Command line programs for busy developers
Project description
Overview
Command line programs for lazy humans.
Decorate a function to be your programs starting point.
Generate command line parser based on function signature.
Search system environment for option default values.
Why begins?
I write a lot of small programs in Python. These programs often accept a small number of simple command line arguments. Having to write command line parsing code in each of these small programs both breaks my train of thought and greatly increases the volume of code I am writting.
Begins was implemented to remove the boilerplate code from these Python programs. It’s not intended to replace the rich command line processing needed for larger applications.
Requirements
For Python versions earlier than Python 3.3, the funcsigs package from the Python Package Index is required.
For Python version 2.6, the argparse package from the Python Package Index is also required.
Both of these dependencies are listed in the package configuration. If using Pip to install begins then the required dependencies will be automatically installed.
Installation
begins is available for download from the Python Package Index. To install using Pip
$ pip install begins
Alternatively, the latest development version can be installed directly from Github.
$ pip install git+https://github.com/aliles/begins.git
Please note that begins is still in an alpha state and therefore the API or behaviour could change.
Setting a programs starting point
The begin.start() function can be used as a function call or a decorator. If called as a function it returns True when called from the __main__ module. To do this it inspects the stack frame of the caller, checking the __name__ global.
This allows the following Python pattern:
>>> if __name__ == '__main__': ... pass
To be replace with:
>>> import begin >>> if begin.start(): ... pass
If used as a decorator to annotate a function the function will be called if defined in the __main__ module as determined by inspecting the current stack frame. Any definitions that follow the decorated function wont be created until after the function call is complete.
Usage of begin.start() as a decorator looks like:
>>> import begin >>> @begin.start ... def run(): ... pass
By deferring the execution of the function until after the remainder of the module has loaded ensures the main function doesn’t fail if depending on something defined in later code.
Parsing command line options
If begin.start() decorates a function accepts parameters begin.start() will process the command for options to pass as those parameters:
>>> import begin >>> @begin.start ... def run(name='Arther', quest='Holy Grail', colour='blue', *knights): ... "tis but a scratch!"
The decorated function above will generate the following command line help:
usage: example.py [-h] [-n NAME] [-q QUEST] [-c COLOUR] [knights [knights ...]] tis but a scratch! positional arguments: knights optional arguments: -h, --help show this help message and exit -n NAME, --name NAME (default: Arther) -q QUEST, --quest QUEST (default: Holy Grail) -c COLOUR, --colour COLOUR (default: blue)
In Python3, any function annotations for a parameter become the command line option help. For example:
>>> import begin >>> @begin.start # doctest: +SKIP ... def run(name: 'What, is your name?', ... quest: 'What, is your quest?', ... colour: 'What, is your favourite colour?'): ... pass
Will generate command help like:
usage: holygrail_py3.py [-h] -n NAME -q QUEST -c COLOUR optional arguments: -h, --help show this help message and exit -n NAME, --name NAME What, is your name? -q QUEST, --quest QUEST What, is your quest? -c COLOUR, --colour COLOUR What, is your favourite colour?
Command line parsing supports:
positional arguments
keyword arguments
default values
variable length arguments
annotations
Command line parsing does not support variable length keyword arguments, commonly written as **kwargs. If variable length keyword arguments are used by the decorated function an exception will be raised.
If a parameter does not have a default, failing to pass a value on the command line will cause running the program to print an error and exit.
For programs that have a large number of options it may be preferable to only use long options. To suppress short options, pass False as the short_args keyword argument to the begin.start decorator:
>>> import begin >>> @begin.start(short_args=False) ... def run(name='Arther', quest='Holy Grail', colour='blue', *knights): ... "tis but a scratch!"
This program will not accept -n, -q or -c as option names.
Similarity, a large number of command line options may be better displayed in alphabetical order. This can be achieved by passing lexical_order as True:
>>> import begin >>> @begin.start(lexical_order=True) ... def main(charlie=3, alpha=1, beta=2): ... pass
This program will list the command line options as alpha, beta, charlie instead of the order in which the function accepts them.
