EDR reader fails when only one step is present

[bosonie]

Expected behavior

It does not fail, reads the EDR without problems

Actual behavior

The following error:

StopIteration                             Traceback (most recent call last)
Cell In [2], line 1
----> 1 a=EDRReader('/home/ebosoni/Downloads/system_npt-skPu.edr', convert_units=False)

File /shared/envs/base_nextmol_func/lib/python3.10/site-packages/MDAnalysis/auxiliary/EDR.py:324, in EDRReader.__init__(self, filename, convert_units, **kwargs)
    322 # attribute to communicate found energy terms to user
    323 self.terms = list(self.data_dict.keys())
--> 324 super(EDRReader, self).__init__(**kwargs)

File /shared/envs/base_nextmol_func/lib/python3.10/site-packages/MDAnalysis/auxiliary/base.py:321, in AuxReader.__init__(self, represent_ts_as, auxname, cutoff, **kwargs)
    319 if self.time_selector is not None and self.constant_dt:
    320     self.auxstep._initial_time = self.time
--> 321     self._read_next_step()
    322     self.auxstep._dt = self.time - self.initial_time
    323     self.rewind()

File /shared/envs/base_nextmol_func/lib/python3.10/site-packages/MDAnalysis/auxiliary/EDR.py:374, in EDRReader._read_next_step(self)
    372 else:
    373     self.rewind()
--> 374     raise StopIteration

StopIteration: 

Code to reproduce the behavior

Unfortunately I can not upload here an .edr, if you need it I can send it through other menas.

WORKAROUND

The crash actually happens in the if self.time_selector is not None and self.constant_dt, therefore I found that setting constant_dt=False avoids the error. However I'm not sure you want to recommend this as a solution...also because I struggled quite a bit to understand the meaning of constant_dt from the documentation and I'm still failing to understand the implications of this parameter. For instance, for the EDR reader we have the time_selector set, isn't it enough to catch possible cases of not constant dt?

Current version of MDAnalysis

• Which version are you using? Version '2.4.1'
• Which version of Python? 3.10.6
• Which operating system? Ubuntu

[orbeckst]

@BFedder could you please have a look at this issue?

[BFedder]

Thanks for raising this issue, bosonie (and thanks for pointing it out to me orbeckst, I completely missed this). I'll have a look later this week.