I'd argue that using argh is just as easy and strictly better:

$ cat test.py
#!/usr/local/bin/python
import argh

def start(width, depth, height):
   print(float(width) * float(depth) * float(height))
   
if __name__ == '__main__':
   p = argh.ArghParser()
   p.set_default_command(start)
   p.dispatch()

$ ./test.py -h
usage: test.py [-h] width depth height

positional arguments:
 width       -
 depth       -
 height      -

options:
 -h, --help  show this help message and exit

$ ./test.py 1 2 3
6.0

$ ./test.py 1 2 3 4
usage: test.py [-h] width depth height
test.py: error: unrecognized arguments: 4

$ ./test.py 1 2 
usage: test.py [-h] width depth height
test.py: error: the following arguments are required: height

And it's even easier if you are willing to use commands (which is often useful when you want to extend the script to do more than one thing):

$ cat test.py
#!/usr/local/bin/python
import argh

def volume(width, depth, height):
   print(float(width) * float(depth) * float(height))

def area(width, height):
   print(float(width) * float(height))
   
if __name__ == '__main__':
   argh.dispatch_commands([volume, area])
   
$ ./test.py -h
usage: test.py [-h] {volume,area} ...

positional arguments:
  {volume,area}
    volume
    area

options:
  -h, --help     show this help message and exit

$ ./test.py volume -h
usage: test.py volume [-h] width depth height

positional arguments:
  width       -
  depth       -
  height      -

options:
  -h, --help  show this help message and exit

$ ./test.py volume 1 2 3
6.0

$ ./test.py area 12 24
288.0