What is main py

Question

What is the   main   py file for  what sort of code should I put into it  and when should I have one

User · Answer

If your script is a directory or ZIP file rather than a single python file    main   py will be executed when the  script  is passed as an argument to the python interpreter

User · Answer

What is the   main   py file for  When creating a Python module  it is common to make the module execute some functionality  usually contained in a main function  when run as the entry point of the program  This is typically done with the following common idiom placed at the bottom of most Python files  if   name         main           execute only if run as the entry point into the program     main    You can get the same semantics for a Python package with   main   py  which might have the following structure     -- demo      --   init   py      --   main   py  To see this  paste the below into a Python 3 shell  from pathlib import Path  demo   Path cwd      demo  demo mkdir     demo      init   py   write text  quot  quot  quot  print  demo   init   py executed    def main        print  main   executed    quot  quot  quot     demo      main   py   write text  quot  quot  quot  print  demo   main   py executed    from demo import main  main    quot  quot  quot    We can treat demo as a package and actually import it  which executes the top-level code in the   init   py  but not the main function    gt  gt  gt  import demo demo   init   py executed  When we use the package as the entry point to the program  we perform the code in the   main   py  which imports the   init   py first    python -m demo demo   init   py executed demo   main   py executed main   executed  You can derive this from the documentation  The documentation says     main       Top-level script environment    main    is the name of the scope in which top-level code executes  A module   s   name   is set equal to    main    when read from standard input  a script  or from an interactive prompt  A module can discover whether or not it is running in the main scope by checking its own   name    which allows a common idiom for conditionally executing code in a module when it is run as a script or with python -m but not when it is imported  if   name         main            execute only if run as a script      main    For a package  the same effect can be achieved by including a   main   py module  the contents of which will be executed when the module is run with -m   Zipped You can also zip up this directory  including the   main   py  into a single file and run it from the command line like this - but note that zipped packages can t execute sub-packages or submodules as the entry point  from pathlib import Path  demo   Path cwd      demo2  demo mkdir     demo      init   py   write text  quot  quot  quot  print  demo2   init   py executed    def main        print  main   executed    quot  quot  quot     demo      main   py   write text  quot  quot  quot  print  demo2   main   py executed    from   init   import main  main    quot  quot  quot    Note the subtle change - we are importing main from   init   instead of demo2 - this zipped directory is not being treated as a package  but as a directory of scripts  So it must be used without the -m flag  Particularly relevant to the question - zipapp causes the zipped directory to execute the   main   py by default - and it is executed first  before   init   py    python -m zipapp demo2 -o demo2zip   python demo2zip demo2   main   py executed demo2   init   py executed main   executed  Note again  this zipped directory is not a package - you cannot import it either

User · Answer

main   py is used for python programs in zip files  The   main   py file will be executed when the zip file in run  For example  if the zip file was as such   test zip        main   py   and the contents of   main   py was   import sys print  hello  s    sys argv 1    Then if we were to run python test zip world we would get hello world out   So the   main   py file run when python is called on a zip file

User · Answer

You create   main   py in yourpackage to make it executable as     python -m yourpackage

User · Answer

Often  a Python program is run by naming a  py file on the command line     python my program py   You can also create a directory or zipfile full of code  and include a   main   py   Then you can simply name the directory or zipfile on the command line  and it executes the   main   py automatically     python my program dir   python my program zip   Or  if the program is accessible as a module   python -m my program   You ll have to decide for yourself whether your application could benefit from being executed like this     Note that a   main   module usually doesn t come from a   main   py file  It can  but it usually doesn t  When you run a script like python my program py  the script will run as the   main   module instead of the my program module  This also happens for modules run as python -m my module  or in several other ways   If you saw the name   main   in an error message  that doesn t necessarily mean you should be looking for a   main   py file

User · Answer

Some of the answers here imply that given a  quot package quot  directory  with or without an explicit   init   py file   containing a   main   py file  there is no difference between running that directory with the -m switch or without  The big difference is that without the -m switch  the  quot package quot  directory is first added to the path  i e  sys path   and then the files are run normally  without package semantics  Whereas with the -m switch  package semantics  including relative imports  are honoured  and the package directory itself is never added to the system path  This is a very important distinction  both in terms of whether relative imports will work or not  but more importantly in terms of dictating what will be imported in the case of unintended shadowing of system modules   Example  Consider a directory called PkgTest with the following structure    PkgTest  tree    -- pkgname         --   main   py         -- secondtest py         -- testmodule py  -- testmodule py  where the   main   py file has the following contents     PkgTest  cat pkgname   main   py import os print   quot Hello from pkgname   main   py  I am the file quot   os path abspath    file       print   quot I am being accessed from quot   os path abspath  os curdir     from  testmodule import main as firstmain      firstmain   from  secondtest import main as secondmain     secondmain     with the other files defined similarly with similar printouts   If you run this without the -m switch  this is what you ll get  Note that the relative import fails  but more importantly note that the wrong testmodule has been chosen  i e  relative to the working directory      PkgTest  python3 pkgname Hello from pkgname   main   py  I am the file   PkgTest pkgname   main   py I am being accessed from   PkgTest Hello from testmodule py  I am the file   PkgTest pkgname testmodule py I am being accessed from   PkgTest Traceback  most recent call last     File  quot  usr lib python3 6 runpy py quot   line 193  in  run module as main      quot   main   quot   mod spec    File  quot  usr lib python3 6 runpy py quot   line 85  in  run code     exec code  run globals    File  quot pkgname   main   py quot   line 10  in  lt module gt      from  secondtest import main as secondmain ImportError  attempted relative import with no known parent package  Whereas with the -m switch  you get what you  hopefully  expected     PkgTest  python3 -m pkgname Hello from pkgname   main   py  I am the file   PkgTest pkgname   main   py I am being accessed from   PkgTest Hello from testmodule py  I am the file   PkgTest testmodule py I am being accessed from   PkgTest Hello from secondtest py  I am the file   PkgTest pkgname secondtest py I am being accessed from   PkgTest    Note  In my honest opinion  running without -m should be avoided  In fact I would go further and say that I would create any executable packages in such a way that they would fail unless run via the -m switch  In other words  I would only import from  in-package  modules explicitly via  relative imports   assuming that all other imports represent system modules  If someone attempts to run your package without the -m switch  the relative import statements will throw an error  instead of silently running the wrong module

[python] What is main.py?

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[python] What is main.py?