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Description:

parsethis 3.0.0

parse_this

Makes it easy to parse command line arguments for any function, method or classmethod.
You just finished writing an awesome piece of code and now comes the boring part: adding the command line parsing to
actually use it ...
So now you need to use the awesome, but very verbose, argparse module. For each argument of your entry point method
you need to add a name, a help message and/or a default value. But wait... Your parameters are correctly named, right!?
They also have type hinting, right!? And you have an awesome docstring for that method. There is probably a way of
creating the ArgumentParser easily right?
Yes and it's called parse_this!
Usage
parse_this contains a simple way to create a command line interface from an entire class. For that you will need to
use the parse_class class decorator.
# script.py
from parse_this import create_parser, parse_class

@parse_class()
class ParseMePlease(object):
"""This will be the description of the parser."""

@create_parser()
def __init__(self, foo: int, ham: int = 1):
"""Get ready to be parsed!

Args:
foo: because naming stuff is hard
ham: ham is good and it defaults to 1
"""
self._foo = foo
self._ham = ham

@create_parser()
def do_stuff(self, bar: int, spam: int = 1):
"""Can do incredible stuff with bar and spam.

Args:
bar: as in foobar, will be multiplied with everything else
spam: goes well with eggs, spam, bacon, spam, sausage and spam

Returns:
Everything multiplied with each others
"""
return self._foo * self._ham * bar * spam

if __name__ == "__main__":
print(ParseMePlease.parser.call())

python script.py --help # Print a comprehensive help and usage message
python script.py 2 do-stuff 2
>>> 4
python script.py 2 --ham 2 do-stuff 2 --spam 2
>>> 16

How does it work TL;DR version?

You need to decorate the methods you want to be usable from the command line using create_parser.
The __init__ method arguments and keyword arguments will be the arguments and options of the script command line i.e. the first arguments and options
The other methods will be transformed into sub-command, again mapping the command line arguments and options to the method's own arguments
All you have to do for this to work is:

Decorate your class with parse_class
Decorate methods with create_parser
Document your class and method with properly formed docstring to get help and usage message
Annotate all parameters with their type
Call <YourClass>.parser.call() and you are done!

If you feel like you may need more customization and details, please read on!

If the __init__ method is decorated it will be considered the first, or top-level, parser this means that all
arguments in your __init__ will be arguments pass right after invoking you script
i.e. python script.py init_arg_1 init_arg_2 etc...
The description of the top-level parser is taken from the class's docstring or overwritten by the keyword
argument description of parse_class.
Each method decorated with create_parser will become a subparser of its own.
The command name of the subparser is the same as the method name with _ replaced by -.
'Private' methods, whose name start with an _, do not have a subparser by default, as this would expose them to the
outside. However if you want to expose them you can set the keyword argument parse_private=True in parse_class. If
exposed their command name will not contain the leading - as this would be confusing for command parsing. Special
methods, such as __str__, can be decorated as well. Their command name will be stripped of all _s resulting in
command names such as str.
When used in a parse_class decorated class create_parser can take an extra parameters name that will be used as
the sub-command name. The same modifications are made to the name replacing _ with -
When calling python script.py --help the help message for every parser will be displayed making easier to find
what you are looking for

Arguments and types
Both parse_this and create_parser need parameters to have type annotations. Any Python builtin type can be used.
There is no need to provide a type for keyword arguments since it is inferred from the default value of the argument. If
your method signature contains arg_with_default=12 parse_this expect an int where arg_with_default is on the
command line.
If this is the content of parse_me.py:
from parse_this import create_parser

class INeedParsing(object):
"""A class that clearly needs argument parsing!"""

def __init__(self, an_argument):
self._an_arg = an_argument

@create_parser(delimiter_chars="--")
def parse_me_if_you_can(self, an_int: int, a_string: str, an_other_int: int = 12):
"""I dare you to parse me !!!

Args:
an_int -- int are pretty cool
a_string -- string aren't that nice
an_other_int -- guess what? I got a default value
"""
return a_string * an_int, an_other_int * self._an_arg

if __name__ == "__main__":
need_parsing = INeedParsing(2)
print(INeedParsing.parse_me_if_you_can.parser.call(need_parsing))

The following would be the output of the command line python parse_me.py --help:
usage: parse_me.py [-h] [--an_other_int AN_OTHER_INT] an_int a_string

I dare you to parse me !!!

positional arguments:
an_int int are pretty cool
a_string string aren't that nice

optional arguments:
-h, --help show this help message and exit
--an_other_int AN_OTHER_INT
guess what? I got a default value

The method parse_me_if_you_can expect an int with the name an_int, a str with the name a_string and
other int with the name an_other_int and a default value of 12. So does the parser as displayed by the --help
command.
Note: create_parser cannot decorate the __init__ method of a class unless the class is itself decorated
with parse_class. A ParseThisException will be raised if you attempt to use the call method of such a parser.
The following would be the output of the command line python parse_me.py 2 yes --default 4:
('yesyes', 8)

Help message
In order to get a help message generated automatically from the method docstring
it needs to be in the specific format described below:
from parse_this import create_parser

@create_parser(delimiter_chars="--")
def method(self, spam: int, ham: int):
"""<description>
<blank_line>
<arg_name><delimiter_chars><arg_help>
<arg_name><delimiter_chars><arg_help>
"""
pass

description: is a multiline description of the method used for the command line
each line of argument help have the following component:

arg_name: the same name as the argument of the method.
delimiter_chars: one or more chars that separate the argument and its help message. Using whitespaces is not
recommended as it could have an expected behavior with multiline help message.
arg_help: is everything behind the delimiter_chars until the next argument, a blank line or the end of the
docstring.

