timestamp_delta.py

#!/usr/bin/env python3

"""Reads from stdin / arguments a sequence of lines which contain an
ISO 8601 timestamp, and outputs to stdout each line with the timestamp
with the delta from the previous line.

In other words, converts a file with absolute timstamps into a file
with relative timestamps since the previous line.

The first line cannot have a delta calculated, so it is reported as
zero.
"""

import datetime
import fileinput
import itertools
import re

ISO8601_REGEX = r"(.*?)(\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d+[+-]\d{2}:\d{2})(.*)"

def parse_line(line):
    m = re.search(ISO8601_REGEX, line)
    if m:
        dt = datetime.datetime.fromisoformat(m.group(2))
        return (m.group(1), dt, m.group(3))
    return None


# Are we processing the first line?
first_line = True

for lineA, lineB in itertools.pairwise(fileinput.input()):
    parsedA = parse_line(lineA)
    parsedB = parse_line(lineB)
    if first_line:
        # There is no previous line for the first line, so
        # we cannot calculate a delta. Report a zero duration.
        print("{}{}{}".format(parsedA[0], datetime.timedelta(), parsedA[2]))
        first_line = False
        continue

    # If we found timestamps in both lines, calculate delta and print,
    # otherwise just emit original line.
    if parsedA and parsedB:
        duration = parsedB[1] - parsedA[1]
        print("{}{}{}".format(parsedB[0], duration, parsedB[2]))
    else:
        print(lineB, end=None)