Python: Compute Difference between Datetime Objects in a Loop

Question

Using Python 2.7, I am trying to create an array of relative time, where relative time is the difference between the middle of a thunderstorm and some other time during that storm. Ultimately, I am hoping that my relative time array will be of the format -minutes, 0, and +minutes (where -minutes are minutes before the middle of the storm, 0 is the middle of the storm, and +minutes are minutes after the middle of the storm). I figured a loop was the most efficient way to do this. I already have a 1-D array, MDAdatetime, filled with the other storm times I mentioned, as strings. I specified the middle of the storm at the beginning of my code, and it is a string, as well.

So far, my code is as follows:

import csv
import datetime

casedate = '06052009'
MDAfile = "/atmomounts/home/grad/mserino/Desktop/"+casedate+"MDA.csv"
stormmidpoint = '200906052226' #midpoint in YYYYMMDDhhmm
relativetime = []
MDAlons = []
MDAlats = []
MDAdatetime = []

with open(MDAfile, 'rU') as f: #open to read in universal-newline mode
reader = csv.reader(f, dialect=csv.excel, delimiter=',')
for i,row in enumerate(reader):
    if i == 0:
        continue #skip header row
    MDAlons.append(float(row[1]))
    MDAlats.append(float(row[2]))
    MDAdatetime.append(str(row[0])) #this is the array I'm dealing with now in the section below; each string is of the format YYYYMMDDhhmmss

## This is the section I'm having trouble with ##
for j in range(len(MDAdatetime)):
    reltime = datetime.datetime.strptime(MDAdatetime[j],'%YYYY%mm%dd%HH%MM%SS') - datetime.datetime(stormmidpoint,'%YYYY%mm%dd%HH%MM')
    retime.strftime('%MM') #convert the result to minutes
    reativetime.append(reltime)
print relativetime

So far, I have been getting the error:

ValueError: time data '20090605212523' does not match format '%YYYY%mm%dd%HH%MM%SS'

I am trying to learn as much as I can about the datetime module. I have seen some other posts and resources mention dateutil, but it seems that datetime will be the most useful for me. I could be wrong, though, and I appreciate any advice and help. Please let me know if I need to clarify anything or provide more information.

Answer 1

%Y matches 2016 . Not %YYYY , similarly for month, date, etc..

So, your format matcher should be %Y%m%d%H%M%S

Something like this:

datetime.datetime.strptime("20090605212523", "%Y%m%d%H%M%S")

Demo

>>> datetime.datetime.strptime("20090605212523", "%YYYY%mm%dd%HH%MM%SS")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/_strptime.py", line 325, in _strptime
    (data_string, format))
ValueError: time data '20090605212523' does not match format '%YYYY%mm%dd%HH%MM%SS'
>>> datetime.datetime.strptime("20090605212523", "%Y%m%d%H%M%S")
datetime.datetime(2009, 6, 5, 21, 25, 23)

Answer 2

After @karthikr pointed out that my strptime formatting was incorrect, I was able to successfully get an answer in minutes. There may be a better way, but I converted to minutes by hand in the code. I also changed my stormmidpoint variable to a string, as it should be. @karthikr was also right about that. Thanks for the help!

for j in range(len(MDAdatetime)):
    reltime = datetime.datetime.strptime(MDAdatetime[j],'%Y%m%d%H%M%S') - datetime.datetime.strptime(stormmidpoint,'%Y%m%d%H%M%S')
    relativetime.append(int(math.ceil(((reltime.seconds/60.0) + (reltime.days*1440.0))))) #convert to integer minutes
print relativetime