Why does ast.literal_eval('5 * 7') fail?

Question

Why does the literal evaluation of 5 * 7 fail, while 5 + 7 doesn\'t?

import ast

print(ast.literal_eval(\'5 + 7\'))
# -> 12

pri         


        

Answer 1

The question is not "why is * not accepted" but rather "why is + accepted at all".

ast.literal_eval can parse literals, but not expressions. However, in Python, complex numbers are not expressed as a single literal value; instead they consist of the real part and imaginary part added together; the imaginary part is signalled with j. literal_eval thus needs to support binary + and - to support complex number constants such as 1 + 2j or -3.4e-5 - 1.72e9j.

In many versions, including Python 3.5, literal_eval is much more lax than it needs to be - it accepts any chain of additions and subtractions for as long as both the left and right-hand sides evaluate to any number, thus (1 + 3) + 2 + (4 - 5) is still parsed, even if it is not complex constant consisting of real + imaginary part.

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+ and - are not accepted unconditionally: if you try to add 2 lists together, it will fail, even though it can parse list literals, and addition is defined for lists:

>>> ast.literal_eval('[1] + [2]')
Traceback (most recent call last):
...
ValueError: malformed node or string: <_ast.BinOp object at 0x7fffffdbe785f8>
>>> ast.literal_eval('[1, 2]')
[1, 2]
>>> [1] + [2]
[1, 2]