Should I put shebang in Python scripts and what form should it take

Question

Should I put the shebang in my Python scripts  In what form      usr bin env python    or     usr local bin python   Are these equally portable  Which form is used most   Note  the tornado project uses the shebang  On the other hand the Django project doesn t

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Absolute vs Logical Path   This is really a question about whether the path to the Python interpreter should be absolute or Logical   usr bin env  in respect to portability   Encountering other answers on this and other Stack sites which talked about the issue in a general way without supporting proofs  I ve performed some really  REALLY  granular testing  amp  analysis on this very question on the unix stackexchange com   Rather than paste that answer here  I ll point those interested to the comparative analysis to that answer   https   unix stackexchange com a 566019 334294  Being a Linux Engineer  my goal is always to provide the most suitable  optimized hosts for my developer clients  so the issue of Python environments was something I really needed a solid answer to  My view after the testing was that the logical path in the she-bang was the better of the  2  options

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You should add a shebang if the script is intended to be executable  You should also install the script with an installing software that modifies the shebang to something correct so it will work on the target platform  Examples of this is distutils and Distribute

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Should I put the shebang in my Python scripts     Put a shebang into a Python script to indicate    this module can be run as a script whether it can be run only on python2  python3 or is it Python 2 3 compatible on POSIX  it is necessary if you want to run the script directly without invoking python executable explicitly      Are these equally portable  Which form is used most    If you write a shebang manually then always use    usr bin env python unless you have a specific reason not to use it  This form is understood even on Windows  Python launcher    Note  installed scripts should use a specific python executable e g    usr bin python or  home me  virtualenvs project bin python  It is bad if some tool breaks if you activate a virtualenv in your shell  Luckily  the correct shebang is created automatically in most cases by setuptools or your distribution package tools  on Windows  setuptools can generate wrapper  exe scripts automatically    In other words  if the script is in a source checkout then you will probably see    usr bin env python  If it is installed then the shebang is a path to a specific python executable such as    usr local bin python   NOTE  you should not write the paths from the latter category manually    To choose whether you should use python  python2  or python3 in the shebang  see PEP 394 - The  python  Command on Unix-Like Systems             python should be used in the shebang line only for scripts that are   source compatible with both Python 2 and 3    in preparation for an eventual change in the default version of   Python  Python 2 only scripts should either be updated to be source   compatible with Python 3 or else to use python2 in the shebang line

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When I installed Python 3 6 1 on Windows 7 recently  it also installed the Python Launcher for Windows  which is supposed to handle the shebang line   However  I found that the Python Launcher did not do this  the shebang line was ignored and Python 2 7 13 was always used  unless I executed the script using py -3    To fix this  I had to edit the Windows registry key HKEY LOCAL MACHINE SOFTWARE Classes Python File shell open command   This still had the value   C  Python27 python exe    1       from my earlier Python 2 7 installation   I modified this registry key value to   C  Windows py exe    1       and the Python Launcher shebang line processing worked as described above

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Use first  which python   This will give the output as the location where my python interpreter  binary  is present   This output could be any such as   usr bin python   or   bin python   Now appropriately select the shebang line and use it   To generalize we can use      usr bin env   or      bin env

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The shebang line in any script determines the script s ability to be executed like a standalone executable without typing python beforehand in the terminal or when double clicking it in a file manager  when configured properly   It isn t necessary but generally put there so when someone sees the file opened in an editor  they immediately know what they re looking at  However  which shebang line you use IS important   Correct usage for Python 3 scripts is        usr bin env python3   This defaults to version 3 latest  For Python 2 7 latest use python2 in place of python3   The following should NOT be used  except for the rare case that you are writing code which is compatible with both Python 2 x and 3 x       usr bin env python   The reason for these recommendations  given in PEP 394  is that python can refer either to python2 or python3 on different systems  It currently refers to python2 on most distributions  but that is likely to change at some point   Also  DO NOT Use      usr local bin python       python may be installed at  usr bin python or  bin python in those   cases  the above    will fail     --    usr bin env python  vs     usr local bin python

User · Answer

Answer   Only if you plan to make it a command-line executable script   Here is the procedure   Start off by verifying the proper shebang string to use   which python   Take the output from that and add it  with the shebang     in the first line   On my system it responds like so    which python  usr bin python   So your shebang will look like      usr bin python   After saving  it will still run as before since python will see that first line as a comment   python filename py   To make it a command  copy it to drop the  py extension     cp filename py filename   Tell the file system that this will be executable   chmod  x filename   To test it  use     filename   Best practice is to move it somewhere in your  PATH so all you need to type is the filename itself    sudo cp filename  usr sbin   That way it will work everywhere  without the    before the filename

