cqlsh: Improve default float precision behavior

Patch by Stefania Alborghetti; reviewed by Tyler Hobbs for
CASSANDRA-9224

CHANGES.txt

@@ -1,4 +1,5 @@
 2.1.6
+ * (cqlsh) Better float precision by default (CASSANDRA-9224)
  * Improve estimated row count (CASSANDRA-9107)
  * Optimize range tombstone memory footprint (CASSANDRA-8603)
  * Use configured gcgs in anticompaction (CASSANDRA-9397)

pylib/cqlshlib/formatting.py

@@ -14,6 +14,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import sys
 import re
 import time
 import calendar
@@ -144,6 +145,13 @@ def format_floating_point_type(val, colormap, float_precision, **_):
     elif math.isinf(val):
         bval = 'Infinity'
     else:
+        exponent = int(math.log10(abs(val))) if abs(val) > sys.float_info.epsilon else -sys.maxint -1
+        if -4 <= exponent < float_precision:
+            # when this is true %g will not use scientific notation,
+            # increasing precision should not change this decision
+            # so we increase the precision to take into account the
+            # digits to the left of the decimal point
+            float_precision = float_precision + exponent + 1
         bval = '%.*g' % (float_precision, val)
     return colorme(bval, colormap, 'float')