Interpreter

The Interpreter pattern defines a grammatical representation for a language along with an interpreter that uses this representation to interpret sentences in the language. Essentially, it allows you to build an interpreter for a simple language without resorting to parsing. It promotes handling requests that are structured as language elements.

This pattern is particularly useful when you have a language with a simple grammar, and you need to execute statements within that language. Common scenarios include evaluating expressions (e.g., mathematical formulas, logical conditions), processing simple command languages (like a text-based game’s commands), or building rule engines. It shines when the grammar’s complexity doesn’t warrant the use of full-fledged parsers and lexers.

Usage

The Interpreter pattern is used in:

• SQL Parsers: Although full SQL parsers exist, simplified versions can use the Interpreter pattern for basic clause evaluations.
• Expression Evaluation: Evaluating complex mathematical or logical expressions without using eval().
• Configuration Files: Interpreting configuration files with a defined syntax.
• Rule Engines: Applying a set of rules based on a defined language to a data set.
• Simple Scripting Languages: Implementation of straightforward scripting used in specific domain contexts.

Examples

1. Python’s ast module: Python’s Abstract Syntax Trees (AST) are a lightweight form of interpretation. While not strictly adhering to the traditional Interpreter pattern in a direct, implementation-focused way, it provides a way to represent code as a tree structure that can then be “interpreted” (executed or analyzed) by other code. The ast module parses Python source code into an AST, which you can then traverse and evaluate.

   python import ast

   expression = ast.parse(“2 + 3 * 4”)

   def evaluate(node): if isinstance(node, ast.Num): return node.n elif isinstance(node, ast.BinOp): left = evaluate(node.left) right = evaluate(node.right) if isinstance(node.op, ast.Add): return left + right elif isinstance(node.op, ast.Mult): return left * right else: raise ValueError(“Unsupported node type”)

   result = evaluate(expression.body[0].value) print(result) # Output: 14

2. JSONPath: JSONPath is a query language for JSON. It’s implemented using an interpreter that traverses the JSON structure based on the path expression. Libraries like jsonpath-ng in Python use the Interpreter pattern to parse and evaluate JSONPath expressions.

   python from jsonpath_ng.ext import parse

   json_data = { “store”: { “book”: [ {“category”: “reference”, “author”: “Nigel Rees”, “title”: “Sayings of the Century”, “price”: 8.95}, {“category”: “fiction”, “author”: “Evelyn Waugh”, “title”: “Sword of Honour”, “price”: 12.99} ] } }

   jsonpath_expression = parse(’$.store.book[?category==“fiction”]..title’) matches = jsonpath_expression.find(json_data)

   for match in matches: print(match.value) # Output: Sword of Honour