---
metadata:
  - name: generator
    content: Diplodoc Platform v5.63.3
alternate:
  - https://ytsaurus.tech/docs/en/flow/python/examples/wordcount.md
  - https://ytsaurus.tech/docs/ru/flow/python/examples/wordcount.md
  - href: https://ytsaurus.tech/docs/en/flow/python/examples/wordcount.md
    type: text/markdown
    title: Markdown version
  - href: https://ytsaurus.tech/docs/en/llms.txt?revision=181542d8c2d9f36be925894a517db8203cafae65
    rel: describedby
---
> **Documentation Index:** Fetch the complete configuration index at https://ytsaurus.tech/docs/en/llms.txt?revision=181542d8c2d9f36be925894a517db8203cafae65

<!-- source: en/_includes/flow/python/examples/wordcount.md -->
# Word Count in YTsaurus Flow (Python)

Use this simple stateful-pipeline example in Python to count how many times each word appears. You’ll use an internal YSON-state for the counts.

[Source code](https://github.com/ytsaurus/ytsaurus/tree/main/yt/yt/flow/examples/python/word_count)

## Structure

The pipeline includes a single transform-computation called `mapper`. You use it to read words from the input stream and update the counter in the state.

## `__main__.py`

This is the entry point. You create the pipeline and register the only computation.

```python
def main():
    log.info("Starting companion execution")

    pipeline = Pipeline()
    pipeline.add("mapper", WordCountMapper())

    log.info("Starting pipeline...")
    pipeline.run()
    log.info("Pipeline completed")
```

## `word_count_mapper.py`

This `RowFunction` uses `ctx.state()` to work with the YSON-state. For each message key, you store a Map with the `word` and `count` fields.

```python
class WordCountMapper(RowFunction):
    """Processes Word messages and maintains word count state."""

    def on_message(self, message, output, ctx):
        state = ctx.state("word-state", message)
        data = state.get_or_default({"word": message.payload["word"], "count": 0})
        data["count"] += 1
        state.set(data)
```

## Key patterns

- A simple stateful pipeline with a single computation.
- Internal YSON-state via `ctx.state()` with `get_or_default` and `set`.
- The state key is defined by `group_by_schema` from the spec (in this case, by the `word` field).
<!-- endsource: en/_includes/flow/python/examples/wordcount.md -->

<!-- source: en/_includes/flow/python/examples/wordcount_also.md -->
## See also

- [Quick start (Python)](https://ytsaurus.tech/docs/en/flow/python/getting-started.md)
- [Computation (Python)](https://ytsaurus.tech/docs/en/flow/python/computation.md)
- [Stateful processing](https://ytsaurus.tech/docs/en/flow/concepts/stateful.md)
<!-- endsource: en/_includes/flow/python/examples/wordcount_also.md -->