How to check if there is no output in python subprocess?

Question

I use Python 3. This is a part of my script:

proc = subprocess.Popen(["git", "ls-remote", "--heads", "origin", "master"],stdout=subprocess.PIPE)
line = proc.stdout.readline()
if line != '':
    print("not empty")
    print(line)
else:
    print(empty)

I want to check if there is a remote master. If the content of line is not empty (= there is a master) it will print "not empty". If there is no master = no output I want to print empty.

But it does not work:

This is the output without python:

$ git ls-remote --heads origin xxx (no output)
$ git ls-remote --heads origin master (output)
a5dd03655381fcee94xx4e759ceba7aeb6456   refs/heads/master

This is the output when I run the script when master exists:

b'a5dd03655381fcee94xx4e759ceba7aeb6456trefs/heads/mastern'

And when master does not exist:

b''

How can I make this work? It seems the pipe seems to take different signs like b'' and n and not only the words.

Asked By: DenCowboy

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Source

Answer 1

You are getting a binary string as output; many programs do internal decoding and present you with the text string, but subprocess.Popen is not one of them.

But you can easily decode it yourself:

out_bin = b'a5dd03655381fcee94xx4e759ceba7aeb6456trefs/heads/mastern'
out_txt = out.decode('utf-8')  # Should be `utf-8` encoded

And the t and n are tab, and newline characters, that will do the respective formatting when you print.

Moreover, all the methods of the text string, works on binary strings too, its just the implementation/presentation that differs.

Also, on your if line != '': condition, you need to either explicitly tell that you are matching against a binary string by:

if line != b'':

Or better as empty strings are falsey in Python, simply do:

if not line:

Answered By: heemayl

How to check if there is no output in python subprocess?

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