Tilde (~) isn't working in subprocess.Popen()

You need to wrap those pathnames with os.path.expanduser():

>>> import os
>>> os.path.expanduser('~/disk_benchmark_file')
'/home/dan/disk_benchmark_file'

In your code the occurrence of:

['sudo', 'dd', 'if=/dev/' + disk, 'of=~/disk_benchmark_file', 'bs=8k', 'count=200k']

should be replaced with:

['sudo', 'dd', 'if=/dev/' + disk, 'of=' + os.path.expanduser('~/disk_benchmark_file'), 'bs=8k', 'count=200k']

import os
import shlex

outfile = os.path.expanduser('~/file')
cmd_string = 'sudo dd if=/dev/sda of=%s bs=8k count=200k; rm -f %s' % (outfile, outfile)
cmd_list = shlex.split(cmd_string)

# Then use cmd_list as argument for Popen

shlex.split is the standard and safest way to produce the list that must be used as command in subprocess. It is able to handle all the exception and make your code easier to read

You can find the home using os.path.expanduser('~').

~ is a shortcut in the shell for home. In order for your command to be interpreted by the shell you need to set shell=True in your Popen.

The shell argument (which defaults to False) specifies whether to use the shell as the program to execute. If shell is True, it is recommended to pass args as a string rather than as a sequence

https://docs.python.org/2/library/subprocess.html

Note, there are some warnings about doing this though.