python/exercises/pov/README.md

# POV

Reparent a graph on a selected node.

This exercise is all about re-orientating a graph to see things from a different
point of view. For example family trees are usually presented from the
ancestor's perspective:

```text
    +------0------+
    |      |      |
  +-1-+  +-2-+  +-3-+
  |   |  |   |  |   |
  4   5  6   7  8   9
```

But the same information can be presented from the perspective of any other node
in the graph, by pulling it up to the root and dragging its relationships along
with it. So the same graph from 6's perspective would look like:

```text
        6
        |
  +-----2-----+
  |           |
  7     +-----0-----+
        |           |
      +-1-+       +-3-+
      |   |       |   |
      4   5       8   9
```

This lets us more simply describe the paths between two nodes. So for example
the path from 6-9 (which in the first graph goes up to the root and then down to
a different leaf node) can be seen to follow the path 6-2-0-3-9

This exercise involves taking an input graph and re-orientating it from the point
of view of one of the nodes.

## Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to
indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not
every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include
a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of
`raise Exception`, you should write:

```python
raise Exception("Meaningful message indicating the source of the error")
```

## Running the tests

To run the tests, run the appropriate command below ([why they are different](https://github.com/pytest-dev/pytest/issues/1629#issue-161422224)):

- Python 2.7: `py.test pov_test.py`
- Python 3.4+: `pytest pov_test.py`

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
`python -m pytest pov_test.py`

### Common `pytest` options

- `-v` : enable verbose output
- `-x` : stop running tests on first failure
- `--ff` : run failures from previous test before running other test cases

For other options, see `python -m pytest -h`

## Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the `$EXERCISM_WORKSPACE/python/pov` directory.

You can find your Exercism workspace by running `exercism debug` and looking for the line that starts with `Workspace`.

For more detailed information about running tests, code style and linting,
please see the [help page](http://exercism.io/languages/python).

## Source

Adaptation of exercise from 4clojure [https://www.4clojure.com/](https://www.4clojure.com/)

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.