# RNA Transcription

Given a DNA strand, return its RNA complement (per RNA transcription).

Both DNA and RNA strands are a sequence of nucleotides.

The four nucleotides found in DNA are adenine (**A**), cytosine (**C**),
guanine (**G**) and thymine (**T**).

The four nucleotides found in RNA are adenine (**A**), cytosine (**C**),
guanine (**G**) and uracil (**U**).

Given a DNA strand, its transcribed RNA strand is formed by replacing
each nucleotide with its complement:

* `G` -> `C`
* `C` -> `G`
* `T` -> `A`
* `A` -> `U`

Your function will need to be able to handle invalid inputs by raising a `ValueError` with a meaningful message.


## 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 rna_transcription_test.py`
- Python 3.3+: `pytest rna_transcription_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 rna_transcription_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/rna-transcription` 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

Hyperphysics [http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html](http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html)

## Submitting Incomplete Solutions

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