generating_output.md

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, [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 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.

Ordinary Output

The cmd2.Cmd.poutput method is similar to the Python built-in print function. 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 allow_style setting. See 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)

Error Messages

When an error occurs in your program, you can display it on sys.stderr by calling the .cmd2.Cmd.perror method. By default this method applies cmd2.ansi.style_error to the output.

Warning Messages

cmd2.Cmd.pwarning is just like cmd2.Cmd.perror but applies 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 Timing setting, the output of how long it took the command to run will be output as feedback. You can use the cmd2.Cmd.pfeedback method to produce this type of output, and several Settings control how it is handled.

If the quiet setting is True, then calling cmd2.Cmd.pfeedback produces no output. If quiet is False, the 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 cmd2.Cmd.pexcept method can help. The default behavior is to just display the message contained within the exception. However, if the 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 .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.

Colored Output

You can add your own ANSI escape sequences 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 documented in [cmd2.ansi][].

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 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.utils.align_left
• cmd2.utils.align_center
• cmd2.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.

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.ansi.style_aware_wcswidth 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.

Pretty Printing Data Structures

The cmd2.Cmd.ppretty method is similar to the Python pprint function from the standard pprint module. cmd2.Cmd.pprint adds the same conveniences as cmd2.Cmd.poutput.

This method provides a capability to “pretty-print” arbitrary Python data structures in a form which can be used as input to the interpreter and is easy for humans to read.

The formatted representation keeps objects on a single line if it can, and breaks them onto multiple lines if they don’t fit within the allowed width, adjustable by the width parameter defaulting to 80 characters.

Dictionaries are sorted by key before the display is computed.