Generating Output ================= A standard ``cmd`` application can produce output by using either of these methods:: print("Greetings, Professor Falken.", file=self.stdout) self.stdout.write("Shall we play a game?\n") While you could send output directly to ``sys.stdout``, ``cmd`` can be initialized with a ``stdin`` and ``stdout`` variables, which it stores as ``self.stdin`` and ``self.stdout``. By using these variables every time you produce output, you can trivially change where all the output goes by changing how you initialize your class. ``cmd2`` extends this approach in a number of convenient ways. See :ref:`features/redirection:Output Redirection And Pipes` for information on how users can change where the output of a command is sent. In order for those features to work, the output you generate must be sent to ``self.stdout``. You can use the methods described above, and everything will work fine. ``cmd2`` also adds a number of output related methods to ``Cmd2.Cmd`` which you may use to enhance the output your application produces. Ordinary Output --------------- The :meth:`.cmd2.Cmd.poutput` method is similar to the Python `built-in print function `_. :meth:`~cmd2.cmd2.Cmd.poutput` adds two conveniences. 1. Since users can pipe output to a shell command, it catches ``BrokenPipeError`` and outputs the contents of ``self.broken_pipe_warning`` to ``stderr``. ``self.broken_pipe_warning`` defaults to an empty string so this method will just swallow the exception. If you want to show an error message, put it in ``self.broken_pipe_warning`` when you initialize ``Cmd2.cmd``. 2. It examines and honors the :ref:`features/settings:allow_ansi` setting. See :ref:`features/generating_output:Colored Output` below for more details. Here's a simple command that shows this method in action:: def do_echo(self, args): """A simple command showing how poutput() works""" self.poutput(args) Colored Output -------------- You may want to generate output in different colors, which is typically done by adding `ANSI escape sequences `_ which tell the terminal to change the foreground and background colors. If you want to give yourself a headache, you can generate these by hand. You could also use another Python color library like `plumbum.colors `_, `colored `_, or `colorama `_. Colorama is unique because when it's running on Windows, it wraps ``stdout``, looks for ANSI escape sequences, and converts them into the appropriate ``win32`` calls to modify the state of the terminal. ``cmd2`` imports and uses Colorama and provides a number of convenience methods for generating colorized output, measuring the screen width of colorized output, setting the window title in the terminal, and removing ANSI escape codes from a string. These functions are all documentated in :mod:`cmd2.ansi`. :mod:`cmd2.cmd2.Cmd` includes an :ref:`features/settings:allow_ansi` setting, which controls whether ANSI escape sequences that instruct the terminal to colorize output are stripped from the output. The recommended approach is to construct your application so that it generates colorized output, and then allow your users to use this setting to remove the colorization if they do not want it. Output generated by any of these methods will honor the :ref:`features/settings:allow_ansi` setting: - :meth:`~.cmd2.Cmd.poutput` - :meth:`~.cmd2.Cmd.perror` - :meth:`~.cmd2.Cmd.pwarning` - :meth:`~.cmd2.Cmd.pexcept` - :meth:`~.cmd2.Cmd.pfeedback` - :meth:`~.cmd2.Cmd.ppaged` Error Messages -------------- When an error occurs in your program, you can display it on ``sys.stderr`` by calling the :meth:`~.cmd2.Cmd.perror` method. Warning Messages ---------------- :meth:`~.cmd2.Cmd.pwarning` is just like :meth:`~.cmd2.Cmd.perror` but applies :meth:`cmd2.ansi.style_warning` to the output. Feedback -------- You may have the need to display information to the user which is not intended to be part of the generated output. This could be debugging information or status information about the progress of long running commands. It's not output, it's not error messages, it's feedback. If you use the :ref:`features/settings:Timing` setting, the output of how long it took the command to run will be output as feedback. You can use the :meth:`~.cmd2.Cmd.pfeedback` method to produce this type of output, and several :ref:`features/settings:Settings` control how it is handled. If the :ref:`features/settings:quiet` setting is ``True``, then calling :meth:`~.cmd2.Cmd.pfeedback` produces no output. If :ref:`features/settings:quiet` is ``False``, the :ref:`features/settings:feedback_to_output` setting is consulted to determine whether to send the output to ``stdout`` or ``stderr``. Exceptions ---------- If your app catches an exception and you would like to display the exception to the user, the :meth:`~.cmd2.Cmd.pexcept` method can help. The default behavior is to just display the message contained within the exception. However, if the :ref:`features/settings:debug` setting is ``True``, then the entire stack trace will be displayed. Paging Output ------------- If you know you are going to generate a lot of output, you may want to display it in a way that the user can scroll forwards and backwards through it. If you pass all of the output to be displayed in a single call to :meth:`~.cmd2.Cmd.ppaged`, it will be piped to an operating system appropriate shell command to page the output. On Windows, the output is piped to ``more``; on Unix-like operating systems like MacOS and Linux, it is piped to ``less``. Centering Text -------------- If you would like to generate output which is centered in the user's terminal, the :meth:`cmd2.utils.center_text` method can help. Pass it a string and it will figure out the width of the terminal and return you a new string, appropriately padded so it will be centered. Columnar Output --------------- When generating output in multiple columns, you often need to calculate the width of each item so you can pad it appropriately with spaces. However, there are categories of Unicode characters that occupy 2 cells, and other that occupy 0. To further complicate matters, you might have included ANSI escape sequences in the output to generate colors on the terminal. The :meth:`cmd2.ansi.ansi_safe_wcswidth` function solves both of these problems. Pass it a string, and regardless of which Unicode characters and ANSI escape sequences it contains, it will tell you how many characters on the screen that string will consume when printed.