make alchemlyb.concat behave like pd.concat (PR #152)

* fix #150
* `alchemlyb.concat([])` now raises ValueError (like pandas.concat())
* Update CHANGES
* add test

CHANGES

@@ -25,6 +25,8 @@ Enhancements
 Deprecations
 
 Fixes
+  - `alchemlyb.concat()` should raise ValueError when given empty list, to be compatible with
+    pandas.concat() (issue #150, PR #152).
   - Fix the support for pandas >= 1.3 (issue #147, PR #148).
   - Fix separate_dhdl not work for multiple columns (issue #149, PR #151).
 

src/alchemlyb/__init__.py

@@ -47,7 +47,11 @@ def concat(objs, *args, **kwargs):
  
     .. versionadded:: 0.5.0'''
     # Sanity check
-    attrs = objs[0].attrs
+    try:
+        attrs = objs[0].attrs
+    except IndexError: # except empty list as input
+        raise ValueError('No objects to concatenate')
+
     for obj in objs:
         if attrs != obj.attrs:
             raise ValueError('All pandas objects should have the same attrs.')

src/alchemlyb/tests/test_units.py

@@ -33,6 +33,11 @@ def test_concat():
     with pytest.raises(ValueError):
         alchemlyb.concat([df1, df2])
 
+def test_concat_empty():
+    '''Test if empty raise the right error.'''
+    with pytest.raises(ValueError):
+        alchemlyb.concat([])
+
 def test_setT():
     '''Test setting temperature.'''
     df = pd.DataFrame(data={'col1': [1, 2]})