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:
- Since users can pipe output to a shell command, it catches
BrokenPipeError
and outputs the contents ofself.broken_pipe_warning
tostderr
.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 inself.broken_pipe_warning
when you initializecmd2.Cmd
. - 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:
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.