Avoid using a temporary variable by returning the value directly.

IPython/core/magics/execution.py

@@ -1284,5 +1284,4 b' def _format_time(timespan, precision=3):'
         order = min(-int(math.floor(math.log10(timespan)) // 3), 3)
     else:
         order = 3
-    ret =  u"%.*g %s" % (precision, timespan * scaling[order], units[order])
-    return ret
+    return u"%.*g %s" % (precision, timespan * scaling[order], units[order])