pythonw.exe or python.exe?

To summarize and complement the existing answers:

• python.exe is a console (terminal) application for launching CLI-type scripts (console applications).

  • Unless run from an existing console window, python.exe opens a new console window.

  • Standard streams sys.stdin, sys.stdout and sys.stderr are connected to the console window.

  • Execution is synchronous when launched from a cmd.exe or PowerShell console window: ^{See eryksun's 1st comment below.}

    • If a new console window was created, it stays open until the script terminates.
    • When invoked from an existing console window, the prompt is blocked until the script terminates.

• pythonw.exe is a GUI app for launching GUI/no-UI-at-all scripts.

  • NO console window is opened.
  • Execution is asynchronous:
    • When invoked from a console window, the script is merely launched and the prompt returns right away, whether the script is still running or not.
  • Standard streams sys.stdin, sys.stdout and sys.stderr are NOT available.
    • Caution: Unless you take extra steps, this has potentially unexpected side effects:
      • Unhandled exceptions cause the script to abort silently.
      • In Python 2.x, simply trying to use print() can cause that to happen (in 3.x, print() simply has no effect).
        • To prevent that from within your script, and to learn more, see this answer of mine.
        • Ad-hoc, you can use output redirection:^{Thanks, @handle.}
          pythonw.exe yourScript.pyw 1>stdout.txt 2>stderr.txt
          (from PowerShell:
          cmd /c pythonw.exe yourScript.pyw 1>stdout.txt 2>stderr.txt) to capture stdout and stderr output in files.
          If you're confident that use of print() is the only reason your script fails silently with pythonw.exe, and you're not interested in stdout output, use @handle's command from the comments:
          pythonw.exe yourScript.pyw 1>NUL 2>&1
          Caveat: This output redirection technique does not work when invoking *.pyw scripts directly (as opposed to by passing the script file path to pythonw.exe). ^{See eryksun's 2nd comment and its follow-ups below.}

You can control which of the executables runs your script by default - such as when opened from Explorer - by choosing the right filename extension:

• *.py files are by default associated (invoked) with python.exe
• *.pyw files are by default associated (invoked) with pythonw.exe

If you don't want a terminal window to pop up when you run your program, use pythonw.exe;
Otherwise, use python.exe

Regarding the syntax error: print is now a function in 3.x
So use instead:

print("a")

See here: http://docs.python.org/using/windows.html

pythonw.exe "This suppresses the terminal window on startup."