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Library for writing command line programs

Project Description

This module contains a lightweight library for creating command line programs, conz.

conz has following features:

  • Simplifies working with pipes
  • Provides methods for handling typical interactive scenarios
  • Supports output colorization
  • Provides tools for working with long-running tasks
  • Controls output in interactive and non-interactive scenarios
  • Manages signals (SIGINT and SIGPIPE)


You can install conz from PyPI using pip or easy_install:

pip install conz

easy_install conz

Quick tour

A quick tour example can be found in examples/

import conz

someval = True

cn = conz.Console(verbose=True)

cn.pstd('This goes to STDOUT')
cn.perr('This goes to STDERR')
cn.pverr(someval, 'This message is related to somevar')

cn.pstd('This message is green'))

with cn.progress('Some long operation'):
    import time

data ='Type something in:')
cn.pstd('You typed in {}'.format(cn.color.yellow(data, style='italic')))

Formatting conventions

Because this library deals with terminal output a lot, we have to somehow tell when something is terminal output and when it is code. Because of this, we use lines to delimit console output. For example:

I'm a sample output

When output is to the STDERR, ‘E’ will be shown in the right corner.:

I'm a sample error

When user enters data, the Entered data will be followed by <Enter> and the right corner will include the ‘I’ character (for ‘interactive session’):

Prompt: some data<Enter>

When value is returned from user input, the value is printed right below the output preceeded by ==>:

Prompt: some data<Enter>
==> 'some data'


The conz package includes a class Console which is the only class you will even need to work with. Simply import and instantiate it at the top of your program.

import conz
cn = conz.Console()

Working with output

The Console() class is, for the most part, a wrapper around the print() function (not print statement, so not compatible with versions of Python that do not support this). It controls how print() is invoked and takes care of some of the edge cases where it may malfunction.

The print() method on a Console object is a very simple wrapper around Python’s print() whic does nothing except pass it’s positional and keywrod arguments to the print() function. We will never use it directly, though, as there are shortcuts for doing specific things with print().

To output to STDOUT, we use the pstd() method. It takes the same arguments as print() function, with the exception of file keyword argument which is set by this method and cannot be overridden.

cn.pstd('This always goes to STDOUT', end='...')
This always goes to STDOUT

To output to STDERR, we use the perr() method. As with pstd(), it overrides the file argument for us.

cn.perr('Mayday, mayday!')
Mayday, mayday!

The main difference between regular print() and pstd()/perr() methods is that the latter will flush the STDOUT/STDERR after writing to it. This can prvent weird issues in some edge cases.

There is a variant of perr() which prints a more structured message to STDERR. The pverr() method takes a value and a message, and prints then in VALUE: Message format.

path = '/foo/bar/baz.txt'
cn.verr(path, 'not found')
/foo/bar/baz.txt: not found

A variant of pstd() is pverb(). It is exactly the same as pstd(), except that it only outputs when verbose flag on the Console object is True. This is useful for programs that need to differentiate between interactive and non-interactive use (e.g., using in pipe vs invoking directly) or wish to have a --verbose switch, etc.

cn.verbose = True
cn.pverb("I'm a talkative program")
I'm a talkative program

cn.verbose = False
cn.pverb("I'm a talkative program")


The verbose flag can be set either as an argument during instantiation, or simply by setting the attribute as in the previous example.

The Console object also provides a outterm property which is False when program is outputting to a pipe rather than the terminal:

if cn.outterm:
    # give full output to the user
    # give a short output that can be parsed by a machine, etc


Before we start, note that this implementation is not cross-platform. If you need something with a bit more punch, you should look at colorama.

To colorize the output, both the conz module and Console class have a color attribute, which provides methods for output colorization. Each piece of text can have the following attributes:

  • foreground color
  • style
  • background color

Foreground and background colors can be:

  • black
  • red
  • green
  • yellow
  • blue
  • purple (magenta)
  • cyan
  • white

Styles can be:

  • bold
  • italic
  • underline
  • blink
  • reverse (inverts foreground and background colors)

Each of these colors have a method on the color attribute. Each color method takes style and bg keyword arguments which set the style and background color respectively. The color() method can be used to specify colors dynamically. Here are some examples:'This is red text')
cn.color.color('This is red text', color='red')'This is blue underlined text', style='underline')
cn.color.color('This is green on yellow', color='green', bg='yellow')

You can find an example script in examples/ which prings all possible combinations of various colors, styles, and backgrounds.

