Welcome to ratus’s documentation!

Ratus is a simple expression language that can be used to easily extend Python applications with an embedded expression language. Evaluating basic expressions is as simple as:

from ratus import Evaluator

evaluator = Evaluator()
evaluator.evaluate("1 + 1") # 1
evaluator.evaluate("1 > 1") # False
evaluator.evaluate("if(1 < 2, 10, 5)") # 5

Expression language

The ratus expression language allows the following constructs:

  • Int, Float and String literals
  • Comparison operations
    • Greater than (>)
    • Greater than or equal (>=)
    • Less than (<)
    • Less than or equal (<=)
    • Equal (=)
    • Not equal (!=)

Function injection

You can add new functions by creating the Evaluator object with a dictionary argument in the constructor that maps the function name to a callable that performs the desired behavior.

from ratus import Evaluator

def f(x):
    return x**2

evaluator = Evaluator({
    "f": f,
    "g": lambda x: x**3
evaluator.evaluate("f(2)") # 4
evaluator.evaluate("g(2)") # 8

Unary/Binary operation override

You can override what the unary and binary operations do by providing a dictionary as the unary_ops and binary_ops argument, respectively, to Evaluator’s constructor. This will update the mapping for the specified operations but will not change any of the others.

from ratus import Evaluator, BinaryOpTypes, UnaryOpTypes

def gt(x, y):
    return x >= y
def neg(x):
    return x

evaluator = Evaluator(
    binary_ops={BinaryOpTypes.GREATER: gt},
    unary_ops={UnaryOpTypes.NEGATIVE: neg}
evaluator.evaluate("-1") # 1
evaluator.evaluate("1 > 1") # True

This is a contrived example and hopefully not something you would need to do but it serves as an example of how you can alter the behavior of all operators.

Indices and tables