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- Verification options

# Verification options

There are 5 options for the `output_verify` argument of the following methods of `HDUList`: `close()`, `writeto()`, and `flush()`, or the `writeto()` method on any HDU object. In these cases, the verification option is passed to a `verify()` call within these methods.

## exception

This option will raise an exception if any FITS standard is violated. This is the default option for output (i.e. when `writeto()`, `close()`, or `flush()` is called. If a user wants to overwrite this default on output, the other options listed below can be used.

## ignore

This option will ignore any FITS standard violation. On output, it will write the HDU List content to the output FITS file, whether or not it is conforming to FITS standard.

The `ignore` option is useful in these situations, for example:

> 1. An input FITS file with non-standard is read and the user wants to copy or write out after some modification to an output file. The non-standard will be preserved in such output file.
> 2. A user wants to create a non-standard FITS file on purpose, possibly for testing purpose.

No warning message will be printed out. This is like a silent warn (see below) option.

## fix

This option will try to fix any FITS standard violations. It is not always possible to fix such violations. In general, there are two kinds of FITS standard violation: fixable and not fixable. For example, if a keyword has a floating number with an exponential notation in lower case ‘e’ (e.g. 1.23e11) instead of the upper case ‘E’ as required by the FITS standard, it’s a fixable violation. On the other hand, a keyword name like `P.I.` is not fixable, since it will not know what to use to replace the disallowed periods. If a violation is fixable, this option will print out a message noting it is fixed. If it is not fixable, it will throw an exception.

The principle behind the fixing is do no harm. For example, it is plausible to ‘fix’ a `Card` with a keyword name like `P.I.` by deleting it, but Astropy will not take such action to hurt the integrity of the data.

Not all fixes may be the “correct” fix, but at least Astropy will try to make the fix in such a way that it will not throw off other FITS readers.

## silentfix

Same as fix, but will not print out informative messages. This may be useful in a large script where the user does not want excessive harmless messages. If the violation is not fixable, it will still throw an exception.

## warn

This option is the same as the ignore option but will send warning messages. It will not try to fix any FITS standard violations whether fixable or not.