Boolean options
If a command line option has a default value that is a bool object. (True or False) The command line option will be a flag rather than an option that accepts a value. Providing the flag on the command line will pass the opposite of the default value to the function:
>>> import begin >>> @begin.start ... def main(enable=False, disable=True): ... pass
Using --enable or --disable when invoking this program will invert the associated option.
Sub-Commands
begins supports using functions as sub-commands with the begin.subcommand() decorator:
>>> import begin >>> @begin.subcommand # doctest: +SKIP ... def name(answer): ... "What is your name?" ... >>> @begin.subcommand # doctest: +SKIP ... def quest(answer): ... "What is your quest?" ... >>> @begin.subcommand # doctest: +SKIP ... def colour(answer): ... "What is your favourite colour?" ... >>> @begin.start ... def main(): ... pass
This example registers three sub-commands for the program:
usage: subcommands.py [-h] {colour,name,quest} ... optional arguments: -h, --help show this help message and exit Available subcommands: {colour,name,quest} colour What is your favourite colour? name What is your name? quest What is your quest?
The main function will always be called with the provided command line arguments. If a sub-command was chosen the associated function will also be called.
Sub-commands can be registered with a specific named group by passing a group argument to the begin.subcommand decorator. The begin.start() decorator can use sub-commands from a named group by passing it a sub_group argument.
Similarly, sub-commands can be load from entry points by passing the name of the entry point through the plugins argument to the begin.start() decorator:
>>> import begin >>> @begin.start(plugins='begins.plugin.demo') ... def main(): ... pass
Any functions from installed packages that are registered with the begins.plugin.demo entry point will be loaded as sub-commands.
Multiple Sub-Commands
Some commands may benefit from being able to be called with multiple subcommands on the command line. The enable multiple sub-commands a command separator value needs to be passed to be passed to begin.start() as the cmd_delim parameter:
>>> import begin >>> @begin.subcommand # doctest: +SKIP ... def subcmd(): ... pass ... >>> @begin.start(cmd_delim='--') ... def main(): ... pass
When this program is called from the command line multiple instances of the sub-command may be called if separated by the command delimiter --.
Sub-Command Context
There are use cases where it is desirable to pass state from the main function to a subsequent sub-command. To support this Begins provides the begin.context object. This object will have the following properties:
return_value, value returned by previous command function.
opts_previous, iterable of options object used by previous commands.
opts_current, options object for current command.
opts_next, iterable of options object for following commands.
Any other properties set on the begin.context object will not be altered by begins.
The return_value property will be set if the value returned from a main function or sub-command function is not the None object.
Environment Variables
Environment variables can be used to override the default values for command line options. To use environment variables pass a prefix string to the begin.start() decorator through the env_prefix parameter:
>>> import begin >>> @begin.start(env_prefix='MP_') ... def run(name='Arther', quest='Holy Grail', colour='blue', *knights): ... "tis but a scratch!"
In the example above, if an environment variable MP_NAME existed, it’s value would be used as the default for the name option. The options value can still be set by explicitly passing a new value as a command line option.
Configuration files
Configuration files can also be used to override the default values of command line options. To use configuration files pass a base file name to the begin.start() decorator through the config_file parameter:
>>> import begin >>> @begin.start(config_file='.camelot.cfg') ... def run(name='Arther', quest='Holy Grail', colour='blue', *knights): ... "tis but a scratch!"
This example will look for configuration files named .camelot.cfg in the current directory and/or the user’s home directory. A command line option’s default value can be changed by an option value in a configuration file. The configuration section used matches the decorated function’s name by default. This can be changed by passing a config_section parameter to begin.start():
>>> import begin >>> @begin.start(config_file='.camelot.cfg', config_section='camelot') ... def run(name='Arther', quest='Holy Grail', colour='blue', *knights): ... "tis but a scratch!"
In this second example the section camelot will be used instead of a section named run.
Argument type casting
Command line arguments are always passed as strings. Sometimes thought it is more convenient to receive arguments of different types. For example, this is a possible function for starting a web application:
>>> import begin >>> @begin.start ... def main(host='127.0.0.1', port='8080', debug='False'): ... port = int(port) ... debug = begin.utils.tobool(debug) ... "Run web application"
Having to convert the port argument to an integer and the debug argument to a boolean is additional boilerplate code. To avoid this begins provides the begin.convert() decorator. This decorator accepts functions as keyword arguments where the argument name matches that of the decorator function. These functions are used to convert the types of arguments.
