Python - unpack arguments from list with star operator

We can use *-operator (star operator) to unpack the arguments out of a list or tuple. Here is an example with python built in range() function :

>>> mylist = [3, 10]
>>> mylist
[3, 10]
>>> range(mylist[0], mylist[1])
[3, 4, 5, 6, 7, 8, 9]
>>> range(*mylist)
[3, 4, 5, 6, 7, 8, 9]
>>> 

Python merge line with line above

Input file:

$ cat file.txt
500:120:100:X
:100:120
200:900:125
120:120
:900
120:345
12:900:1234:34
:90

Required: Join the lines which startswith : with the previous line.
i.e. required output is:

500:120:100:X:100:120
200:900:125
120:120:900
120:345
12:900:1234:34:90

The python script to achieve this:

data=open("file.txt").read().split("\n")
for i,line in enumerate(data):
    if line.startswith(":"):
        data[i-1]= data[i-1]+line
        data.pop(i)
print '\n'.join(data),

Executing it:

$ python merge-lines.py
500:120:100:X:100:120
200:900:125
120:120:900
120:345
12:900:1234:34:90

The solution using UNIX Awk can be found here

More about python enumerate function can be found here. Mentioned below is a small example on python enumerate function

>>> for i, student in enumerate(['Alex', 'Ryan', 'Deb']):
...     print i, student
...
0 Alex
1 Ryan
2 Deb
>>>

Related Posts:
- Print section of file using line number - Python
- Print line next to pattern using Python
- Print line above pattern using Python

New learning:
Python list pop method:
list.pop([i])
Remove the item at the given position in the list, and return it. If no index is specified, a.pop() removes and returns the last item in the list.

Python list append example - divide by two

Input file:

$ cat file.txt
h1|u|1
h2|5|1|1
rec1|1239400800|Sat|fan1|AX|2|10035|-|2|50
rec2|1239400800|Sat|fan1|AX|2|-|-|2|17
rec5|1239400801|Sat|fan3|AY|5|10035|-|2|217
rec8|1239400804|Sat|fan5|AX|2|5|-|2|970

Required Output:
- Lines starting with "h1" or "h2", no action required, just print.
- Lines starting with "rec", divide the values starting from 6th field by 2.

Required output is:

h1|u|1
h2|5|1|1
rec1|1239400800|Sat|fan1|AX|1|5017|-|1|25
rec2|1239400800|Sat|fan1|AX|1|-|-|1|8
rec5|1239400801|Sat|fan3|AY|2|5017|-|1|108
rec8|1239400804|Sat|fan5|AX|1|2|-|1|485

The python script:

fp = open("file.txt", "rU")
lines = fp.readlines()
fp.close()

for line in lines:
       if line.startswith("h1"):
               print line,
       if line.startswith("h2"):
               print line,
       if line.startswith("rec"):
               f=line.split("|")
               r = f[5:]
               l = []
               for each in r:
                       try:
                               l.append(str(int(each)/2))
                       except ValueError:
                               l.append(each)

               t = "|".join(f[0:5]) + "|" + "|".join(l)
               print t.rstrip()