How to evaluate python string boolean expression using eval()?

Question

I get boolean expression like below :

string = '!True && !(True || False || True)'

I know eval('1+2') returns 3. But when I am executing eval(string) it is throwing me error as in invalid syntax.

Is there any other way I can execute the above expression?

python doesn't use `&&` and `||` - you want `and`, `or`, `not`. — georg, Mar 25 '15 at 10:18
`!True` is not valid Python. The `!` operator doesn't exist in Python, use `not` instead. — Martijn Arts, Mar 25 '15 at 10:19

score 4 · Answer 1 · answered Mar 25 '15 at 10:19

None of !, && and || are valid Python operators; eval() can only handle valid Python expressions.

You'd have to replace those expressions with valid Python versions; presumably ! is not, && is and, and || is or, so you could just replace those with the Python versions:

eval(string.replace('&&', 'and').replace('||', 'or').replace('!', 'not '))

Note the space after not because Python requires this.

The better approach would be to not use the wrong spelling for those operators (they look like Java or JavaScript or C).

score 2 · Answer 2 · edited Apr 19 '21 at 15:01

If you want to parse boolean logic (in contrast to control flow) take a look at this stackoverflow post. It mentions pyparsing

The pyparsing module is an alternative approach to creating and executing simple grammars

and sympy.

The logic module for SymPy allows to form and manipulate logic expressions using symbolic and boolean value.

There also seems to be a parser implemented with pyparsing.

Of course you could also write a parser yourself.

How to evaluate python string boolean expression using eval()?

2 Answers2

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