Rewriting the example above using the begin.convert() decorator:
>>> import begin >>> @begin.start ... @begin.convert(port=int, debug=begin.utils.tobool) ... def main(host='127.0.0.1', port=8080, debug=False): ... "Run web application"
The module begin.utils contains useful functions for converting argument types.
Automatic casting
For simple, built-in types begins can automatically type cast arguments. This is achieved by passing the parameter _automatic to begin.convert():
>>> import begin >>> @begin.start ... @begin.convert(_automatic=True) ... def main(host='127.0.0.1', port=8080, debug=False): ... "Run web application"
This example is functionally equivalent to the example above.
Automatic type casting works for the following built-in types.
int or long
float
boolean
tuple or list
Additional casting functions can be provided with the same call to the begin.convert() decorator.
Alternatively, use of begin.convert() can be dispensed by passing True to begin.start() via the auto_convert parameter:
>>> import begin >>> @begin.start(auto_convert=True) ... def main(host='127.0.0.1', port=8080, debug=False): ... "Run web application"
Again, this example is functionally equivalent to the example above.
The limitation of using auto_convert is that it is not longer possible to provide additional casting functions.
Command Line Extensions
There are behaviours that are common to many command line applications, such as configuring the logging and cgitb modules. begins provides function decorators that extend a program’s command line arguments to configure these modules.
begin.tracebacks()
begin.logging()
To use these decorators they need to decorate the main function before begin.start() is applied.
Tracebacks
The begin.tracebacks() decorator adds command line options for extended traceback reports to be generated for unhandled exceptions:
>>> import begin >>> @begin.start ... @begin.tracebacks ... def main(*message): ... pass
The example above will now have the following additional argument group:
tracebacks: Extended traceback reports on failure --tracebacks Enable extended traceback reports --tbdir TBDIR Write tracebacks to directory
Passing --tracebacks will cause extended traceback reports to be generated for unhandled exceptions.
Traceback options may also be set using configuration files, if Configuration files are supported. The follow options are used.
enabled: use any of true, t, yes, y, on or 1 to enable tracebacks.
directory: write tracebacks to this directory.
Options are expected to be in a tracebacks section.
Logging
The begin.logging() decorator adds command line options for configuring the logging module:
>>> import logging >>> import begin >>> @begin.start ... @begin.logging ... def main(*message): ... for msg in message: ... logging.info(msg)
The example above will now have two additional optional arguments as well as an additional argument group:
optional arguments: -h, --help show this help message and exit -v, --verbose Increse logging output -q, --quiet Decrease logging output logging: Detailed control of logging output --loglvl {DEBUG,INFO,WARNING,ERROR,CRITICAL} Set explicit log level --logfile LOGFILE Ouput log messages to file --logfmt LOGFMT Log message format
The logging level defaults to INFO. It can be adjusted by passing --quiet, --verbose or explicitly using --loglvl.
The default log format depends on whether log output is being directed to standard out or file. The raw log text is written to standard out. The log message written to file output includes:
Time
Log level
Filename and line number
Message
The message format can be overridden using the --logfmt option.
Logging options may also be set using configuration files, if Configuration files are supported. The follow options are used.
level: log level, must be one of DEBUG, INFO, WARNING, ERROR or CRITICAL.
file: output log messages to this file.
format: log message format.
Options are expected to be in a logging section.
Entry Points
The setuptools package supports automatic script creation to automatically create command line scripts. These command line scripts use the entry points system from setuptools.
To support the use of entry points, functions decorated by begin.start() have an instance method called start() that must be used to configure the entry point:
setup( # ... entry_points = { 'console_scripts': [ 'program = package.module:main.start' ] }
Use of the start() method is required because the main function is not called from the __main__ module by the entryp points system.
Issues
Any bug reports or feature requests can be made using GitHub’ issues system.
Project details
Download files
Download the file for your platform. If you're not sure which to choose, learn more about installing packages.