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Playwright · 21 Reading Traces

Stage: Asynchrony, Isolation, and Diagnosis · External example · preview instructions · Storage ID: playwright-step-21

Goals and learning outcomes

Learning outcome: explain the role of Execution evidence. Input cases: an intentionally incorrect document expectation.
Deliverable: diagnostic notes linking actions, DOM, and requests. Explain the rationale for handling Diagnosis, linking it to the code.

Prerequisites

Prepare the experiment results for Signal collection and Filter rationale from Unit 20, “Console and PageError”.

Key concepts and explanations

Execution evidence

A trace provides actions and DOM·network clues. It shows events before and after a failure more fully than a single screenshot.

Diagnosis

A trace does not automatically establish the cause. Compare the last successful condition and the first failed condition.

Practice setup

First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. Write and run all tests in a separate Node project using @playwright/test and a dedicated local test origin. The existing playwright-01·02·03 previews only display test source; they do not run Playwright in the iframe. Write new test files yourself for the assignments and judge whether they pass only from actual runner output. Do not use a user profile or storage used for everyday work; preserve CSP and isolation.

The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.

Practice tasks

In tests/day-21.spec.ts in a separate practice project, set trace:on-first-retry or retain-on-failure and open the failure file. Refer to the existing Unit 1·14·28 examples only as starting points.
Input conditions: an intentionally incorrect document expectation. Baseline: off. Run under the baseline conditions, then record observations and errors in notes/day-21.md.
Variable: recording; changed value: retain on failure. Run again through the same input path. Deliverable to save: diagnostic notes linking actions, DOM, and requests. Hypothesis to check: Inspect the state immediately before the failure. Compare it with the actual results, and record whether they agree and why they differ.

Variables to observe

Variable	Baseline	Change	Expected change / hypothesis
recording	off	retain on failure	Inspect the state immediately before the failure.

Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.

Completion criteria

Required deliverable: diagnostic notes linking actions, DOM, and requests. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
The explanation of differences between the two conditions connects both concepts: Execution evidence and Diagnosis. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.

Self-check

Which processing stage does Execution evidence affect in this exercise? Input cases: an intentionally incorrect document expectation.
Variable: recording. Hypothesis to check: Inspect the state immediately before the failure. Did the actual results agree with the hypothesis? Explain the evidence in terms of Diagnosis.

Official references

Linked example and execution scope

Example source · Runtime environments and limits

Runnable standalone Playwright spec and HTTP fixture; preview does not run tests.

Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.

Next learning

Previous: Console and PageError · Next: Fixtures and Setup Responsibility

Open code, practice, and notes →