feat: Get PromptTemplate template variables at run-time (#1502)

.changeset/rotten-fans-mix.md

+---
+"@llamaindex/core": patch
+---
+
+Get PromptTemplate template variables at run-time

packages/core/src/prompts/base.ts

@@ -32,6 +32,10 @@ export abstract class BasePromptTemplate<
   const Vars extends readonly string[] = string[],
 > {
   metadata: Metadata = {};
+  /**
+   * Set of template variables used in the prompt template. Used for type hints only.
+   * To get the list of template variables used in the prompt at run-time, use the `vars` method.
+   */
   templateVars: Set<string> = new Set();
   options: Partial<Record<TemplatesVar[number] | (string & {}), string>> = {};
   outputParser: BaseOutputParser | undefined;
@@ -223,4 +227,13 @@ export class PromptTemplate<
   get template(): Template {
     return this.#template;
   }
+
+  /**
+   * Returns all the template variables used in the prompt template.
+   */
+  vars(): string[] {
+    const template = this.template;
+    const matches = template.match(/\{([^}]+)\}/g) || [];
+    return [...new Set(matches.map((match) => match.slice(1, -1)))];
+  }
 }

packages/core/tests/prompts.test.ts

@@ -158,4 +158,15 @@ describe("PromptTemplate", () => {
     const formatted = prompt.format({ text: "world", foo: "bar" });
     expect(formatted).toBe("hello world bar\noutput_instruction");
   });
+
+  test("should return all unique template vars of a prompt", () => {
+    const prompt = new PromptTemplate({
+      template: "Hello {name}! Your age is {age}. Nice to meet you {name}!",
+    });
+
+    const vars = prompt.vars();
+    expect(vars).toHaveLength(2);
+    expect(vars).toContain("name");
+    expect(vars).toContain("age");
+  });
 });