Searching for a sub string in a String

It is usual to check for an item in a list, tuple, or dict using 'in' operator.

List=[ 'l' , 'i' , 's' , 't']

if 's' in List:

    print "success"

for string we usually go with 'find' function. Its quite easier to go with 'in' operator for string too.

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Fancy Formatting using Python

Its quite easy in python to pretty print the output...
Here are two ways to write a table of squares and cubes:
for x in range(1, 6):
...     print repr(x).rjust(2), repr(x*x).rjust(3),repr(x*x*x).rjust(4)
...
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125

>>> for x in range(1,6):
...     print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)
...
 1   1    1
 2   4    8
 3   9   27
 4  16   64
 5  25  125
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similar to rjust function, there are ljsut and center, which justifies the string with spaces (if filler is not specified)

>>> print 'PYTHON'.center(15)
     PYTHON  
>>>
>>> print 'PYTHON'.center(15, '*' )
*****PYTHON****
_____________________________________________________________________________________________

We can use the brackets ( {} ) to format the data.
The brackets and characters within them (called format fields) are replaced  with the objects passed
into the str.format() method.
A number in the brackets refers to the position of the object passed into the str.format() method.

>>> print 'The story of {0}, {1}, and {other}' .format('Bill', 'Manfred', other='Georg')
The story of Bill, Manfred, and Georg
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An optional ':' and format specifier can follow the field name. This allows greater control over how the value is formatted. The following example rounds Pi to three places after the decimal.

>>> import math
>>> print 'The value of PI is approximately {0:.3f}.'.format(math.pi)
The value of PI is approximately 3.142.


Passing an integer after the ':' will cause that field to be a minimum number of characters wide. This is useful for making tables pretty.

>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
...             print '{0:10} ==> {1:10d}'.format(name, phone)
...
Jack        ==>       4098
Dcab       ==>       7678
Sjoerd     ==>       4127

___________________________________________________________________________

The % operator can also be used for string formatting in place of str.format() . but it is old fashion. this old style of formatting will eventually be removed from the language. 

>>> import math

>>> print 'The value of PI is approximately %.3f' % math.pi

The value of PI is approximately 3.142

Ref: Python v2.7 Documentation

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Some Optimization Tips/Tricks in Python

Looping:
Use xrange for looping across long ranges; it uses much less memory than range, and may save time as well. Both versions are likely to be faster than a while loop:


xrange is a generator. The performance improvement from the use of generators is the result of the lazy generation of values, means values are generated on demand. Furthermore, we do not need to wait until all the elements have been generated before we start to use them.

You can often eliminate a loop by calling map instead.

Strings:

Building up strings with the concatenation operator + can be slow, because it often involves copying strings several times. Formatting using the % operator is generally faster, and uses less memory.

For example:


If you are building up a string with an unknown number of components, consider using string.join to combine them all, instead of concatenating them as you go:


Sample code :



Here are the results for above codes:

	range	xrange	list_range	list_xrange
real	0m4.136s	0m2.863s	0m1.867s	0m1.569s
user	0m3.960s	0m2.804s	0m1.732s	0m1.516s
system	0m0.160s	0m0.048s	0m0.124s	0m0.048s

File Operation: (Ext to my previous post)

# Each call to a file’s readline method is quite slow:


# It is much faster to read the entire file into memory by calling readlines; however, this uses up a lot of RAM.


# Another approach is to read blocks of lines.


#Best of all is to use the xreadlines method of a file:


Sample Codes:



Here are the results for above codes:

	readline	readlines	readblock	xreadlines
real	0m9.948s	0m13.037s	0m9.880s	0m9.574s
user	0m7.144s	0m4.316s	0m2.716s	0m2.372s
system	0m0.332s	0m1.092s	0m0.404s	0m0.328s

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Best way to open files in Python

C like syntax is as follows,

f = open("file", 'r')
line = f.readline( )
print line
close(f)

some one may forget to close f. this leads to leak, which is considerable in large scale programs. in order to avoid that we  can make use of 'with' or 'for' keywords.


with open("file", 'r') as f:
        line = f.readline()
        print line
or

for line in open("file", 'r'):
       print line


in these scenarios, file will get closed when f is out of scope.

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