Fix incorrect self.atom assignment in SingleFrameReaderBase (#5055)

- fix n_atom bug in SingleFrameReaderBase
- conform lammps init function to SingleFrameReaderBase

Author: yuxuanzhuang

package/CHANGELOG

@@ -21,6 +21,7 @@ The rules for this file:
  * 2.10.0
 
 Fixes
+ * Fix incorrect `self.atom` assignment in SingleFrameReaderBase (Issue #5052, PR #5055)
  * Fixes bug in `analysis/hydrogenbonds.py`: `_donors` and `_hydrogens`
    were not updated when the `acceptors_sel` and `hydrogens_sel` were provided
    after initialization. (Issue #5010, PR #5018)

package/MDAnalysis/coordinates/LAMMPS.py

@@ -240,11 +240,11 @@ class DATAReader(base.SingleFrameReaderBase):
 
     @store_init_arguments
     def __init__(self, filename, **kwargs):
-        self.n_atoms = kwargs.pop("n_atoms", None)
-        if self.n_atoms is None:  # this should be done by parsing DATA first
+        n_atoms = kwargs.pop("n_atoms", None)
+        if n_atoms is None:  # this should be done by parsing DATA first
             raise ValueError("DATAReader requires n_atoms keyword")
         self.atom_style = kwargs.pop("atom_style", None)
-        super(DATAReader, self).__init__(filename, **kwargs)
+        super(DATAReader, self).__init__(filename, n_atoms=n_atoms, **kwargs)
 
     def _read_first_frame(self):
         with DATAParser(self.filename) as p:

package/MDAnalysis/coordinates/base.py

@@ -1655,6 +1655,9 @@ class SingleFrameReaderBase(ProtoReader):
        Calling `__iter__` now rewinds the reader before yielding a
        :class:`Timestep` object (fixing behavior that was not
        well defined previously).
+    .. versionchanged:: 2.10.0
+       Fixed a typo in the attribute assignment (`self.atom` → `self.atoms`),
+       which may affect subclasses relying on this value.
     """
     _err = "{0} only contains a single frame"
 
@@ -1666,7 +1669,7 @@ def __init__(self, filename, convert_units=True, n_atoms=None, **kwargs):
         self.convert_units = convert_units
 
         self.n_frames = 1
-        self.n_atom = n_atoms
+        self.n_atoms = n_atoms
 
         ts_kwargs = {}
         for att in ('dt', 'time_offset'):