Python subprocess Popen with a modified environment

Question

I believe that running an external command with a slightly modified environment is a very common case  That s how I tend to do it   import subprocess  os my env   os environ my env  PATH       usr sbin  sbin     my env  PATH   subprocess Popen my command  env my env    I ve got a gut feeling that there s a better way  does it look alright

User · Answer

you might use my env get  PATH       instead of my env  PATH   in case PATH somehow not defined in the original environment  but other than that it looks fine

User · Answer

In certain circumstances you may want to only pass down the environment variables your subprocess needs  but I think you ve got the right idea in general  that s how I do it too

User · Answer

To temporarily set an environment variable without having to copy the os envrion object etc  I do this   process   subprocess Popen   env    RSYNC PASSWORD foobar    rsync      rsync   username foobar com      stdout subprocess PIPE

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With Python 3 5 you could do it this way   import os import subprocess  my env      os environ   PATH     usr sbin  sbin     os environ  PATH     subprocess Popen my command  env my env    Here we end up with a copy of os environ and overridden PATH value   It was made possible by PEP 448  Additional Unpacking Generalizations    Another example  If you have a default environment  i e  os environ   and a dict you want to override defaults with  you can express it like this   my env      os environ    dict with env variables

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The env parameter accepts a dictionary  You can simply take os environ  add a key  your desired variable   to a copy of the dict if you must  to that and use it as a parameter to Popen

User · Answer

I think os environ copy   is better if you don t intend to modify the os environ for the current process   import subprocess  os my env   os environ copy   my env  PATH       usr sbin  sbin     my env  PATH   subprocess Popen my command  env my env

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I know this has been answered for some time  but there are some points that some may want to know about using PYTHONPATH instead of PATH in their environment variable  I have outlined an explanation of running python scripts with cronjobs that deals with the modified environment in a different way  found here   Thought it would be of some good for those who  like me  needed just a bit more than this answer provided

User · Answer

That depends on what the issue is  If it s to clone and modify the environment one solution could be   subprocess Popen my command  env dict os environ  PATH  path      But that somewhat depends on that the replaced variables are valid python identifiers  which they most often are  how often do you run into environment variable names that are not alphanumeric underscore or variables that starts with a number     Otherwise you ll could write something like   subprocess Popen my command  env dict os environ                                             Not valid python name   value       In the very odd case  how often do you use control codes or non-ascii characters in environment variable names   that the keys of the environment are bytes you can t  on python3  even use that construct   As you can see the techniques  especially the first  used here benefits on the keys of the environment normally is valid python identifiers  and also known in advance  at coding time   the second approach has issues  In cases where that isn t the case you should probably look for another approach

[python] Python subprocess/Popen with a modified environment

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