maximum

The maximum reducer picks the maximum value of the field provided during the reduction interval.

maximum value for a field

from flume import *

(
    emit(limit=10, start='2015-01-01')
    | put(count=count())
    | reduce(maximum=maximum('count'))
    | write('stdio')
).execute()

The above would produce the output:

{"maximum": 10, "time": "2015-01-01T00:00:00.000Z"}