## Editing Textual Output

The safest advice about editing the textual output from astWriteastWrite (or astShowastShow) is “don't!”—unless you know what you are doing.

Having given that warning, however, it is sometimes possible to make changes to the text, or even to write entire ObjectObject descriptions from scratch, and to read the results back in to construct new Objects. Normally, simple changes to numerical values are safest, but be aware that this is a back door method of creating Objects, so you are on your own! There are a number of potential pitfalls. In particular:

- astReadastRead is intended for retrieving data written by astWrite and not for reading data input by humans. As such, the data validation provided is very limited and is certainly not foolproof. This makes it quite easy to construct Objects that are internally inconsistent by this means. In contrast, the normal programming interface incorporates numerous checks designed to make it impossible to construct invalid Objects. You should not necessarily think you have found a bug if your changes to an Object's textual description fail to produce the results you expected!

- In many instances the names associated with values in textual output will correspond with Object attributes. Sometimes, however, these names may differ from the attribute name. This is mainly because of length restrictions imposed by other common external formats, such as FITS headers. Some of the names used do not correspond with attributes at all.

- It is safest to change single numerical or string values. Beware of changing the size or shape of Objects (e.g. the number of axes in a FrameFrame). Often, these values must match others stored elsewhere within the Object and changing them in a haphazard fashion will not produce useful results.

- Be wary about un-commenting default values. Sometimes this will work, but often these values are derived from other Objects stored more deeply in the structure and the proper place to insert a new value is not where the default itself appears.