What are the uses of the python 'pass' statement?

In some languages, we can have empty (do nothing) blocks like below.
if a==ture
{
   //do something
}
else
{
  //do nothing
}


#Python doesn't support empty blocks.

In python, following block gives error

a = True

if a is True:
    print ('condition is true')
else:
     

A 'pass' will need to be used in such cases.

a = True

if a is True:

    print ('condition is true')

#a block without any statements is invalid in python. In such cases,

# we need to use 'pass'

else:

    pass

Note: this is one of the reason why 'pass' is needed. There may be others.

3W LED emergency light Circuit

These days, many LED emergency lights are seen in the market. They are handy, because, they  are compact, easy to carry and give good 1-2 hr lighting Most of them can be charged using 5V mobile chargers. Ever wondered what is inside them! Here is a quick look.

The device has a 4V battery, A diode, few resister and a rotary switch. A very simple circuit!

Battery is connected to the +ve of the 5V DC Socket through a diode. The diode is connected to such that the battery +ve doesn't get the supply -ve by accident. The resistor is used to limit the current to the 'Charging' indication LED. A rotary switch with a bunch of registers is used to control the intensity of light

Simple, yet very useful!

Happy Reading!! 

Python, Working with Directories and Files: Basics

import os

#print the current working directory
print "CWD", os.getcwd()

#change the current working directory
os.chdir("C:\\temp")

print "CWD after changing ", os.getcwd()

#Given a directroy path, list all its contents.

""" listdir function is used for this purpose

the below line prints the list of contents in the
current working directory
"""

print os.listdir(os.getcwd())

#change CWD to some other directory and try
os.chdir("c:\\program files")

print os.listdir(os.getcwd())


#print only the file names
for f in os.listdir(os.getcwd()):
    if(os.path.isfile(f)):
        print "filename", f

#change to some other directory and try
os.chdir("C:\\program files\\7-zip")
for f in os.listdir(os.getcwd()):
    if(os.path.isfile(f)):
        print "filename", f


#print only directory names
os.chdir("c:\\program files")
for f in os.listdir(os.getcwd()):
    if(os.path.isdir(f)):
        print "Dir Name ", f


Back to Python Home Page

Python, Pickle Module

import pickle

#create a list, We are going to pickle this list and store it in disk
mylist = ['Apple', 'Banana', 'Orange', 'Pineapple']

#open a temp file in write mode
fh = open("C:\\Temp\\test.txt", 'w')

# dump the list l into file
pickle.dump(mylist, fh)

#close the file
fh.close()

#now if we open and see our file, we can see that python has saved this list
#but with some extra characters

""" Now, load the file back into the python data structure"""

#opne the file for reading

fh = open("C:\\Temp\\test.txt", 'r')

#create and empty list
mylist2 = []

#load the data from file to mylist2
mylist2 = pickle.load(fh)

print mylist2

""" We'll see that the out put is

['Apple', 'Banana', 'Orange', 'Pineapple']

In this way pickle can be used to save and load python data structure
to and from disk

Some of the uses of the pickling

1) saving a program's state data to disk so that it can carry on where it left off when restarted (persistence)

2) sending python data over a TCP connection in a multi-core or distributed system (marshalling)

3) storing python objects in a database

4) converting an arbitrary python object to a string so that it can be used as a dictionary key (e.g. for caching & memoization).

"""


Back to Python Home Page