Expression Language

The Expression Language is a domain-specific language (DSL) that allows for the evaluation of expressions with a syntax similar to Python. It provides a restricted subset of Python’s features to ensure safe and controlled execution.

See the inference documentation for details on how to use these at run time.

Syntax

The Expression Language supports the following syntax elements:

Literals

• Numbers: Integer and floating-point numbers are supported.

  • Examples: 42, 3.14

• Strings: String literals are enclosed in double quotes (").

  • Example: "Hello, world!"

• Booleans: Boolean values are represented by True and False.

• None: The None value represents the absence of a value.

Arithmetic Operations

• Addition: +

  • Example: 1 + 2

• Subtraction: -

  • Example: 5 - 3

• Multiplication: *

  • Example: 2 * 4

• Division: /

  • Example: 10 / 2

• Exponentiation: ^ or **

  • Example: 2^3 or 2**3

• Bitwise XOR: ^

  • Example: 5 ^ 3

Comparison Operations

• Equal to: ==

  • Example: x == y

• Less than: <

  • Example: x < y

• Less than or equal to: <=

  • Example: x <= y

• Greater than: >

  • Example: x > y

• Greater than or equal to: >=

  • Example: x >= y

Logical Operations

• Logical AND: and

  • Example: x and y

• Logical OR: or

  • Example: x or y

• Logical NOT: not

  • Example: not x

Variables

Variables can be referenced by their names. They are resolved from the context in which the expression is evaluated.

• Example: x + y

If a variable is not defined or has a value of None, it will be treated as None in the expression.

Functions

The Expression Language provides a set of built-in functions:

• max(arg1, arg2, ...): Returns the maximum value among the arguments.

• min(arg1, arg2, ...): Returns the minimum value among the arguments.

• len(arg): Returns the length of a string or a list.

• str(arg): Converts the argument to a string.

• case(cond1, val1, cond2, val2, ..., default): Evaluates the conditions in order and returns the value associated with the first true condition. If no condition is true, the default value is returned.

Attribute Access

Attributes of objects can be accessed using dot notation (object.attribute). Attribute access can be chained to navigate through nested objects.

• Example: person.name, person.address.street

When an attribute is accessed on a list, the operation is distributed over the elements of the list.

• Example: persons.name (returns a list of names)

Conditional Expressions

Conditional expressions allow for the evaluation of different expressions based on a condition.

• Syntax: <expression_if_true> if <condition> else <expression_if_false>

• Example: "Positive" if x > 0 else "Non-positive"

Operator Precedence

The Expression Language follows the same operator precedence rules as Python. Parentheses can be used to override the default precedence order.

Limitations

The Expression Language has the following limitations:

• It does not support control flow statements such as loops or function definitions.

• It does not allow the use of certain potentially unsafe operations, such as __import__.

• It is designed for simple expressions and does not provide the full range of features available in Python.

Examples

Here are a few examples of expressions in the Expression Language:

• 1 + 2 * 3: Evaluates to 7.

• "Hello, " + "world!": Evaluates to "Hello, world!".

• x > 10 and y < 5: Evaluates to True if x is greater than 10 and y is less than 5, otherwise False.

• max(x, y, z): Returns the maximum value among x, y, and z.

• person.name: Returns the value of the name attribute of the person object.

• "Positive" if x > 0 else "Non-positive": Returns "Positive" if x is greater than 0, otherwise returns "Non-positive".

This documentation provides an overview of the Expression Language and its supported features. It is independent of any specific implementation or programming language.