Python subprocess.run inside Flask app failure [Errno 2] No such file or directory: 'ls': 'ls'

What is needed to use subprocess.run() inside a Flask app?

Even a simple example from https://docs.python.org/3.6/library/subprocess.html fails.

process = subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
output = process.stdout
app.logger.info(f"Process output: {output}")

  File "./main.py", line 209, in process_pdf
    process = subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/usr/lib/python3.6/subprocess.py", line 729, in __init__
    restore_signals, start_new_session)
  File "/usr/lib/python3.6/subprocess.py", line 1364, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'ls': 'ls'

Python 3.6.8 (on Ubuntu 18.04LTS)

Flask is being served by uwsgi (from nginx)

I started from more complicated examples, trying with shell=True and other arguments, but nothing seems to be working.

subprocess.run() works just fine when called from command line sub.py

process = subprocess.run(["ls", "-l", "/dev/null"], stdout=subprocess.PIPE)
output = process.stdout
print(f"Results {output}")

Results b'crw-rw-rw- 1 root root 1, 3 Nov 28 15:10 /dev/null\n'

Within Flask I can use old os.popen - no results though

stream = os.popen('ls -l /dev/null')
output = stream.readlines()

app.logger.info(f"Process output: {output}")

EDIT: Thanks to @furas and @Dursug for pointing me in the right direction. It seems like lack of shell issue for www-data.

So what would be the most Pythonic / Flask way of solving this?

PS I want to run specific external programs such as imagemagick, pdftotext, but I want to avoid wrappers/bindings (sometimes there are none).

This did turn out to be an enviroment issue for www-data which only had access to virtualenv path where Flask app was residing.

Solved by editing

/etc/systemd/system/myproject.service and adding :/usr/bin:/bin
as in

Environment="PATH=/home/myname/myproject/myprojectenv/bin:/usr/bin:/bin"

Then restarted nginx and service

As suggested by one of the answers on
https://stackoverflow.com/questions/45906997/uwsgi-python-subprocess-chrome-firefox-failed.

The question remains open whether this is the best practice for Flask.

Theoretically this opens up a potential vulnerability if client can find a way to run an arbitrary command.