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If you have more than one version of Python and the script needs to run under a specific version  the she-bang can ensure the right one is used when the script is executed directly  for example      usr bin python2 7   Note the script could still be run via a complete Python command line  or via import  in which case the she-bang is ignored   But for scripts run directly  this is a decent reason to use the she-bang      usr bin env python is generally the better approach  but this helps with special cases   Usually it would be better to establish a Python virtual environment  in which case the generic    usr bin env python would identify the correct instance of Python for the virtualenv

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Sometimes  if the answer is not very clear  I mean you cannot decide if yes or no   then it does not matter too much  and you can ignore the problem until the answer is clear   The    only purpose is for launching the script  Django loads the sources on its own and uses them  It never needs to decide what interpreter should be used  This way  the    actually makes no sense here   Generally  if it is a module and cannot be used as a script  there is no need for using the     On the other hand  a module source often contains if   name         main         with at least some trivial testing of the functionality  Then the    makes sense again   One good reason for using    is when you use both Python 2 and Python 3 scripts -- they must be interpreted by different versions of Python  This way  you have to remember what python must be used when launching the script manually  without the    inside   If you have a mixture of such scripts  it is a good idea to use the    inside  make them executable  and launch them as executables  chmod        When using MS-Windows  the    had no sense -- until recently  Python 3 3 introduces a Windows Python Launcher  py exe and pyw exe  that reads the    line  detects the installed versions of Python  and uses the correct or explicitly wanted version of Python  As the extension can be associated with a program  you can get similar behaviour in Windows as with execute flag in Unix-based systems

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The purpose of shebang is for the script to recognize the interpreter type when you want to execute the script from the shell   Mostly  and not always  you execute scripts by supplying the interpreter externally  Example usage  python-x x script py  This will work even if you don t have a shebang declarator   Why first one is more  portable  is because   usr bin env contains your PATH declaration which accounts for all the destinations where your system executables reside   NOTE  Tornado doesn t strictly use shebangs  and Django strictly doesn t  It varies with how you are executing your application s main function   ALSO  It doesn t vary with Python

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If you have different modules installed and need to use a specific  python install  then shebang appears to be limited at first  However  you can do tricks like the below to allow the shebang to be invoked first as a shell script and then choose python  This is very flexible imo      bin sh     Choose the python we need  Explanation    a        translates to   in shell  and starts a python multi-line string   b     strings are treated as string concat by python  shell ignores them   c   true  command ignores its arguments   c  exit before the ending     so the shell reads no further   d  reset set docstrings to ignore the multiline comment code    true        PREFERRED PYTHON  Library Frameworks Python framework Versions 2 7 bin python ALTERNATIVE PYTHON  Library Frameworks Python framework Versions 3 6 bin python3 FALLBACK PYTHON python3  if   -x  PREFERRED PYTHON    then     echo Using preferred python  PREFERRED PYTHON     exec  PREFERRED PYTHON   0       elif   -x  ALTERNATIVE PYTHON    then     echo Using alternative python  ALTERNATIVE PYTHON     exec  ALTERNATIVE PYTHON   0       else     echo Using fallback python  FALLBACK PYTHON     exec python3   0       fi exit 127        doc        What this file does    print   doc    import platform print platform python version      Or better yet  perhaps  to facilitate code reuse across multiple python scripts      bin bash  true         source   cd   dirname   BASH SOURCE       amp  gt  dev null  amp  amp  pwd  select sh  exec  CHOSEN PYTHON   0        exit 127        and then select sh has   PREFERRED PYTHON  Library Frameworks Python framework Versions 2 7 bin python ALTERNATIVE PYTHON  Library Frameworks Python framework Versions 3 6 bin python3 FALLBACK PYTHON python3  if   -x  PREFERRED PYTHON    then     CHOSEN PYTHON  PREFERRED PYTHON elif   -x  ALTERNATIVE PYTHON    then     CHOSEN PYTHON  ALTERNATIVE PYTHON else     CHOSEN PYTHON  FALLBACK PYTHON fi

User · Answer

It s really just a matter of taste  Adding the shebang means people can invoke the script directly if they want  assuming it s marked as executable   omitting it just means python has to be invoked manually   The end result of running the program isn t affected either way  it s just options of the means

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