Testing for fragment attribute

[PicoCentauri]

Expected behavior

Checking for the fragment attribute

from MDAnalysisTests.core.util import UnWrapUniverse
u = UnWrapUniverse(have_molnums=False, have_bonds=False)

print(hasattr(u,"fragments"))
print(hasattr(u.atoms, "fragments"))

should give the following consistent output

False
False

Actual behavior

However, while the first works fine and returns False the second raises a NoDataError.

/usr/local/lib/python3.7/site-packages/MDAnalysis/core/groups.py in __getattr__(self, attr)
   2222             # raise NDE(_ATTR_ERRORS[attr])
   2223             raise NoDataError("AtomGroup.{} not available; this requires Bonds"
-> 2224                               "".format(attr))
   2225         raise AttributeError("{cls} has no attribute {attr}".format(
   2226             cls=self.__class__.__name__, attr=attr))

NoDataError: AtomGroup.fragments not available; this requires Bonds

Current version of MDAnalysis

• Which version are you using? 0.20.2-dev0
• Which version of Python: 3.7
• Which operating system? macOS

[lilyminium]

Interesting. I guess you could define __hasattr__ but... would making NoDataError a subclass of AttributeError fix this?

Edit: yes, but then it breaks a bunch of properties. Also __hasattr__ doesn't exist and hasattr just calls __getattr__ and checks for an AttributeError (from Python 3+) so there's no getting around this without raising an AttributeError...