
> > If you're not the one who created the string, it might wipe your hard
> > drive or empty your bank account.  If you _are_ the one who created
> > the string, then generate the desired result instead.
> >
> > --
> > Grant
> I didn't evaluate the input string directly. It's the translated "digit" string been evaluated, so shouldn't have any danger on using eval().

Replacing the first five letters of the alphabet is not sufficient to
sanitize untrusted input for eval. Here's a simple example that avoids
using any of those letters:

py> eval(re.sub(r'[a-e]', '0',
"__import__('su\\x62pro\\x63\\x65ss').run('\\x65\\x63ho rm -rf /',
**{'sh\\x65ll': 1})"))
rm -rf /
CompletedProcess(args='echo rm -rf /', returncode=0)

Now, if you remove *all* the characters that could possibly start
identifiers (https://docs.python.org/3/reference/lexical_analysis.html#identifiers)
then you might be safe. Possibly just removing all the ones in ASCII
(A-Z + a-z + _) would suffice. I make no guarantees either way.

I wish I could say you should just use ast.literal_eval instead.
Unfortunately it doesn't seem to support ==:

py> ast.literal_eval('10 + 20 == 30')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.5/ast.py", line 84, in literal_eval
    return _convert(node_or_string)
  File "/usr/lib/python3.5/ast.py", line 83, in _convert
    raise ValueError('malformed node or string: ' + repr(node))
ValueError: malformed node or string: <_ast.Compare object at 0x78172bee5358>

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