PythonTutorial

Generating Python parser & usage code

This tutorial will demonstrate how to use the The program interface definition framework to generate a Python command line parser for your program.

• [Generating Python parser & usage code](#Generating Python parser & usage code)
  • [Step 1. Defining the options](#Step 1. Defining the options)
  • [Step 2. Generate parser and program description](#Step 2. Generate parser and program description)
  • [Step 3. Write your program](#Step 3. Write your program)
  • [Step 4. Run!](#Step 4. Run!)
  • [Step 5. Beyond the Python program](#Step 5. Beyond the Python program)
    • [Auto complete](#Auto complete)
  • [See also](#See also)

Step 1. Defining the options

The first step is to describe the program options in a XML file. The XML elements have to follow the program interface XML Schema. miniapp.xml

#!xml
<?xml version="1.0" encoding="utf-8"?>
<prg:program version="2.0" xmlns:prg="http://xsd.nore.fr/program">
	<prg:name>miniapp</prg:name>
	<prg:version>1.0</prg:version>
	<prg:options>
		<prg:switch>
			<prg:databinding>
				<prg:variable>displayHelp</prg:variable>
			</prg:databinding>
			<prg:names>
				<prg:long>help</prg:long>
				<prg:short>h</prg:short>
			</prg:names>
		</prg:switch>
		<prg:argument>
			<prg:databinding>
				<prg:variable>arg</prg:variable>
			</prg:databinding>
			<prg:names>
				<prg:long>some-arg</prg:long>
				<prg:short>a</prg:short>
			</prg:names>
		</prg:argument>
	</prg:options>
</prg:program>

You can use any editor to write this file but an XML editor with auto-completion and XML Schema support will greatly increase the writing speed.

Step 2. Generate parser and program description

Using the build-python command line tool

#!bash
${NS_XML_PATH}/ns/sh/build-python.sh --xml-description miniapp.xml --embed --output miniapp_info.py

This command line will generate [miniapp_info.py](https://github.com/noresources/ns-xml/wiki/miniapp_info.py) which will contains

• A copy of the Python parser classes
• A miniappProgramInfo class, extending ProgramInfo, which is a Python representation of the XML file.

Step 3. Write your program

Let's write a little and useless Python program [miniapp.py](https://github.com/noresources/ns-xml/wiki/miniapp.py)

#!python
#!/usr/bin/env python
import sys

from miniapp_info import *
"""
Include the parser program interface definition
generated by build-python
"""

info = miniappProgramInfo();
"""
Represents the miniapp interface description
"""

parser = Parser(info);
"""
Create a command line parser
"""

result = parser.parse(sys.argv, 1);
"""
Parse all command line arguments except the first (the program path)
The returned value is an instance of ProgramResult
populated dynamically with the value of miniapp's options
"""

usg = UsageFormat();
"""
Define several display options for program usg such as
- maximum line length
- indentation style
- usg verbosity
"""

if not result():
	"""
	PregramResult redefines the __call__ magic method.
	This will return True if no error occurs
	"""
	
	for m in result.getMessages():
		print " - ", m
	# Prints all warnings and errors
	
	usg.format = UsageFormat.SHORT_TEXT
	print info.usg(usg)
	# Prints a short program usg
	
	exit (1)

if result.displayHelp():
	"""*
	ProgramResult redefines __getattr__, so option variables
	can be accessed directly 
	
	Thus, OptionResult redefines __call__ to return the value of the option.
	In this case, a switch will return a boolean to indicates if it was present or not
	
	Other available syntax are:
	 result.displayHelp.isSet
	or
	 result["displayHelp"].isSet
	or
	 result["displayHelp"].value()
	...
	"""
	
	usg.format = UsageFormat.DETAILED_TEXT
	print info.usage(usg)
	# Prints a detailed program usg
	
	exit (0)


if result["arg"].isSet:
	print "Value of arg: ", result.arg()
	# Here, __call__ returs the option argument value

Step 4. Run!

#!bash
python ./miniapp.py -- --help --some-arg "Bleeeh Blaaah"

You should try to type invalid options or forget the --some-arg argument to see what's happen.

Step 5. Beyond the Python program

Auto complete

To get a bash auto-completion command file, use the bashcompletion.xsl style sheet to transform the option specification file

#!bash
xsltproc -o miniapp-autocomplete.inc.sh ${NS_XML_PATH}/ns/xsl/program/${SCHEMA_VERSION}/bashcompletion.xsl miniapp.xml

Then, include the generated file in your current environment

#!bash
. miniapp-autocomplete.inc.sh

And try typing

#!bash
./miniapp -<TAB>

The shell will propose...

#!bash
$ ./miniapp -
-a		   -h		   --help	   --some-arg   

See the Bash auto-complete file generation for more details.

See also

• The Python parser API details
• build-python command line tool usage

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The program interface definition framework