Just Some Python Things

[c0dist]

Hello,

This post is here so people can share their Python tips, tricks, hacks and best practices. This post follows the footsteps of bash cheat sheet here - http://garage4hackers.com/showthread.php?t=3100
So, all the Pythonistas, share your tips, tricks and whatever you think is awesome about python.

Regards,
c0dist

[c0dist]

Hi,

Want to share files between PCs? Want to test your html page? Need a quick simple http server? Call python.

Here's the one liner that will start serving a web server on port 8000

batman@gothamcity ~ $ python -m SimpleHTTPServer

Want to serve on a different port? Here's how

batman@gothamcity ~ $ python -m SimpleHTTPServer <port_number>

Yup, that's all you need to do. Just open your browser and use it.
Hope this will help you.
Note: Tested on a Linux machine. Should work on Windows too.

Regards,
c0dist

[c0dist]

In Python, you can reverse a string simply by using this syntax,

>>> string = "Garage4Hackers"
>>> print string[::-1]
srekcaH4egaraG

The way this works is by using something known as "slicing". In above example above, we ask Python to print the string, using the negative step. This is explained in a better way here - http://stackoverflow.com/a/509295

Fun Thing:

Remember writing code to check if a string is a palindrome or not? Here's how to do, Python's way.

>>> string = "radar"
>>> string == string[::-1]
True

Simple, right?

Regards,
c0dist

[c0dist]

Just came across a handy Python cheat sheet via @Punter. Posting this here so others could benefit from it and also so that I don't forget.

Here it is - http://overapi.com/python/
Enjoy.

Regards,
c0dist

[c0dist]

In Python, if you want to read/write from a file, you might call "open()" first, and then read() or write() accordingly and then finally a close() to close that file handle.

For example, one way to read from a local file is,

#!/usr/bin/env python

f = open("/etc/passwd")
print f.read()
f.close()

This code will simply output the content of file. However, downside with this code is if some error occurs between open and close call, the file handle might stay open and in some cases, the file might get corrupted too. A better way to handle files is using the "with" statement in Python. According to the official page https://www.python.org/dev/peps/pep-0343/, we see that

This PEP adds a new statement "with" to the Python language to make
it possible to factor out standard uses of try/finally statements.

In this PEP, context managers provide __enter__() and __exit__()
methods that are invoked on entry to and exit from the body of the
with statement.

What this means in simple words is, if you open a file using "with" and some error occurs, the __exit__() i.e. close() in this case will be called before the program goes out of this block.
Another good definition/advantage is defined here http://preshing.com/20110920/the-pyt...nt-by-example/ -

The advantage of using a with statement is that it is guaranteed to close the file no matter how the nested block exits. If an exception occurs before the end of the block, it will close the file before the exception is caught by an outer exception handler. If the nested block were to contain a return statement, or a continue or break statement, the with statement would automatically close the file in those cases, too.

The syntax for using "with" statement from official standard is,

with EXPR as VAR:
BLOCK

Here, 'with' and 'as' are new keywords; EXPR is an arbitrary
expression (but not an expression-list) and VAR is a single
assignment target. It can *not* be a comma-separated sequence of
variables, but it *can* be a *parenthesized* comma-separated
sequence of variables.

An example of reading and writing a file using "with" statement,

#!/usr/bin/env python

with open("/etc/passwd", "r") as fin:
    content = fin.read()
    if "root" in content:
        print "root word found inside file"

with open("/tmp/test.txt", "w") as fout:
    fout.write("I wrote this using 'with' in Python")

I hope this helps.

Cheers,
c0dist

[Anant Shrivastava]

Couple of entries from my side

Python simple server in version 3

python -m http.server 8000

Make python os independent.
1. Avoid using os specific modules.
1. use os.pathsep to seperate path's in path's


Some helpful python modules that are life savers

1. argparse : useful for taking commandline arguments from commandline.
1. beautifulsoup : html parsing and processing module.



Note: I will keep editing this same post to add more details in it.

[c0dist]

There are various situations where instead of keeping configurations in the python script, we might need to keep an external config file. The advantage of keeping a config file is that then multiple scripts can access and use that conf file. The way Python provides this facility is via a library "ConfigParser". This library provides all the methods necessary to parse your config file.A great tutorial for the library can be found here - https://docs.python.org/2/library/configparser.html and here - https://pymotw.com/2/ConfigParser/. In this post, I'll show a basic usage case of ConfigParser.

Example of config file (example.conf):

[global]
path = /tmp/files/
time = 90

[app]
restart = True

The above config file contains two sections, global and app. We'll see how to parse this in following py scripts.

#!/usr/bin/env python

import ConfigParser

conf_parser = ConfigParser.ConfigParser() # Making a ConfigParser object
conf_parser.read("example.conf") # The config file to parse

path = conf_parser.get("global", "path") # Getting `path` setting from `global` section
app_time = conf_parser.getint("global", "time") # Getting integer value
restart = conf_parser.getboolean("app", "restart") # Getting boolean value

print path, app_time, restart # Prints the config values

That's it, pretty simple. For more detailed explanations and examples, please check out the mentioned resources. Hope this helps.

Cheers,
c0dist

["vinnu"]

A simple trick to save the script form loading "struct" module and converting string or binary data to number is as :

num = int(data[:4][::-1].encode("hex"),16)

and vice-versa would be :

data = str(('%08X' % num).decode("hex")[::-1])

..."vinnu"