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# Adding custom arguments to `k4run`

It is possible to extend `k4run` with custom arguments from a steering file
using `k4FWCore.parseArgs`.

Example:

```python
from k4FWCore.parseArgs import parser
parser.add_argument("--trackingOnly", action="store_true", help="Run only track reconstruction", default=False)
my_opts = parser.parse_known_args()[0]

# later
if my_opts.trackingOnly:
    # only run track reconstruction
```

The example steering file can then be executed as follows:

```
k4run example_steering_file.py --trackingOnly --some-other-args
```

Behind the scenes `parser` is just a normal instance of python's
[`argparse.ArgumentParser`](https://docs.python.org/3/library/argparse.html),
please refer to its documentation for usage details. Use `parse_known_args()`
instead of `parse_args()` so that the normal `k4run` arguments keep working. The
`[0]` is necessary because the added arguments will be in the first element of
the tuple returned from `parse_known_args`.