Empty histrange means history of current session

When empty string is given to
`IPython.core.history.extract_hist_ranges`, it will now return a range
corresponding to the whole history of the current session.

This means that `HistoryManager.get_range_by_str` will now also return
everything if given an empty string - which has implications for other
magics like `%save`, `%pastebin`, etc.

Commits that follow this one will introduce that fact to each magic
docstring and implementation, where required.

IPython/core/history.py

@@ -445,8 +445,11 b' class HistoryAccessor(HistoryAccessorBase):'
         Parameters
         ----------
         rangestr : str
-          A string specifying ranges, e.g. "5 ~2/1-4". See
-          :func:`magic_history` for full details.
+          A string specifying ranges, e.g. "5 ~2/1-4". If empty string is used,
+          this will return everything from current session's history.
+
+          See the documentation of :func:`%history` for the full details.
+
         raw, output : bool
           As :meth:`get_range`
 
@@ -851,11 +854,18 b' $""", re.VERBOSE)'
 def extract_hist_ranges(ranges_str):
     """Turn a string of history ranges into 3-tuples of (session, start, stop).
 
+    Empty string results in a `[(0, 1, None)]`, i.e. "everything from current
+    session".
+
     Examples
     --------
     >>> list(extract_hist_ranges("~8/5-~7/4 2"))
     [(-8, 5, None), (-7, 1, 5), (0, 2, 3)]
     """
+    if ranges_str == '':
+        yield (0, 1, None)  # Everything from current session
+        return
+
     for range_str in ranges_str.split():
         rmatch = range_re.match(range_str)
         if not rmatch:

IPython/core/tests/test_history.py

@@ -160,6 +160,14 b' def test_extract_hist_ranges():'
     actual = list(extract_hist_ranges(instr))
     nt.assert_equal(actual, expected)
 
+
+def test_extract_hist_ranges_empty_str():
+    instr = ''
+    expected = [(0, 1, None)]  # 0 == current session, None == to end
+    actual = list(extract_hist_ranges(instr))
+    nt.assert_equal(actual, expected)
+
+
 def test_magic_rerun():
     """Simple test for %rerun (no args -> rerun last line)"""
     ip = get_ipython()