moving from os to subprocess

[Barrowman]

I am working on a Linux Mint 18.2 laptop.
I used python 2.7 in the past but moved to 3.5 and now 3.6. I have carried on using os.system calls as I cannot figure out the correct syntax for using subprocess calls.
Here is an example of what I am struggling with.
with os.system I can do

vid = home + "/myvids/videos/" + vidstore[thevid]
vid2play = "mpv --fs  " + vid
os.system(vid2play)

 i get the variable home from

from os.path import expanduser
home = expanduser(~)

I have no idea what can replace expanduser and try as I might I cannot find any combination of " ' , and so on which can be passed to subprocess.call which it accepts. I have googled until my eyes swap places but all the suggestions I have tried so far have failed me.
something like this example is fine

import subprocess
subprocess.call(["ls", "-l"])

but how can I use variables as in the os.system call?
I tried 

subprocess.call('mpv', '--fs', vid)

also

subprocess.call('mpv', '--fs', 'vid')

along with various other combinations but get

Exception in Tkinter callback
Traceback (most recent call last):
  File "/usr/lib/python3.6/tkinter/__init__.py", line 1702, in __call__
    return self.func(*args)
  File "./sign-songsPCC.py", line 143, in play_song
    subprocess.call('mpv', '--fs', 'vid') #,  'mwbv_BSL_201712_05_r720P.mp4')
  File "/usr/lib/python3.6/subprocess.py", line 267, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/usr/lib/python3.6/subprocess.py", line 609, in __init__
    raise TypeError("bufsize must be an integer")
TypeError: bufsize must be an integer

So I do hope someone can point me in the right direction please.