dyn_est

from collections import defaultdict
from itertools import imap

from boto import dynamodb
from magnitude import mg

REGION='us-west-1'
conn = dynamodb.connect_to_region(REGION)

WANT_PROPS = (
    ('item_count', None),
    ('size_bytes', 'B'),
    ('write_units', 'kB/s'),
)

def main():
    stats = []
    totals = defaultdict(int)
    for table in imap(conn.get_table, conn.list_tables(10000)):
        if table.status != 'ACTIVE':
            continue
        tstat = {}
        for (k,unit) in WANT_PROPS:
            v = getattr(table,k)
            tstat.update({k:v})
            totals[k] += v
        stats.append({table.name:tstat})
    return (stats,totals)

def fu(value, units, do_units=True):
    if units is None or not do_units:
        return mg(value)
    else: 
        return mg(value,units).auto_si()

if __name__ == '__main__':
    stats,totals = main()
    print('--totals')
    for (k,units) in WANT_PROPS:
        print('%20s %-35s'%(k,fu(totals[k],units)))

mag_auto_si

diff --git a/magnitude.py b/magnitude.py
index d6e2ee4..00bf4c9 100644
--- a/magnitude.py
+++ b/magnitude.py
@@ -213,7 +213,9 @@ _prefix = {'y': 1e-24,  # yocto
            'Pi': 2 ** 50, # Pebi (<- peta, 10^15)
            'Ei': 2 ** 60  # Exbi (<- exa, 10^18)
            }
-
+_r_si_prefix = dict([(math.log10(v),k) for (k,v) in sorted(
+    _prefix.items(),key=lambda x:x[1]) if len(k)==1]
+)
 
 ###### Default print formatting options
 
@@ -417,7 +419,10 @@ class Magnitude():
 
         if not _prn_units:
             return st
+        st += self.get_unit_str()
+        return st.strip()
 
+    def get_unit_str(self):
         u = self.unit
         num = ' '  # numerator
         for i in range(len(_unames)):
@@ -434,10 +439,24 @@ class Magnitude():
         if den:
             if num == ' ':
                 num += '1 '
-            st += (num + '/ ' + den)
+            return (num + '/ ' + den).strip()
         elif num != ' ':
-            st += num
-        return st.strip()
+            return num.strip()
+        return ''
+
+    def auto_si(self):
+        r = self.copy()
+        r.to_base_units()
+        bu = r.get_unit_str()
+        if bu == 'b': bu = 'B'
+        assert bu, 'no base unit'
+        
+        e = int(math.log10(abs(r.val)))
+        use_prefix = _r_si_prefix.get((e/3)*3)
+        if use_prefix is not None:
+            return r.ounit(use_prefix+bu)
+        else:
+            return r
 
     def term2mag(self, s):
         """Converts a string with units to a Magnitude.