Are there undocumented ast.literal_eval limitations?

Question

Using, python 3.4, that works:

>>> ast.literal_eval("2 - 1")
1

but that doesn't:

>>> ast.literal_eval("1 | 2")
raise ValueError('malformed node or string: ' + repr(node))
ValueError: malformed node or string: <_ast.BinOp object at 0x0000000003338978>

>>> ast.literal_eval("1 ^ 2")
raise ValueError('malformed node or string: ' + repr(node))
ValueError: malformed node or string: <_ast.BinOp object at 0x0000000003338400>

All values are literals, there's no difficulty performing logical operations when it can perform additions, substractions...

Why can't I perform logical operations using ast.literal_eval ?

I thought it was related to constant folding, but that does not seem to be a case, as `1|2` seems to be correctly folded to `3` (according to `dis` module). — Łukasz Rogalski, Dec 07 '16 at 13:51
@ŁukaszRogalski Additionally, >>> ast.dump(ast.parse("1 ^ 2")) 'Module(body=[Expr(value=BinOp(left=Num(n=1), op=BitXor(), right=Num(n=2)))])' — pradyunsg, Dec 07 '16 at 13:56
@pradyunsg: `ast` does not do any constant folding, so I'm not sure what point you are trying to make — Eric, Dec 07 '16 at 13:59
@Eric That the constant folding does not happen at the ast level. At least not until the result from `ast.parse`. — pradyunsg, Dec 07 '16 at 14:01

Eric · Accepted Answer · 2016-12-07T14:11:21.750

4

Because it's not implemented that way in Python 2.7.

At a glance, addition is only supported in order to enable complex literals like 1 + 2j, since it rejects 2j + 1.

In python 3.5, the implementation was made a bit more lenient, and does not restrict addition in this way.

edited Dec 07 '16 at 14:11

answered Dec 07 '16 at 13:51

Eric

95,302
53
242
374

Hah. You beat me to it. – pradyunsg Dec 07 '16 at 13:55
No. Python 3.5 still restricts it. – pradyunsg Dec 07 '16 at 13:57
@pradyunsg: Try `ast.literal_eval('2j + 1')`. Works in 3.5, but not 2.7. So 3.5 is less restrictive – Eric Dec 07 '16 at 13:57
Ah. I thought you were comparing 3.4 and 3.5. – pradyunsg Dec 07 '16 at 14:00
You're right, support for Add and Sub was added to support complex literals, see [issue4907](http://bugs.python.org/issue4907). – mata Dec 07 '16 at 14:10

pradyunsg · Answer 2 · 2016-12-07T13:59:32.867

1

It's just not implemented that way. The only operators "white-listed" are Add and Sub. 3.4, 3.5

~~This could be filed as a bug-report to http://bugs.python.org, asking for support of more operators.~~

edited Dec 07 '16 at 13:59

answered Dec 07 '16 at 13:54

pradyunsg

18,287
11
43
96

Are there undocumented ast.literal_eval limitations?

2 Answers2