import argparse

import cv2

def argument_parser():
    parser = argparse.ArgumentParser(description="Change color space of the \
            input video stream using keyboard controls. The control keys are: \
            Grayscale - 'g', YUV - 'y', HSV - 'h'")
    return parser

if __name__=='__main__':
    args = argument_parser().parse_args()

    cap = cv2.VideoCapture(0)

    # Check if the webcam is opened correctly
    if not cap.isOpened():
        raise IOError("Cannot open webcam")

    cur_char = -1
    prev_char = -1

    while True:
        # Read the current frame from webcam
        ret, frame = cap.read()

        # Resize the captured image
        frame = cv2.resize(frame, None, fx=0.5, fy=0.5, interpolation=cv2.INTER_AREA)

        c = cv2.waitKey(1)

        if c == 27:
            break

      ...