Python: How to loop through blocks of lines

How to go through blocks of lines separated by an empty line? The file looks like the following:

ID: 1
Name: X
FamilyN: Y
Age: 20

ID: 2
Name: H
FamilyN: F
Age: 23

ID: 3
Name: S
FamilyN: Y
Age: 13

ID: 4
Name: M
FamilyN: Z
Age: 25

I want to loop through the blocks and grab the fields Name, Family name and Age in a list of 3 columns:

Y X 20
F H 23
Y S 13
Z M 25

Here's another way, using itertools.groupby. The function groupy iterates through lines of the file and calls isa_group_separator(line) for each line. isa_group_separator returns either True or False (called the key), and itertools.groupby then groups all the consecutive lines that yielded the same True or False result.

This is a very convenient way to collect lines into groups.

import itertools

def isa_group_separator(line):
    return line=='\n'

with open('data_file') as f:
    for key,group in itertools.groupby(f,isa_group_separator):
        # print(key,list(group))  # uncomment to see what itertools.groupby does.
        if not key:
            data={}
            for item in group:
                field,value=item.split(':')
                value=value.strip()
                data[field]=value
            print('{FamilyN} {Name} {Age}'.format(**data))

# Y X 20
# F H 23
# Y S 13
# Z M 25

import re
result = re.findall(
    r"""(?mx)           # multiline, verbose regex
    ^ID:.*\s*           # Match ID: and anything else on that line 
    Name:\s*(.*)\s*     # Match name, capture all characters on this line
    FamilyN:\s*(.*)\s*  # etc. for family name
    Age:\s*(.*)$        # and age""", 
    subject)

Result will then be

[('X', 'Y', '20'), ('H', 'F', '23'), ('S', 'Y', '13'), ('M', 'Z', '25')]

which can be trivially changed into whatever string representation you want.

Use a generator.

def blocks( iterable ):
    accumulator= []
    for line in iterable:
        if start_pattern( line ):
            if accumulator:
                yield accumulator
                accumulator= []
        # elif other significant patterns
        else:
            accumulator.append( line )
     if accumulator:
         yield accumulator

If your file is too large to read into memory all at once, you can still use a regular expressions based solution by using a memory mapped file, with the mmap module:

import sys
import re
import os
import mmap

block_expr = re.compile('ID:.*?\nAge: \d+', re.DOTALL)

filepath = sys.argv[1]
fp = open(filepath)
contents = mmap.mmap(fp.fileno(), os.stat(filepath).st_size, access=mmap.ACCESS_READ)

for block_match in block_expr.finditer(contents):
    print block_match.group()

The mmap trick will provide a "pretend string" to make regular expressions work on the file without having to read it all into one large string. And the find_iter() method of the regular expression object will yield matches without creating an entire list of all matches at once (which findall() does).

I do think this solution is overkill for this use case however (still: it's a nice trick to know...)

import itertools

# Assuming input in file input.txt
data = open('input.txt').readlines()

records = (lines for valid, lines in itertools.groupby(data, lambda l : l != '\n') if valid)    
output = [tuple(field.split(':')[1].strip() for field in itertools.islice(record, 1, None)) for record in records]

# You can change output to generator by    
output = (tuple(field.split(':')[1].strip() for field in itertools.islice(record, 1, None)) for record in records)

# output = [('X', 'Y', '20'), ('H', 'F', '23'), ('S', 'Y', '13'), ('M', 'Z', '25')]    
#You can iterate and change the order of elements in the way you want    
# [(elem[1], elem[0], elem[2]) for elem in output] as required in your output

Python: How to loop through blocks of lines

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