Code style, docstrings and the README

Code is read many more times than it is written — often by you, three months from now, when you have forgotten everything.

Names are the cheapest documentation there is: number_passed beats n2. Function names should be verbs (read_csv, compute_mean).

A docstring explains what and why, not how (that is visible in the code):

def mean(values, ignore_empty=False):
    """The mean of values.

    An empty list raises ValueError unless ignore_empty=True (then 0.0).
    Used by the report, which must not crash on empty groups.
    """

Comments should explain the unexpected: why does it say + 1e-8 here? Not # increase i by 1.

A README that is enough — five headings:

# Project name
One sentence about what it does and who it is for.

## Installation
    python3 -m venv .venv && source .venv/bin/activate
    pip install -r requirements.txt

## Usage
    python -m myapp.cli --data data/train.csv --out report.md

## Development
    pytest -q          # tests
    ruff check .       # the linter
    ruff format .      # the formatter

## Limitations
Handles Swedish text only; a maximum of 100 MB of input.

Tools do the rest automatically: ruff format (or black) formats it the same way every time — then diffs stop being about whitespace. ruff check finds unused imports, undefined names and common mistakes. Run both in CI and nobody has to nag about style in code review.