Allow both history indices to be negative

Author: kotfu

cmd2/history.py

@@ -167,9 +167,7 @@ def span(self, span: str) -> List[HistoryItem]:
             end = int(end)
 
         if start is not None and end is not None:
-            # we have both start and end, return a slice of history, unless both are negative
-            if start < 0 and end < 0:
-                raise ValueError
+            # we have both start and end, return a slice of history
             result = self[start:end]
         elif start is not None and sep is not None:
             # take a slice of the array

docs/freefeatures.rst

@@ -310,8 +310,7 @@ last number, it will continue to the end::
         3  alias create three !echo three
         4  alias create four !echo four
 
-You can use negative numbers as either the first or second number of the range
-(but not both). If you want to display the last three commands entered::
+If you want to display the last three commands entered::
 
     (Cmd) history -- -3:
         2  alias create two !echo two

tests/test_history.py

@@ -77,11 +77,14 @@ def test_history_class_span(hist):
 
     assert hist.span('1..3') == ['first', 'second', 'third']
     assert hist.span('1:3') == ['first', 'second', 'third']
+    assert hist.span('2:-1') == ['second', 'third']
+    assert hist.span('-3:4') == ['second', 'third','fourth']
+    assert hist.span('-4:-2') == ['first', 'second']
 
     assert hist.span(':-2') == ['first', 'second']
     assert hist.span('..-2') == ['first', 'second']
 
-    value_errors = ['fred', 'fred:joe', 'a..b', '2 ..', '1 : 3', '-2..-3' ]
+    value_errors = ['fred', 'fred:joe', 'a..b', '2 ..', '1 : 3']
     for tryit in value_errors:
         with pytest.raises(ValueError):
             hist.span(tryit)