The delimiter_chars can be passed to both parse_this and create_parser as the keywords argument delimiter_chars.
It defaults to : since this is the convention I most often use.
If no docstring is specified a generic - not so useful - help message will be generated for the command line and
arguments.
Using None as a default value and bool as flags
Using None as a default value is common practice in Python but for parse_this and create_parser to work properly
the type of the argument which defaults to None needs to be specified. Otherwise a ParseThisException will be
raised.
from parse_this import create_parser

@create_parser()
def parrot(ham: str, spam=None):
if spam is not None:
return ham * spam
return ham

# Will raise ParseThisException: To use default value of 'None' you need to specify
# the type of the argument 'spam' for the method 'parrot'

Specifying the type of spam will allow create_parser to work properly
from parse_this import create_parser

@create_parser()
def parrot(ham: str, spam: int = None):
if spam is not None:
return ham * spam
return ham

# Calling function.parser.call(args="yes".split()) -> 'yes'
# Calling function.parser.call(args="yes --spam 3".split()) -> 'yesyesyes'

An other common practice is to use bools as flags or switches. All arguments of type bool, either typed directly or
inferred from the default value, will become optional arguments of the command line. A bool argument without default
value will default to True as in the following example:
from parse_this import create_parser

@create_parser()
def parrot(ham: str, spam: bool):
if spam:
return ham, spam
return ham

# Calling parrot.parser.call(args="yes".split()) -> 'yes', True
# Calling parrot.parser.call(args="yes --spam".split()) -> 'yes'

Adding --spam to the arguments will act as a flag/switch setting spam to False. Note that spam as become
optional and will be given the value True if --spam is not among the arguments to parse.
Arguments with a boolean default value will act as a flag to change the default value:
from parse_this import create_parser

@create_parser()
def parrot(ham: str, spam: bool = False):
if spam:
return ham, spam
return ham

# Calling parrot.parser.call(args="yes".split()) -> 'yes'
# Calling parrot.parser.call(args="yes --spam".split()) -> ('yes', True)

Here everything works as intended and the default value for spam is False
and passing --spam as an argument to be parsed will assign it True.
Decorator
As a decorator create_parser will create an argument parser for the decorated function. A parser attribute will be
added to the method and can be used to parse the command line argument.
from parse_this import create_parser

@create_parser()
def concatenate_str(one: str, two: int = 2):
"""Concatenates a string with itself a given number of times.

Args:
one: string to be concatenated with itself
two: number of times the string is concatenated, defaults to 2
"""
return one * two

if __name__ == "__main__":
print(concatenate_str.parser.call())

Calling this script from the command line, python script.py yes --two 3 will return 'yesyesyes' as expected and all
the parsing has been done for you.
Note that the function can still be called as any other function from any python file. Also it is not possible to
stack create_parser with any decorator that would modify the signature of the decorated function e.g.
using functools.wraps.
Function
As a function parse_this will handle the command line arguments directly.
from parse_this import parse_this

def concatenate_str(one, two=2):
"""Concatenates a string with itself a given number of times.

Args:
one: string to be concatenated with itself
two: number of times the string is concatenated, defaults to 2
"""
return one * two

if __name__ == "__main__":
print(parse_this(concatenate_str))

Calling this script with the same command line arguments yes --two 3 will also return 'yesyesyes' as expected.
Classmethods
In a similar fashion you can parse command line arguments for classmethods:
from parse_this import create_parser

class MyClass(object):

@classmethod
@create_parser(delimiter_chars="--")
def parse_me_if_you_can(cls, an_int: int, a_string: str, default: int = 12):
"""I dare you to parse me !!!

Args:
an_int -- int are pretty cool
a_string -- string aren't that nice
default -- guess what I got a default value
"""
return a_string * an_int, default * default

MyClass.parse_me_if_you_can.parser.call(MyClass)

The output will be the same as using create_parser on a regular method.
Notes:

The classmethod decorator is placed on top of the create_parser decorator in order for the method to still be
a considered a class method.
A classmethod decorated with create_parser in a class decorated with parse_class will not be accessible through
the class command line.

Installing parse_this
parse_this can be installed using the following command:
pip install parse_this

RUNNING TESTS
To check that everything is running fine you can run the following command after cloning the repo:
python -m pip install --upgrade pip && python -m pip install -r requirements.txt && pytest

CAVEATS

parse_this and create_parser are not able to be used on methods with *args and **kwargs
A subsequent effect of the previous caveat is that create_parser cannot be stacked with other decorator that would
alter the callable's signature
Classmethods cannot be access from the command line in a class decorated with parse_class
When using create_parser on a method that has an argument with None as a default value its type must be past in
the list of types. A ParseThisException will be raised otherwise.

TO DO

Handle file arguments
Handle list/tuple arguments i.e. argparse's nargs

License
parse_this is released under the MIT Licence. See the bundled LICENSE file for details.
Contributing and dev
python3 -m venv --clear --upgrade-deps --prompt "parse-this-39" venv && \
source venv/bin/activate && \
pip install -r requirements.txt && \
pre-commit install && \
pytest