---
title: "CompetitionOutput"
description: "The starting point for the output of another competition: you describe the solution, the rest is done."
---

> Documentation Index
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> Use this file to discover all available pages before exploring further.

# CompetitionOutput

## What it does

The base of the competition output observers. It writes the `o` lines while solving and the `s` line at
the end; a subclass only says which file it writes, which instance formats it applies to, and how the
solution looks.

## Use it

Subclass it, set `name` (the output file is `logs/<run_id>.<name>.out`) and `formats`, and return the
solution's lines (without `v `) from `solution`:

```python
from cpbenchy import formats
from cpbenchy.observers import CompetitionOutput

class SchedulingOutput(CompetitionOutput):
    name = "schedule"
    formats = ("jsplib", "rcpsp")

    def solution(self, ctx):  # v start[0] = 3, v start[1] = 0, ...
        solution = formats.solution_dict(formats.instance_variables(ctx.model))
        return [f"{name} = {value}" for name, value in solution.items()]
```

Use it like any observer: `-p myoutput.py:SchedulingOutput`, or in rules. Under
[`cpbenchy solve`](/library/solve/), it writes to stdout instead, for submissions.

## What it gives you

| | |
|---|---|
| `solution(ctx)` | override: the solution, as the lines after `v ` |
| `on_solution` | writes `o <objective>` for each solution found while solving |
| `on_finish` | writes the `s` line and the solution, or `s UNSATISFIABLE`, or a comment and `s UNKNOWN` |
| `write(ctx, *lines)` | appends lines to the output (or stdout) |
| `output(ctx_or_run)` | the output file, from the worker's `ctx` or the parent's `run` |

[`cpbenchy.formats`](/library/solution-formats/) has the pieces for common formats.

## Implementation

Source: https://docs.cpbenchy.com/library/competition-output/index.mdx