# Generating Output

A standard `cmd` application can produce output by using either of these methods:

```py
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`, [cmd2.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.Cmd` extends this approach in a number of convenient ways. See
[Output Redirection and Pipes](./redirection.md#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.Cmd][] also includes a number of output related methods which you
may use to enhance the output your application produces.

Since `cmd2` has a dependency on the [rich](https://github.com/Textualize/rich) library, the
following [cmd2.Cmd][] output methods can natively render `rich`
[renderable objects](https://rich.readthedocs.io/en/latest/protocol.html), enabling beautiful and
complex output:

- [poutput][cmd2.Cmd.poutput]
- [perror][cmd2.Cmd.perror]
- [psuccess][cmd2.Cmd.psuccess]
- [pwarning][cmd2.Cmd.pwarning]
- [pfeedback][cmd2.Cmd.pfeedback]
- [ppaged][cmd2.Cmd.ppaged]

!!! tip "Advanced output customization"

      Each of the above methods accepts additional optional parameters that help control how the output is
      formatted:

      - `sep`: string to write between printed text. Defaults to " "
      - `end`: string to write at end of printed text. Defaults to a newline
      - `style`: optional style to apply to output
      - `soft_wrap`: Enable soft wrap mode. If True, lines of text will not be word-wrapped or cropped to fit the terminal width. Defaults to True
      - `justify`: justify method ("left", "center", "right", "full"). Defaults to None.
      - `emoji`: If True, Rich will replace emoji codes (e.g., 😃) with their corresponding Unicode characters. Defaults to False
      - `markup`: If True, Rich will interpret strings with tags (e.g., [bold]hello[/bold]) as styled output. Defaults to False
      - `highlight`: If True, Rich will automatically apply highlighting to elements within strings, such as common Python data types like numbers, booleans, or None.
      - `rich_print_kwargs`: optional additional keyword arguments to pass to `console.print()`
      - `kwargs`: arbitrary keyword arguments to support extending the print methods. These are not passed to `console.print()`.

## Ordinary Output

The [poutput][cmd2.Cmd.poutput] method is similar to the Python built-in
[print](https://docs.python.org/3/library/functions.html#print) function. `poutput` adds a few
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 [allow_style](./settings.md#allow_style) setting. See
   [Colored Output](#colored-output) below for more details.
3. It allows printing arbitrary `rich` renderable objects which can get visually quite complex.

Here's a simple command that shows this method in action:

```py
def do_echo(self, args):
    """A simple command showing how poutput() works"""
    self.poutput(args)
```

## Error Messages

When an error occurs in your program, you can display it on `sys.stderr` by calling the
[perror][cmd2.Cmd.perror] method. By default this method applies
[Cmd2Style.ERROR][cmd2.styles.Cmd2Style.ERROR] to the output.

## Warning Messages

[pwarning][cmd2.Cmd.pwarning] is just like `cmd2.Cmd.perror` but applies `Cmd2Style.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 status. If you use the
[Timing](./settings.md#timing) setting, the output of how long it took the command to run will be
output as to `stderr` without any styling. You can use the [perror][cmd2.Cmd.perror] method to
produce this type of output by passing it `style=None`.

If the [quiet](./settings.md#quiet) setting is `True`, then calling `cmd2.Cmd.pfeedback` produces no
output. If [quiet](./settings.md#quiet) is `False`,the `pfeedback` method sends output to `stdout`.
Hence, `pfeedback` is useful for non-essential output that you want the ability to silence when
`quiet` is `True`.

## Exceptions

If your app catches an exception and you would like to display the exception to the user, the
[pexcept][cmd2.Cmd.pexcept] method can help. The default behavior is to just display the message
contained within the exception. However, if the [debug](./settings.md#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 [ppaged][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](https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/more); on
Unix-like operating systems like MacOS and Linux, it is piped to
[less](https://man7.org/linux/man-pages/man1/less.1.html).

## Colored Output

You can add your own [ANSI escape sequences](https://en.wikipedia.org/wiki/ANSI_escape_code#Colors)
to your output which tell the terminal to change the foreground and background colors.

`cmd2` provides a number of convenience functions and classes for adding color and other styles to
text. These are all based on [rich](https://github.com/Textualize/rich) and are documented in the
following sections:

- [cmd2.colors][]
- [cmd2.rich_utils][]
- [cmd2.string_utils][]

The [color.py](https://github.com/python-cmd2/cmd2/blob/main/examples/color.py) example demonstrates
all colors available to your `cmd2` application.

### Custom Themes

`cmd2` uses a `rich` [Theme](https://rich.readthedocs.io/en/stable/reference/theme.html) object to
define styles for various UI elements. You can define your own custom theme using
[cmd2.theme.update_theme][]. See the
[rich_theme.py](https://github.com/python-cmd2/cmd2/blob/main/examples/rich_theme.py) example for
more information.

After adding the desired escape sequences to your output, you should use one of these methods to
present the output to the user:

- [cmd2.Cmd.poutput][]
- [cmd2.Cmd.perror][]
- [cmd2.Cmd.pwarning][]
- [cmd2.Cmd.pexcept][]
- [cmd2.Cmd.pfeedback][]
- [cmd2.Cmd.ppaged][]

These methods all honor the [allow_style](./settings.md#allow_style) setting, which users can modify
to control whether these escape codes are passed through to the terminal or not.

## Aligning Text

If you would like to generate output which is left, center, or right aligned within a specified
width or the terminal width, the following functions can help:

- [cmd2.string_utils.align_left][]
- [cmd2.string_utils.align_center][]
- [cmd2.string_utils.align_right][]

These functions differ from Python's string justifying functions in that they support characters
with display widths greater than 1. Additionally, ANSI style sequences are safely ignored and do not
count toward the display width. This means colored text is supported. If text has line breaks, then
each line is aligned independently.

!!! tip "Advanced alignment customization"

      You can also control output alignment using the `justify` parameter when calling
      `cmd2`'s print methods.

## 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 [cmd2.string_utils.str_width][] function solves both of these problems. Pass it a string, and
regardless of which Unicode characters and ANSI text style escape sequences it contains, it will
tell you how many characters on the screen that string will consume when printed.
