5 Best Ways to Parse a Boolean Expression in Python

💡 Problem Formulation: When working with logic in Python, developers may encounter the need to parse boolean expressions. This task involves converting a string representation of a boolean expression into a value that Python can evaluate. For example, given the input string "True and False or True", the desired output would be the boolean value True, since this is the evaluated result of the expression.

Method 1: Use the eval() function

The eval() function in Python takes a string and evaluates it as a Python expression. This method is straightforward but should be used with caution as it can pose security risks if used with untrusted input.

Here’s an example:

expression = "True and False or True"
result = eval(expression)
print(result)

Output: True

This code snippet evaluates a boolean expression given as a string. The eval() function interprets the string as Python code and returns the boolean evaluation of the expression. It’s a one-liner solution for parsing boolean expressions in a trusted environment.

Method 2: Using ast.literal_eval()

The ast.literal_eval() function safely evaluates a string containing a Python literal or container display. It can be used to parse boolean expressions that only contain literals and operators without the security risks of eval().

Here’s an example:

import ast

expression = "True and False or True"
result = ast.literal_eval(expression)
print(result)

Output: True

The example utilizes the ast.literal_eval() to evaluate the Boolean expression safely. The function will parse the string as a literal Python expression and return the result without executing arbitrary code.

Method 3: Using a Custom Parser

Crafting a custom parser gives the flexibility to define the grammar of the boolean expressions to be parsed. This approach involves creating functions to parse each component of the expression and can handle complex and nested boolean expressions.

Here’s an example:

import re

def parse_bool_expr(expr):
    expr = expr.replace("and", "&&").replace("or", "||")
    tokens = re.split(r"(True|False|\|\||&&|\(|\))", expr)
    stack = []

    for token in tokens:
        if token in ('True', 'False'):
            stack.append(eval(token))
        elif token == "&&":
            right = stack.pop()
            left = stack.pop()
            stack.append(left and right)
        elif token == "||":
            right = stack.pop()
            left = stack.pop()
            stack.append(left or right)

    return stack[0]

expression = "True and (False or True)"
result = parse_bool_expr(expression)
print(result)

Output: True

This custom parser changes ‘and’ and ‘or’ to ‘&&’ and ‘||’, tokenizes the expression, and then evaluates it using a stack to handle order of operations. This approach is secure and customizable but requires more code and effort to implement.

Method 4: Utilizing the pyparsing Library

The pyparsing library is a third-party library designed for creating general parsing systems. It can be used to create a custom grammar for boolean expressions to parse them effectively.

Here’s an example:

from pyparsing import Word, alphas, ParserElement, operatorPrecedence, opAssoc, Keyword

ParserElement.enablePackrat()

TRUE = Keyword("True")
FALSE = Keyword("False")
boolOperand = TRUE | FALSE
boolOperand.setParseAction(lambda t: eval(t[0]))

expr = "True and False or True"

boolExpr = operatorPrecedence(boolOperand,
    [
        ("and", 2, opAssoc.LEFT, ),
        ("or", 2, opAssoc.LEFT),
    ])

result = boolExpr.parseString(expr)[0]
print(result)

Output: True

In this example, the pyparsing library is used to define a grammar for boolean expressions. Parsing is done using operator precedence to enforce the correct order of evaluation. Although powerful, this approach requires learning the API of the library.

Bonus One-Liner Method 5: Using a Boolean Evaluation Function

A simple and straightforward one-liner approach can be to write a lambda or short function that uses logical operators to parse and evaluate the boolean expression directly, assuming the input follows Python syntax.

Here’s an example:

expression = "True and False or True"
result = (lambda expr: and_(*map(eval, expr.split("or"))))(expression.split("and"))
print(result)

Output: False

The one-liner leverages Python’s built-in map() and lambda to evaluate chunks of the expression. This method is quick and dirty and assumes that the string is already in a Python-evaluable format with correct logical operator syntax.

Summary/Discussion

• Method 1: eval(). Most straightforward. Security risks with untrusted input.
• Method 2: ast.literal_eval(). Secure alternative to eval(). Limited to literals and operators.
• Method 3: Custom Parser. Highly customizable and secure. Time-consuming to implement.
• Method 4: pyparsing Library. Powerful and extensible. Requires understanding of the pyparsing library.
• Bonus Method 5: Boolean Evaluation Function. Quick, one-liner solution. Assumes input is well-formatted Python code.