Working with input

There are two types of input you can work with: interactive user input, and pipes.

To read the user input, use read() method. This method takes two optional arguments. One is the prompt argument, which we use to set the prompt. It is an empty string by default. The other argument is a data-cleaning function. When you pass the clean argument, user input is passed through the function before it is retuned. For example:'Exit? [y/N] ', clean=lambda x: x.lower()[:1] == 'y')
Exit? [y/N] y<Enter>
==> True

Note that this method uses raw_input() on Python 2.7.x and input() on Python 3.x.

To work with pipes, we use the readpipe() method. This method reads from the STDIN pipe one line at a time and returns an iterator that allows us to iterate over the lines.

for l in cn.readpipe():
    l = l.strip()
    cn.pstd('Received: {}'.format())

Note that line-feed characters are not stripped from the output so it is up to us to strip it away.

When working with a large number of lines coming down the pipe, we may sometimes need to work in batches, rather than one line at a time. The chunk argument can be set to an integer value that specifies the number of lines we want buffered before they are returned to us. When using chunks, the lines are returned as a list of strings, rather than strings. The following example will return pipe input in groups of 500:

for lines in cn.readpipe(500):
    # do something with 500 lines

If we need to know whether input will come from a pipe or not, we can use the interm property.

if cn.interm:
    # possibly interactive version
    # we are on the receiving end of a pipe

Advanced interactive input

So far we have looked at simpe user input. However in most cases, input is not the only thing we want. We normally also need to show notes, validate the input, construct menus, etc. The Console class provides three methods that are useful for different scenarios.

You will find examples of code discussed here in examples/ and examples/


RVPL (pead validate print loop) is a loop in which some data is read from the user, validated, and error message printed. This loop continues as long as data is invalid. The rvpl() method is used to start such a loop.

At bare minimum, rvpl() is called with a prompt that should be shown to the user.

cn.rvpl('Please enter your name:')
Please enter your name: My name<enter>
==> 'My name'

Like read(), rvpl() also takes a clean argument, which is used to control how the value is cleaned. In addition, it takes validator argument, which is a function that validates the cleaned data. The default validator simply makes sure the input is not an empty string.

For invalid input, error message is displayed:

cn.rvpl('Please enter your name:')
Please enter your name: <Enter>
Entered value is invalid
Please enter your name: Mike<Enter>
==> 'Mike'

Error message can be customized using the error argument. If error argument is is a callable, it will be called with entered value and it must return the message to be shown.

valid_input = ('a', 'b', 'c')
error = lambda x: '{} is not one of the {}'.format(
    x, ', '.join(valid_input))
validator = lambda x: x in valid_input
cn.rvpl('Type one of the first 3 characters of English alphabet:')
Type one of the first 3 characters of English alphabet: e<Enter>
e is not one of the a, b, c
Type one of the first 3 characters of English alphabet: b<Enter>
==> 'b'

An intro message can be passed which is shown above the prompt. Unlinke the prompt itself, intro message is not repeated in the loop.

cn.rvpl('>', intro='Please enter your name:')
Please enter your name:
> <Enter>
Entered value is invalid
> Mike<Enter>
==> 'Mike'

When requesting optional input, the strict validation can be turned off using the strict argument. When this argument is False, then the loop exists even when validation fails. The value returned when validation fails is controlled by default argument, which defaults to None.

cn.rvpl('Please enter your name:', strict=False, default='Bob')
Please enter your name: <Enter>
==> 'Bob'

Yes/No input

The yesno() method provides a specialized version the RVPL limited to yes and no answer, and returnin True or False.

cn.yesno('Are you all right?')
Are you all right? (y/n): y<Enter>
==> True

The prompt passed to yesno() is automatically appended the ‘(y/n):’ string. The appearance of this string depends on the default value discussed further below.

Since it is a wrapper around rvpl() it takes the same error and intro arguments which behave the same way.

