Retrieving the output of subprocess.call()
Question:
How can I get the output of a process run using subprocess.call()
?
Passing a StringIO.StringIO
object to stdout
gives this error:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 444, in call
return Popen(*popenargs, **kwargs).wait()
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 588, in __init__
errread, errwrite) = self._get_handles(stdin, stdout, stderr)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 945, in _get_handles
c2pwrite = stdout.fileno()
AttributeError: StringIO instance has no attribute 'fileno'
>>>
Answers:
Output from subprocess.call()
should only be redirected to files.
You should use subprocess.Popen()
instead. Then you can pass subprocess.PIPE
for the stderr, stdout, and/or stdin parameters and read from the pipes by using the communicate()
method:
from subprocess import Popen, PIPE
p = Popen(['program', 'arg1'], stdin=PIPE, stdout=PIPE, stderr=PIPE)
output, err = p.communicate(b"input data that is passed to subprocess' stdin")
rc = p.returncode
The reasoning is that the file-like object used by subprocess.call()
must have a real file descriptor, and thus implement the fileno()
method. Just using any file-like object won’t do the trick.
See here for more info.
I recently just figured out how to do this, and here’s some example code from a current project of mine:
#Getting the random picture.
#First find all pictures:
import shlex, subprocess
cmd = 'find ../Pictures/ -regex ".*(JPG|NEF|jpg)" '
#cmd = raw_input("shell:")
args = shlex.split(cmd)
output,error = subprocess.Popen(args,stdout = subprocess.PIPE, stderr= subprocess.PIPE).communicate()
#Another way to get output
#output = subprocess.Popen(args,stdout = subprocess.PIPE).stdout
ber = raw_input("search complete, display results?")
print output
#... and on to the selection process ...
You now have the output of the command stored in the variable "output". "stdout = subprocess.PIPE" tells the class to create a file object named ‘stdout’ from within Popen. The communicate() method, from what I can tell, just acts as a convenient way to return a tuple of the output and the errors from the process you’ve run. Also, the process is run when instantiating Popen.
If you have Python version >= 2.7, you can use subprocess.check_output
which basically does exactly what you want (it returns standard output as string).
Simple example (linux version, see note):
import subprocess
print subprocess.check_output(["ping", "-c", "1", "8.8.8.8"])
Note that the ping command is using linux notation (-c
for count). If you try this on Windows remember to change it to -n
for same result.
As commented below you can find a more detailed explanation in this other answer.
I have the following solution. It captures the exit code, the stdout, and the stderr too of the executed external command:
import shlex
from subprocess import Popen, PIPE
def get_exitcode_stdout_stderr(cmd):
"""
Execute the external command and get its exitcode, stdout and stderr.
"""
args = shlex.split(cmd)
proc = Popen(args, stdout=PIPE, stderr=PIPE)
out, err = proc.communicate()
exitcode = proc.returncode
#
return exitcode, out, err
cmd = "..." # arbitrary external command, e.g. "python mytest.py"
exitcode, out, err = get_exitcode_stdout_stderr(cmd)
I also have a blog post on it here.
Edit: the solution was updated to a newer one that doesn’t need to write to temp. files.
In Ipython
shell:
In [8]: import subprocess
In [9]: s=subprocess.check_output(["echo", "Hello World!"])
In [10]: s
Out[10]: 'Hello World!n'
Based on sargue’s answer. Credit to sargue.
For python 3.5+ it is recommended that you use the run function from the subprocess module. This returns a CompletedProcess
object, from which you can easily obtain the output as well as return code.
from subprocess import PIPE, run
command = ['echo', 'hello']
result = run(command, stdout=PIPE, stderr=PIPE, universal_newlines=True)
print(result.returncode, result.stdout, result.stderr)
The key is to use the function subprocess.check_output
For example, the following function captures stdout and stderr of the process and returns that as well as whether or not the call succeeded. It is Python 2 and 3 compatible:
from subprocess import check_output, CalledProcessError, STDOUT
def system_call(command):
"""
params:
command: list of strings, ex. `["ls", "-l"]`
returns: output, success
"""
try:
output = check_output(command, stderr=STDOUT).decode()
success = True
except CalledProcessError as e:
output = e.output.decode()
success = False
return output, success
output, success = system_call(["ls", "-l"])
If you want to pass commands as strings rather than arrays, use this version:
from subprocess import check_output, CalledProcessError, STDOUT
import shlex
def system_call(command):
"""
params:
command: string, ex. `"ls -l"`
returns: output, success
"""
command = shlex.split(command)
try:
output = check_output(command, stderr=STDOUT).decode()
success = True
except CalledProcessError as e:
output = e.output.decode()
success = False
return output, success
output, success = system_call("ls -l")
How can I get the output of a process run using subprocess.call()
?
