Find a specific header in a CSV file using python 3 code

Use the csv module to split your data into columns. Use the csv.DictReader() object to make it easier to select a column by the header:

import csv

source = r'C:\Users\jk\Desktop\helloworld.csv'
dest = 'test.csv'

with open(source, newline='') as inf, open(dest, 'w', newline='') as outf:
    reader = csv.DictReader(inf)
    writer = csv.DictWriter(outf, fieldnames=reader.fieldnames)
    for row in reader:
        words = row['Keyword'].split()
        row['Keyword'] = words[0]
        writer.writerow(row)
        writer.writerows({'Keyword': w} for w in words[1:])

The DictReader() will read the first row from your file and use it as the keys for the dictionaries produced for each row; so a row looks like:

{'Keyword': 'Lions Tigers Bears', 'Source': 'US', 'Number': '3'}

Now you can address each column individually, and update the dictionary with just the first word of the Keyword column before producing additional rows for the remaining words.

I'm assuming here that your files are comma separated. If a different delimiter is needed, then set the delimiter argument to that character:

reader = csv.DictReader(inf, delimiter='\t')

for a tab-separated format. See the module documentation for the various options, including pre-defined format combinations called dialects.

Demo:

>>> import sys
>>> import csv
>>> from io import StringIO
>>> sample = StringIO('''\
... Keyword,Source,Number
... Lions Tigers Bears,US,3
... Dogs Zebra,Canada,5
... Sharks Guppies,US,2
... ''')
>>> output = StringIO()
>>> reader = csv.DictReader(sample)
>>> writer = csv.DictWriter(output, fieldnames=reader.fieldnames)
>>> for row in reader:
...     words = row['Keyword'].split()
...     row['Keyword'] = words[0]
...     writer.writerow(row)
...     writer.writerows({'Keyword': w} for w in words[1:])
... 
12
15
13
>>> print(output.getvalue())
Lions,US,3
Tigers,,
Bears,,
Dogs,Canada,5
Zebras,,
Sharks,US,2
Guppies,,