Python convert string to tuple & list

Let's check the use of python 'tuple' and 'list' in-built functions.

tuple([iterable])

It returns a 'tuple' whose items are the same and in the same order as iterable‘s items. iterable may be a sequence, a container that supports iteration, or an iterator object.

tuple('xyz') returns ('x', 'y', 'z') and tuple([1, 2, 3]) returns (1, 2, 3)

e.g.

$ cat file.txt
Python Prog
Readline
Programming

Now:

>>> for line in open("file.txt"):
...     t = tuple(line)
...     print t
...
('P', 'y', 't', 'h', 'o', 'n', ' ', 'P', 'r', 'o', 'g', '\n')
('R', 'e', 'a', 'd', 'l', 'i', 'n', 'e', '\n')
('P', 'r', 'o', 'g', 'r', 'a', 'm', 'm', 'i', 'n', 'g', '\n')
>>>

list([iterable])

It returns a list whose items are the same and in the same order as iterable‘s items. iterable may be either a sequence, a container that supports iteration, or an iterator object. If iterable is already a list, a copy is made and returned, similar to iterable[:]. For instance, list('xyz') returns ['x', 'y', 'z'] and list( (1, 2, 3) ) returns [1, 2, 3].

>>>
>>> for line in open("file.txt"):
...     l = list(line)
...     print l
...
['P', 'y', 't', 'h', 'o', 'n', ' ', 'P', 'r', 'o', 'g', '\n']
['R', 'e', 'a', 'd', 'l', 'i', 'n', 'e', '\n']
['P', 'r', 'o', 'g', 'r', 'a', 'm', 'm', 'i', 'n', 'g', '\n']
>>>

Split a file into sub files in python

Input file 'file.txt' is basically a log file containing running information of certain device interfaces in the following format:

$ cat file.txt
debug: on
max allowed connection: 3
tr#45
Starting: interface 78e23
Fan Status: On
Speed: -
sl no: 3431212-2323-90
vendor: aledaia
Stopping: interface 78e23
tr#90
newdebug received
Starting: interface 78e24
Fan Status: Off
Speed: 5670
sl no: 3431212-2323-90
vendor: aledaia
Stopping: interface 78e24
Starting: interface 68e73
Fan Status: On
Speed: 1200
sl no: 3431212-2323-90
vendor: aledaia
Stopping: interface 68e73
tr#99

Required:

Split the above file into sub-files such that
- Each sub file conatins information of an interface (basically information from 'Starting' and 'Stopping' of the interface)
- Sub-file name should be of the format: interface-name_someSLno.txt

The python script:

flag=0;c=0
for line in open("file.txt"):
    line=line.strip()
    if line.startswith("Stopping"):
        flag=0
        o.close()
    if line.startswith("Starting"):
        interface=line.split(" ")[2]
        flag=1;c=c+1
        o=open(interface+"_"+str(c)+".txt","w")
    if flag and not line.startswith("Starting"):
        print >>o, line

Output:

$ cat 78e23_1.txt
Fan Status: On
Speed: -
sl no: 3431212-2323-90
vendor: aledaia

$ cat 78e24_2.txt
Fan Status: Off
Speed: 5670
sl no: 3431212-2323-90
vendor: aledaia

$ cat 68e73_3.txt
Fan Status: On
Speed: 1200
sl no: 3431212-2323-90
vendor: aledaia

Python - print last few characters

Input file:

$ cat file.txt
sldadop233masdsa213313131ada121
sltadop233masdsa813313133cso128
slyadop233masdsa11331313Kada134
slqadop233masdsa31331313tada162

Required: Print last 6 characters of each line of the above input file.

The python script:

$ cat extract-last.py
import sys
for line in sys.stdin:
    print '%s' % (line[-7:-1])

Executing it:

$ python extract-last.py < file.txt
ada121
cso128
ada134
ada162

Things to learn:
- How to read a file in python from stdin

Other alternatives in UNIX are:

#Using bash parameter substitution
$ while read line ; do echo ${line: -6}; done < file.txt

#Since all lines are of fixed length, we can use 'cut' command
$ cut -c26-31 file.txt

#Using sed
$ sed 's/^.*\(......\)$/\1/' file.txt

#Using awk
$ awk '{ print substr( $0, length($0) - 5, length($0) ) }'  file.txt