take standard deviation of datetime in python

Question

I am importing the datetime library in my python program and am taking the duration of multiple events. Below is my code for that:

d1 = datetime.datetime.strptime(starttime, '%Y-%m-%d:%H:%M:%S')
d2 = datetime.datetime.strptime(endtime, '%Y-%m-%d:%H:%M:%S')
duration = d2 - d1
print str(duration)

Now I have a value in the variable "duration". The output of this will be:

0:00:15
0:00:15
0:00:15
0:00:15
0:00:15
0:00:05
0:00:05
0:00:05
0:00:05
0:00:05
0:00:10
0:00:10
0:00:10
0:00:10
0:45:22

I want to take the standard deviation of all the durations and determine if there is an anomaly. For example, the 00:45:22 is an anomaly and I want to detect that. I could do this if I knew what format datetime was in, but it doesn't appear to be digits or anything..I was thinking about splitting the values up from : and using all the values in between, but there might be a better way.

Ideas?

Answer 1

You have datetime.timedelta() objects. These have .microseconds, .seconds and .days attributes, all 3 integers. The str() string representation represents those as [D day[s], ][H]H:MM:SS[.UUUUUU] as needed to fit all values present.

You can use simple arithmetic on these objects. Summing and division work as expected, for example:

>>> (timedelta(seconds=100) + timedelta(seconds=200)) / 2
datetime.timedelta(0, 150)

Unfortunately, you cannot multiply two timedeltas and calculating a standard deviation thus becomes tricky (no squaring of offsets).

Instead, I'd use the .total_seconds() method, to give you a floating point value that is calculated from the days, seconds and microseconds values, then use those values to calculate a standard deviation.