Prompt templates and variables

Understand the concept before the code

Start from zero: Prompt templates and variables is part of learning Haystack as a practical tool, not just memorizing commands. Use Haystack in practical AI, data science and model development workflows. This beginner lesson shows how to identify the object you work with while keeping every input small enough to inspect. Imagine a teammate asks why a calculation, prediction, image annotation or service response changed. A good answer identifies which data entered the step, which operation ran, and which evidence proves the result.

Before using a real account or a large dataset, write your intended input and output on paper. Work with a tiny synthetic sample, confirm the interpretation, then decide whether the technique belongs in your project. A running program is only the first check: it is possible to run a script successfully and still obtain the wrong answer because the units, labels or assumptions were incorrect.

The purpose of Prompt templates and variables is to make a useful engineering decision in the context of Haystack. You will see concrete input and output rather than a slogan about artificial intelligence. The programs below use ordinary Python so a complete beginner can check the reasoning before attempting a larger or paid library workflow. They are concept labs, not claims that Haystack executed them.

python3 --version
python3 -m venv .venv
source .venv/bin/activate

Worked code examples and actual outputs

Both Python concept labs below were executed locally during packaging. They demonstrate reasoning skills useful before moving to the named third-party library; they do not substitute for a library API tutorial.

Example 1 — inspect a simple result for Prompt templates and variables

The code uses a deliberately small example. Read the input values, calculate the predicted result without a computer, then run it and confirm the output. Pay attention to the data type of the result.

from statistics import mean,median
readings=[3,5,7,9]
print('mean:',mean(readings),'median:',median(readings))

mean: 6 median: 6.0

Why it works: Python evaluates the statements in order. Every printed value comes from the input records shown, so you can check the calculation manually. In a Haystack project, the comparable principle is to identify which object represents the data and which operation changes it.

Example 2 — independent verification

This second program checks a different operation. Compare its input assumptions with the first, then modify the values and observe which parts of the output change. Notice that a convincing result requires both a working command and the correct interpretation.

versions=['v1','v2','v2','v3']
print('unique:',sorted(set(versions)))

unique: ['v1', 'v2', 'v3']

Interpretation: the displayed result is the direct, captured standard output of the shown program. It is not a screenshot, a fabricated model prediction or a live cloud response. Connect this to Prompt templates and variables by identifying the analogous input, transformation and validation procedure for Haystack.

Six-step practical learning workflow

Hands-on project

Create three mock requests, known reference answers and a declared tool permission boundary. Use the concept from “Prompt templates and variables” to perform one measurable operation in Haystack when that feature is available, and compare it to the pure-Python demonstration below. Note whether the library handles validation, persistence, model execution, annotation or visualization itself, because those are different responsibilities.

1. Write one expected result before running anything; use numbers or text that can be verified manually.
2. Run the small demonstration; change one input and predict exactly how the output should change.
3. Use the maintained documentation of Haystack to locate the corresponding feature or API, and identify its input contract.
4. Test an empty input, mismatched type, invalid setting or unavailable dependency. Record the precise error rather than suppressing it.
5. Save a short report that distinguishes what you executed from what you only proposed; include version information when you used a third-party package.

Chapter summary

In this lesson you explored Prompt templates and variables as one specific capability or engineering concern in Haystack. You worked through Python code with observed outputs, learned how to distinguish an explanation from a real library execution, and designed an experiment around inputs, checks and failure cases. Keep the code and the verification notes together so you can repeat the result later.

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