Python script header

Question

The typical header should be      usr bin env python   But I found below also works when executing the script like  python   my script py     usr bin python   python   What s difference between these 2 headers  What could be the problem for 2nd one  Please also discussing the case for python interpreter is in PATH or not  Thanks

User · Answer

The Python executable might be installed at a location other than  usr bin  but env is nearly always present in that location so using  usr bin envis more portable

User · Answer

The  usr bin env python becomes very useful when your scripts depend on environment settings for example using scripts which rely on python virtualenv  Each virtualenv has its own version of python binary which is required for adding packages installed in virtualenv to python path  without touching PYTHONPATH env    As more and more people have started to used virtualenv for python development prefer to use  usr bin env python unless you don t want people to use their custom python binary   Note  You should also understand that there are potential security issues  in multiuser environments  when you let people run your scripts in their custom environments  You can get some ideas from here

User · Answer

I d suggest 3 things in the beginning of your script   First  as already being said use environment      usr bin env python   Second  set your encoding     - - coding  utf-8 - -   Third  set some doc string      This is a awesome     python script       And for sure I would use         4 spaces  for ident  Final header will look like      usr bin env python   - - coding  utf-8 - -     This is a awesome         python script       Best wishes and happy coding

User · Answer

From the manpage for env  GNU coreutils 6 10    env - run a program in a modified environment   In theory you could use env to reset the environment  removing many of the existing environment variables  or add additional environment variables in the script header  Practically speaking  the two versions you mentioned are identical   Though others have mentioned a good point  specifying python through env lets you abstractly specify python without knowing its path

User · Answer

Yes  there is - python may not be in  usr bin  but for example in  usr local bin  BSD    When using virtualenv  it may even be something like   projects env bin python

User · Answer

First  any time you run a script using the interpreter explicitly  as in    python   my script py   ksh   bin redouble sh   lua5 1  usr local bin osbf3   the    line is always ignored   The    line is a Unix feature of executable scripts only  and you can see it documented in full on the man page for execve 2    There you will find that the word following    must be the pathname of a valid executable   So     usr bin env python   executes whatever python is on the users  PATH   This form is resilient to the Python interpreter being moved around  which makes it somewhat more portable  but it also means that the user can override the standard Python interpreter by putting something ahead of it in  PATH   Depending on your goals  this behavior may or may not be OK   Next      usr bin python   deals with the common case that a Python interpreter is installed in  usr bin   If it s installed somewhere else  you lose   But this is a good way to ensure you get exactly the version you want or else nothing at all   fail-stop  behavior   as in      usr bin python2 5   Finally     python   works only if there is a python executable in the current directory when the script is run   Not recommended

[python] Python script header

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