Lightweight Expressions: Example of simple SQLAlchemy expression compiler. For more information see https://github.com/Stiivi/expressions

sqlalchemy_expression_compiler.py

class SQLAlchemyExpressionCompiler(object):
    def __init__(self, statement):
        # Context of this compiler is a SQLAlchemy statement object
        self.statement = statement

    def compile_literal(self, literal):
        return literal

    def compile_variable(self, variable):
        # Get a column object from the statement
        return self.statement.c[variable]

    def compile_operator(self, operator, op1, op2):
        if operator == "+":
            return op1 + op2
        elif operator == "-":
            return op1 - op2
        elif operator == "*":
            return op1 * op2
        elif operator == "/":
            return op1 / op2
        else:
            raise SyntaxError("Unknown operator '%s'" % operator)

    def compile_function(self, function, args):
        raise NotImplementedError

# Assume that we have a `table` object which is instance of sqlalchemy `Table` class

compiler = SQLAlchemyExpressionCompiler(table)

expr = "(amount / transactions) * 2"
expr = Expression(expr)
selection = expr.compile(compiler)

statement = sqlalchemy.sql.expression.select([selection], table)
print(statement)

# Prints:
#
# SELECT (data.amount / data.transactions) * ? AS anon_1 
# FROM data