print first paragraph in python

I have a book in a text file and I need to print first paragraph of each section. I thought that if I found a text between \n\n and \n I can find my answer. Here is my codes and it didn’t work. Can you tell me that where am I wrong ?

lines = [line.rstrip('\n') for line in open('G:\\aa.txt')]

check = -1
first = 0
last = 0

for i in range(len(lines)):
    if lines[i] == "": 
            if lines[i+1]=="":
                check = 1
                first = i +2
    if i+2< len(lines):
        if lines[i+2] == "" and check == 1:
            last = i+2
while (first < last):
    print(lines[first])
    first = first + 1

Also I found a code in stackoverflow I tried it too but it just printed an empty array.

f = open("G:\\aa.txt").readlines()
flag=False
for line in f:
        if line.startswith('\n\n'):
            flag=False
        if flag:
            print(line)
        elif line.strip().endswith('\n'):
            flag=True

I shared a sample section of this book in belown.

I

THE LAY OF THE LAND

There is a vast field of fascinating human interest, lying only just outside our doors, which as yet has been but little explored. It is the Field of Animal Intelligence.

Of all the kinds of interest attaching to the study of the world’s wild animals, there are none that surpass the study of their minds, their morals, and the acts that they perform as the results of their mental processes.

II

WILD ANIMAL TEMPERAMENT & INDIVIDUALITY

What I am trying to do here is, find the uppercase lines, and put them all in an array. Then, using the index method, I will find the first and last paragraphs of each section by comparing the indexes of these elements of this array I created.

Output should be like this :

There is a vast field of fascinating human interest, lying only just outside our doors, which as yet has been but little explored. It is the Field of Animal Intelligence.

What I am trying to do here is, find the uppercase lines, and put them all in an array. Then, using the index method, I will find the first and last paragraphs of each section by comparing the indexes of these elements of this array I created.

Best answer

If you want to group the sections you can use itertools.groupby using empty lines as the delimiters:

from itertools import groupby
with open("in.txt") as f:
    for k, sec in groupby(f,key=lambda x: bool(x.strip())):
        if k:
            print(list(sec))

With some more itertools foo we can get the sections using the uppercase title as the delimiter:

from itertools import groupby, takewhile

with open("in.txt") as f:
    grps = groupby(f,key=lambda x: x.isupper())
    for k, sec in grps:
        # if we hit a title line
        if k: 
            # pull all paragraphs
            v = next(grps)[1]
            # skip two empty lines after title
            next(v,""), next(v,"")

            # take all lines up to next empty line/second paragraph
            print(list(takewhile(lambda x: bool(x.strip()), v)))

Which would give you:

['There is a vast field of fascinating human interest, lying only just outside our doors, which as yet has been but little explored. It is the Field of Animal Intelligence.\n']
['What I am trying to do here is, find the uppercase lines, and put them all in an array. Then, using the index method, I will find the first and last paragraphs of each section by comparing the indexes of these elements of this array I created.']

The start of each section has an all uppercase title so once we hit that we know there are two empty lines then the first paragraph and the pattern repeats.

To break it into using loops:

from itertools import groupby  
from itertools import groupby
def parse_sec(bk):
    with open(bk) as f:
        grps = groupby(f, key=lambda x: bool(x.isupper()))
        for k, sec in grps:
            if k:
                print("First paragraph from section titled :{}".format(next(sec).rstrip()))
                v = next(grps)[1]
                next(v, ""),next(v,"")
                for line in v:
                    if not line.strip():
                        break
                    print(line)

For your text:

In [11]: cat -E in.txt

THE LAY OF THE LAND$
$
$
There is a vast field of fascinating human interest, lying only just outside our doors, which as yet has been but little explored. It is the Field of Animal Intelligence.$
$
Of all the kinds of interest attaching to the study of the world's wild animals, there are none that surpass the study of their minds, their morals, and the acts that they perform as the results of their mental processes.$
$
$
WILD ANIMAL TEMPERAMENT & INDIVIDUALITY$
$
$
What I am trying to do here is, find the uppercase lines, and put them all in an array. Then, using the index method, I will find the first and last paragraphs of each section by comparing the indexes of these elements of this array I created.

The dollar signs are the new lines, the output is:

In [12]: parse_sec("in.txt")
First paragraph from section titled :THE LAY OF THE LAND
There is a vast field of fascinating human interest, lying only just outside our doors, which as yet has been but little explored. It is the Field of Animal Intelligence.

First paragraph from section titled :WILD ANIMAL TEMPERAMENT & INDIVIDUALITY
What I am trying to do here is, find the uppercase lines, and put them all in an array. Then, using the index method, I will find the first and last paragraphs of each section by comparing the indexes of these elements of this array I created.