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
>>>

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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.