Passing a StringIO.StringIO
object to stdout
gives this error:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 444, in call
return Popen(*popenargs, **kwargs).wait()
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 588, in __init__
errread, errwrite) = self._get_handles(stdin, stdout, stderr)
File "/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/subprocess.py", line 945, in _get_handles
c2pwrite = stdout.fileno()
AttributeError: StringIO instance has no attribute 'fileno'
>>>
Output from subprocess.call()
should only be redirected to files.
You should use subprocess.Popen()
instead. Then you can pass subprocess.PIPE
for the stderr, stdout, and/or stdin parameters and read from the pipes by using the communicate()
method:
from subprocess import Popen, PIPE
p = Popen(['program', 'arg1'], stdin=PIPE, stdout=PIPE, stderr=PIPE)
output, err = p.communicate(b"input data that is passed to subprocess' stdin")
rc = p.returncode
The reasoning is that the file-like object used by subprocess.call()
must have a real file descriptor, and thus implement the fileno()
method. Just using any file-like object won’t do the trick.
See here for more info.
I recently just figured out how to do this, and here’s some example code from a current project of mine:
#Getting the random picture.
#First find all pictures:
import shlex, subprocess
cmd = 'find ../Pictures/ -regex ".*(JPG|NEF|jpg)" '
#cmd = raw_input("shell:")
args = shlex.split(cmd)
output,error = subprocess.Popen(args,stdout = subprocess.PIPE, stderr= subprocess.PIPE).communicate()
#Another way to get output
#output = subprocess.Popen(args,stdout = subprocess.PIPE).stdout
ber = raw_input("search complete, display results?")
print output
#... and on to the selection process ...
You now have the output of the command stored in the variable "output". "stdout = subprocess.PIPE" tells the class to create a file object named ‘stdout’ from within Popen. The communicate() method, from what I can tell, just acts as a convenient way to return a tuple of the output and the errors from the process you’ve run. Also, the process is run when instantiating Popen.
If you have Python version >= 2.7, you can use subprocess.check_output
which basically does exactly what you want (it returns standard output as string).
Simple example (linux version, see note):
import subprocess
print subprocess.check_output(["ping", "-c", "1", "8.8.8.8"])
Note that the ping command is using linux notation (-c
for count). If you try this on Windows remember to change it to -n
for same result.
As commented below you can find a more detailed explanation in this other answer.
I have the following solution. It captures the exit code, the stdout, and the stderr too of the executed external command:
import shlex
from subprocess import Popen, PIPE
def get_exitcode_stdout_stderr(cmd):
"""
Execute the external command and get its exitcode, stdout and stderr.
"""
args = shlex.split(cmd)
proc = Popen(args, stdout=PIPE, stderr=PIPE)
out, err = proc.communicate()
exitcode = proc.returncode
#
return exitcode, out, err
cmd = "..." # arbitrary external command, e.g. "python mytest.py"
exitcode, out, err = get_exitcode_stdout_stderr(cmd)
I also have a blog post on it here.
Edit: the solution was updated to a newer one that doesn’t need to write to temp. files.
In Ipython
shell:
In [8]: import subprocess
In [9]: s=subprocess.check_output(["echo", "Hello World!"])
In [10]: s
Out[10]: 'Hello World!n'
Based on sargue’s answer. Credit to sargue.
For python 3.5+ it is recommended that you use the run function from the subprocess module. This returns a CompletedProcess
object, from which you can easily obtain the output as well as return code.
from subprocess import PIPE, run
command = ['echo', 'hello']
result = run(command, stdout=PIPE, stderr=PIPE, universal_newlines=True)
print(result.returncode, result.stdout, result.stderr)
The key is to use the function subprocess.check_output
For example, the following function captures stdout and stderr of the process and returns that as well as whether or not the call succeeded. It is Python 2 and 3 compatible:
from subprocess import check_output, CalledProcessError, STDOUT
def system_call(command):
"""
params:
command: list of strings, ex. `["ls", "-l"]`
returns: output, success
"""
try:
output = check_output(command, stderr=STDOUT).decode()
success = True
except CalledProcessError as e:
output = e.output.decode()
success = False
return output, success
output, success = system_call(["ls", "-l"])
If you want to pass commands as strings rather than arrays, use this version:
from subprocess import check_output, CalledProcessError, STDOUT
import shlex
def system_call(command):
"""
params:
command: string, ex. `"ls -l"`
returns: output, success
"""
command = shlex.split(command)
try:
output = check_output(command, stderr=STDOUT).decode()
success = True
except CalledProcessError as e:
output = e.output.decode()
success = False
return output, success
output, success = system_call("ls -l")