How do you use subprocess check output in Python

Question

I have found documentation about subprocess check output   but I cannot find one with  arguments and the documentation is not very in depth  I am using Python 3  but am trying to run a Python 2 file through Python 3   I am trying to run this command  python py2 py -i test txt  -i is a positional argument for argparse  test txt is what the -i is  py2 py is the file to run  I have tried a lot of  non working  variations including  py2output   subprocess check output  str  python py2 py     -i    test txt     py2output   subprocess check output  str  python    py2 py   -i   test txt

User · Answer

Adding on to the one mentioned by  abarnert  a better one is to catch the exception   import subprocess try      py2output   subprocess check output   python    py2 py    -i    test txt   stderr  subprocess STDOUT         print  py2 said    py2output      print  here  except subprocess CalledProcessError as e      print  Calledprocerr    this stderr  subprocess STDOUT is for making sure you dont get the filenotfound error in stderr- which cant be usually caught in filenotfoundexception  else you would end up getting  python  can t open file  py2 py    Errno 2  No such file or directory   Infact a better solution to this might be to check  whether the file scripts exist and then to run the file script

User · Answer

The right answer  using Python 2 7 and later  since check output   was introduced then  is   py2output   subprocess check output   python   py2 py   -i    test txt      To demonstrate  here are my two programs   py2 py   import sys print sys argv   py3 py   import subprocess py2output   subprocess check output   python    py2 py    -i    test txt    print  py2 said    py2output    Running it     python3 py3 py py2 said  b   py2 py    -i    test txt   n    Here s what s wrong with each of your versions   py2output   subprocess check output  str  python py2 py     -i    test txt      First  str  python py2 py   is exactly the same thing as  python py2 py    you re taking a str  and calling str to convert it to an str  This makes the code harder to read  longer  and even slower  without adding any benefit   More seriously  python py2 py can t be a single argument  unless you re actually trying to run a program named  say   usr bin python  py2 py  Which you re not  you re trying to run  say   usr bin python with first argument py2 py  So  you need to make them separate elements in the list   Your second version fixes that  but you re missing the   before test txt   This should give you a SyntaxError  probably saying EOL while scanning string literal   Meanwhile  I m not sure how you found documentation but couldn t find any examples with arguments  The very first example is    gt  gt  gt  subprocess check output   echo    Hello World     b Hello World  n    That calls the  echo  command with an additional argument   Hello World     Also      -i is a positional argument for argparse  test txt is what the -i is   I m pretty sure -i is not a positional argument  but an optional argument  Otherwise  the second half of the sentence makes no sense

User · Answer

Since Python 3 5  subprocess run   is recommended over subprocess check output      gt  gt  gt  subprocess run   cat    tmp text txt    stdout subprocess PIPE  stdout b First line nSecond line n    Since Python 3 7  instead of the above  you can use capture output true parameter to capture stdout and stderr    gt  gt  gt  subprocess run   cat    tmp text txt    capture output True  stdout b First line nSecond line n    Also  you may want to use universal newlines True or its equivalent since Python 3 7 text True to work with text instead of binary    gt  gt  gt  stdout   subprocess run   cat     tmp text txt    capture output True  text True  stdout  gt  gt  gt  print stdout  First line Second line   See subprocess run   documentation for more information

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