THE PYTHON STD LIB BY EXAMPLE – APPLICATION BUILDING BLOCKS

JohnThursday, April 13, 2023

Brief introduction• This lesson covers some of the more frequently

reused building blocks that solve problems common to many applications

• Command-line argument parsing: module getopt,optparse,argparse.

• Interactive programs: readline, cmd, shlex,fileinput

• Manage application configuration: ConfigParser• Manage log file: logging• Others: atexit, sched etc

Module getopt – option parsing

• The getopt() take 3 arguments:1. The sequeence of arguments to be parsed.

Usually it is sys.argv[1:]2. Single-character option, following a colon(:)

mean it require an argument. E.g. ab:c:, it mean a is -a simple flag, while –b and –c require an argument.

3. It is optional. If used, it is a list of long-style option names. Having suffix “=“ means ruquire an argument. E.g [‘noarg’,’witharg=‘]

Quick example 1# file test.pyimport getoptimport sysopts,args = getopt.getopt(sys.argv[1:],’ab:c:’)for opt in opts:

print opt

•Then run command line “python test.py –a –b valb –c valc”, the output should be ('-a', '')('-b', 'val')('-c', 'val')

quick example 2

# file test.pyimport getoptimport sysopts,args = getopt.getopt(sys.argv[1:],’’,[‘opta’,’optb=‘,’optc=‘])for opt in opts:

print opt•run command line “python test.py --opta --optb val --optc val”('--opta', '')('--optb', 'val')('--optc', 'val')

Module optparse

• it is a modern alternative for module getopt

Quick example

Example 2:

More options in add_option method• option dest: the attribute name• option default: the default value. also can be set in

set_defaults method. • option type: convert the argument string to specific type,

e.g. int, float,string• option choices: validation use a list of candidate strings. e.g.

choices=[‘a’,’b’,’c’] . the valid value should be ‘a’, or ‘b’ or ‘c’• option help: help message• option action: store, store_const,store_true,append,acount

Module argparse

• argparse added to python 2.7 as a replacement of optparse.

• Change the OptionParser by ArgumentParser, and add_option by add_argument in previous example.

More Options of ArgumentParser

• add help automatically: parser = argparse.ArgumentParser(add_help=True)

• version control: version=‘1.0’ (-v or --version will show the version number)

More options of add_argument method• All options in optparse.add_option method.• option nargs: number of expected

arguments. (? mean 0 or 1 arguments, * means 0 or all, + means 1 or all).exampleparser.add_argument(‘-c’,nargs=3)we need write command line “perl -c 1 2 3”

Module getpass: handle passowrd prompts securely• print a promt and then read input from the user.

• We can change the prompt: getpass(prompt=‘what is your favorite color?’)

MODULE CMD: BUILD COMMAND LINE PROCESSORS

brief introduction

• the Cmd class is used as base class for interactive shells and other command interpreters.

• It can provide interactive prompt, command-line editing, and command completion.

quick example

• method do_greet() connect with command “greet”• method help_greet() connect with command “help greet”

auto-completion

• auto-completion is already enabled in previous example.

• if user want to do user-defined on auto-completion, use complete_<comand> (e.g. complete_greet() )

Overrite Base Class method

• Method cmploop(intro) is the main processing loop. Each iteration in cmdloop call parseline and onecmd.

• Method emptyline: behavior if emptyline. Default behavior is that rerun previous command.

• Method default: default method if command is not found.

Running shell command

• An exclamation point(!) map to the do_shell() method and is intended “shelling out” to run other commands.

• First import subprocess module.• Second, define do_shell() method

THE LOGGING MODULE

Brief introduction

• The logging module define standard API for reporting error and status information.

• Both application developer and module author benefit from this module, but has different considerations.

Logging to file

• Use basicConfig set the log file name

The level of the events logging tracks

Level When it’s used Numeric value

DEBUG

Detailed information, typically of interest only when diagnosing problems. 10

INFO Confirmation that things are working as expected. 20

WARNING

An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected.

30

ERROR

Due to a more serious problem, the software has not been able to perform some function. 40

CRITICAL

A serious error, indicating that the program itself may be unable to continue running. 50

Best practice: logging in single file

import logginglogging.basicConfig(level=logging.WARNING) # Only the level equal or alove this level can be displayed.logging.debug('This message should go to the log file')logging.info('So should this')logging.warning('And this, too')

•Write the log to file: logging.basicConfig(filename='example.log',level=logging.DEBUG)

Best practice: Logging from multiple modules• Define logging config

in main file

• Write log in other files