I assume crontab's cwd (Current Working Directory) is not same as where the script is stored.
this would solve your problem:
import os
script_dir = os.path.dirname(os.path.realpath(__file__))
os.chdir(script_dir)
You can get the directory where you script is by calling "os.path.dirname(os.path.realpath(file))"
if you change the current working directory "os.chdir(...dir...)" you can access you config/config.json by relative path,
Otherwise you will have to use a absolute path
Try running this and check your output file:
import os
script_dir = os.path.dirname(os.path.realpath(__file__))
print (os.getcwd())
print(script_dir)
os.chdir(script_dir)
print (os.getcwd())
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