A lookup file operation using python

Details of input files:

details.txt contains the details of some students. The details of a student starts with its #ID,Name,Class,Year ans Status
idlist.txt contains the IDs of the students who has passed the exam.

$ cat idlist.txt
ID55
ID12
ID90

$ cat details.txt
#ID10
Name:Mr A
Class:IX
Year:1985
Satus=Nill
#ID12
Name:Mr B
Class:X
Year:1987
#ID10
Name:Mr X
Class:X
Year:1983
#ID90
Name:Mr Y
Class:IX
Year:1984
#ID55
Name:Mr Z
Class:X
Year:1985

Required: Pull out the details of the students(from details.txt) who has passed the exam(whose ID is present in idlist.txt).

idlist = open("idlist.txt").readlines()
idlist = [i.strip() for i in idlist]
detailslist = open("details.txt").readlines()
flag = 0
fp = open("filter.out", "w")
for id in idlist:
        for lines in detailslist:
                if lines.startswith("#") and id in lines:
                        flag = 1
                if lines.startswith("#") and not id in lines:
                        flag = 0
                if flag:
                        fp.write(lines)
fp.close

Executing the filter.py:

$ python filter.py
$ cat filter.out
#ID55
Name:Mr Z
Class:X
Year:1985
#ID12
Name:Mr B
Class:X
Year:1987
#ID90
Name:Mr Y
Class:IX
Year:1984

So filter.out contains the required output.

Related concepts and functions:

>>> idlist = open("idlist.txt").readlines()
>>> idlist
['ID55\n', 'ID12\n', 'ID90\n']
>>> idlist = [i.strip() for i in idlist]
>>> idlist
['ID55', 'ID12', 'ID90']