Using f-strings:

plot.savefig(f'hanning{num}.pdf')

This was added in 3.6 and is the new preferred way.

Using str.format():

plot.savefig('hanning{0}.pdf'.format(num))

String concatenation:

plot.savefig('hanning' + str(num) + '.pdf')

Conversion Specifier:

plot.savefig('hanning%s.pdf' % num)

Using local variable names (neat trick):

plot.savefig('hanning%(num)s.pdf' % locals())

Using string.Template:

plot.savefig(string.Template('hanning${num}.pdf').substitute(locals()))

See also:

• Fancier Output Formatting - The Python Tutorial
• Python 3's f-Strings: An Improved String Formatting Syntax (Guide) - RealPython

Make it a shell function instead. Paste this into your terminal, or add to your shell's configuration files (e.g. ~/.bashrc for bash)

pipeline_test(){
  python -c 'from sys import argv; from pipeline_runs.z_full_pipeline import test; test(argv[1])' "$@"
}

And then run as

pipeline_test "$a"

There are multiple methods to do that.:

fstring

data_file_format = f'{sourcesystem}_{batchid}_{extractname}_{loadtype}'

or using .format

data_file_format = '{}_{}_{}_{}'.format(sourcetype,batchid,extractname,loadtype)

Your example code seems to work as it should. I can only duplicate your error if I added 'cd' to the value of str such that it tried to change to a directory called 'cd ~/Library'. That is also what seems to have happened based on the error message you show.

Broken

str = "cd ~/Library"
with cd(str):
   subprocess.call("ls")

Fine

str = "~/Library"
with cd(str):
   subprocess.call("ls")