call python function from shell and get only the returned value

Question

How to get only the returned value from a function, in the below example, I want to get only foorbar and not this is a test in the bash shell.

$ cat foo.py
print "this is a test"
def bar():
 return "foobar"
$ output=$(python -c 'import foo;print foo.bar()')
$ echo $output
this is a test foobar

• juanpa.arrivillaga: .... suppress stdout, then re-enable it only for that function call maybe?

Answer 1

I'd recommend if you require for whatever reason the print "this is a test" section, you check if the file is being run itself, such as by checking the __name__ property as follows:

foo.py:

def bar():
 return "foobar"
if __name__ == "__main__":
 print("this is a test")

This should mean that the only time "this is a test" is printed is when you call python foo.py, and not when foo is imported as a module. Hope this helps!

• rodee: in my case, I cannot modify foo.py, as its coming from different team, so I will have to do something from shell. thanks for the reply.

Answer 2

If you want to keep the module as-is and get what you want, you need to suppress the module level string being printed by temporarily redirecting the STDOUT to someplace else (e.g. None, /dev/null or any other file descriptor as a dumping zone), and duplicate again to the original STDOUT once the import is done.

% cat spam.py
print("this is a test")
def bar():
 return "foobar"
% python3 -c 'import sys;sys.stdout=None;import spam;sys.stdout=sys.__stdout__;print(spam.bar())'
foobar
% out=$(python3 -c 'import sys;sys.stdout=None;import spam;sys.stdout=sys.__stdout__;print(spam.bar())')
% echo "$out"
foobar

Expanded form:

import sys
sys.stdout = None
import spam
sys.stdout = sys.__stdout__
print(spam.bar())

• kindall: Ooh, I didn't realize that you could use None for sys.stdout without complaint from Python.
• heemayl: @kindall Funny thing is I also come to know this today :)
• kindall: Well, since this obviates the need for my answer, I'll at least share my one-liner to create a dummy file-like class with a write method that doesn't do anything: sys.stdout = type("DummyStdout", (object,), {"write": lambda self, s: None})()
• rodee: Thanks, but foo.py was written for python2, so it throws some error with python3, any solution with python2 is appreciated. thanks again.

Answer 3

In your specific case, where the desired output is the final line of the python's stdout, you can use tail:

output=$(python -c 'import foo;print foo.bar()' | tail -1)