Calculate FStops Difference Between Two Images

getFStops.py

import itertools
import math
import re

def unpackDefault(data, length=3, default=None):
	"""
	This definition unpacks given sequence enforcing a given length.

	:param data: Sequence to unpack. ( Tuple / List )
	:param length: Default length to unpack to. ( Integer )
	:param default: Default fill object. ( Object)
	:return: Unpacked sequence. ( Iterator )
	"""

	return itertools.islice(itertools.chain(iter(data), itertools.repeat(default)), length)

def parseShutterSpeed(shutterSpeed):
	"""
	This definition parses given shutter speed data.

	:param shutterSpeed: Shutter speed data. ( String )
	:return: Parser shutter data. ( Float )
	"""

	shutterSpeed = tuple(float(re.match(r"[-+]?\d*\.?\d+([eE][-+]?\d+)?", token).group(0)) for token in unpackDefault(shutterSpeed.split("/"), 2, str(1)))
	return shutterSpeed[1] != 0 and shutterSpeed[0] / shutterSpeed[1] or None

def getFStops(metadataA, metadataB):
	"""
	This definition returns the fStops difference between given data.

	:param metadataA: Shutter speed / aperture data. ( Tuple / List )
	:param metadataB: Shutter speed / aperture data. ( Tuple / List )
	:return: Exposure ratio left / right, fStops. ( Tuple )
	"""

	shutterSpeedA, apertureA, = (float(data) for data in metadataA)
	shutterSpeedB, apertureB, = (float(data) for data in metadataB)

	shutterSpeedRatio = shutterSpeedA / shutterSpeedB
	apertureRatio = apertureB / apertureA
	exposureRatio = shutterSpeedRatio * apertureRatio * apertureRatio

	if exposureRatio > 0.999 and exposureRatio < 1.001:
		exposureRatio=1

	if exposureRatio > 1:
		left = round(exposureRatio * 1000) / 1000
		right = 1
	elif exposureRatio < 1:
		left = 1
		exposureRatio = 1 / exposureRatio
		right = round(exposureRatio * 1000) / 1000
	else:
		left = right = 1

	fStops = math.log(exposureRatio) / math.log(2)
	return left, right, fStops

#print parseShutterSpeed("1/125")
#print parseShutterSpeed("1.29684")
#print parseShutterSpeed("1/3.08442 Sec.")

print getFStops((parseShutterSpeed("1/125"), 4),
				(parseShutterSpeed("1/250"), 2.8))