Checksum not verified until loading HDUs

[saimn]

Since the addition of lazy loading, checksum are verified only for the primary HDU when opening the file, and the other HDUs are verified only when (and if) there are loaded. So if an invalid HDU is not explicitly loaded, it will not be verified.
I think this is wrong, and specifying checksum=True in fits.open should load all HDUs to verify them. The documentation also states that opening is enough to verify the file: http://docs.astropy.org/en/latest/io/fits/usage/verification.html#verification-using-the-fits-checksum-keyword-convention

(example after adding prints in _ValidHDU._verify_checksum_datasum).

In [1]: %astropy
Numpy 1.15.4
Astropy 3.2.dev23320

In [2]: hdu = fits.ImageHDU(data=np.arange(10))

In [3]: hdu.writeto('test.fits', checksum=True, overwrite=True)

In [4]: hdul = fits.open('test.fits', checksum=True)
verify checksum <astropy.io.fits.hdu.image.PrimaryHDU object at 0x7fedcefbf240>
verify datasum <astropy.io.fits.hdu.image.PrimaryHDU object at 0x7fedcefbf240>

In [5]: len(hdul)
verify checksum <astropy.io.fits.hdu.image.ImageHDU object at 0x7fedcefc6320>
verify datasum <astropy.io.fits.hdu.image.ImageHDU object at 0x7fedcefc6320>
Out[5]: 2

[pllim]

Seems implied but not explicitly stated -- Are you saying that if both checksum and lazy_load_hdus are True, some sort of exception should be raised?

[saimn]

No, the code should just force the loading of all HDUs to trigger the verification. That was the default behavior before lazy loading.