Although it takes the default argument like rvpl(), the behavior is different. When default is None it automatically turns on strict validation. The argument can also be either True or False, in which case the default value is respectively ‘yes’ and ‘no’.

cn.yesno('Are you all right?', default=True)
Are you all right? (Y/n): <Enter>
==> True

cn.yesno('Are you all right?', default=False
Are you all right? (y/N): <Enter>
==> False

Working with progress

Progress is a more complex construct that we use to notify user of some activity that may take a while. Each progress has a start banner, which is printed before we begin, and two end banners, one for success and one for failure.

Before we can use the progress context manager, we must enable verbose mode.

cn.verbose = True

A progress is started using the progress() method, which is a context manager.

with cn.progress("Let's get this show on the road"):
    # do something

This is the simplest form. When an exception of any kind is triggered inside the context, it is trapped, the failure banner is printed, and the conz.ProgressAbrt exception is raised. (This exception is also available as an attribute on Console objects for convenience.) If everything goes well, then the success banner will be printed. With the previous code snippet, sucess output may look like this:

Let's get this show on the road...DONE

And failure would look like this:

Let's get this show on the road...FAIL

The end banners can be customized by using the end and abrt arguments:

with cn.progress('Almost there', end='finally!', abrt='awww, bummer'):
    # do something

The outputs would look like this:

Almost there...finally!


Almost there...awww, bummer

The elipsis (three dots) can be customized using the sep argument:

with cn.progress('File check', sep=': '):
    # do something

This results in:

File check: DONE


File check: FAIL

By default, the progress context manager will trap any exception. This may or may not make sense for a particular situation. This behavior can therefore be customized using the excs argument, which takes a tuple of exception classes that we are expecting. Passing exceptions explicitly like this allows the context manager to propagate unhandled exceptions and reval subtle flaws in our logic.

We can also specify a callback that runs each time an exception (other than ProgressAbrt and ProgressOK are raised inside the context. This callback is specified using onerror argument, and defaults to an error handler that prints ‘Program error: ERROR MESSAGE’ to STDERR. For convenience, the Console object has a error() method which creates such handlers.

By default, the tracebacks raised during progress is suppressed. To see the full traceback, Console constructor takes a debug argument, which can be set to True to prevent traceback suppression.

To create a handler, we call the error() method like so:

handler = cn.error('Ouch!', exit=1)
with cn.progress('Ouch progress', onerror=handler):
    raise RuntimeError()

The above results in:

Outch progress...FAIL

The message may have a {err} placeholder, which gets replaced by the string representation of the exception that was raised in the block.

To completely suppress the error handler, simply pass it a function that does nothing.

with cn.progress('No ouch', onerror: lambda exc: None):
    raise RuntimeError()
No ouch...FAIL


Note that passing None as onerror value simply causes the default error handler to be used instead.

The progress context manager returns a Progress object, which provides methods for explicitly terminating the progress, and printing the progress indicator. This object has end() and abrt() methods, which are called to terminate with success and error status respectively. For example:

with cn.progress('Something') as prg:
    if not success:

The end() and abrt() methods raise ProgressOK and ProgressAbrt exceptions repsectively. We can suppress raising of the exceptions using noraise argument and setting it to True. Both of the methods will use the default end banners. We can also use any banner we want by passing it as the first positional argument. This can be useful in cases where the end banner should indicate different outcomes.


Default banners are colorized (green for success, red for failure). Any custom banners passed directly to end() and abrt() will not be colorized, though.

The ProgressOK exception is not meant to be handled by us in any way, and it’s simply there to facilitate flow control. ProgressAbrt is, by default, reraised so that code outside the context manager can handle it. Therefore, we normally wrap the context block in a try-except:

    with cn.progress('Something'):
        # do something
except cn.ProgressAbrt:
    # something went wrong

This reraising of the ProgressAbrt exception can be suppressed by using the reraise argument which can be True or False. Setting this flag to False silences the ProgressAbrt exception. At that point, we are still able to do error handling using the onerror callback.

You can find a script in examples/ which demonstrates a few typical cases.


To quit the program, we call the quit() method on the Console object. This method works the same way as sys.exit() (except that it takes one less import to use it).

Signal handling

The default implementation of Console class automatically takes care of SIGINT (keyboard interrupt) and SIGPIPE (broken pipe) signals. You can customize the way those are handled by overloading the onint() and onpipe() methods. You can also customize the registration of signals themselves by overloading the register_signals() method.

Reporting bugs

Please report any bugs or feature requests to the issue tracker.

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