To read only one line instead of multiple lines in a Python file, use the method .readline() on a file object that is returned from the open() function. Every subsequent .readline() will extract the next line in the file if it exists.

with open('story.txt') as story_object:
  print(story_object.readline())

will print only the first line in story.txt.

Writing to an opened file with the 'w' flag overwrites all previous content in the file. To avoid this, we can append to a file instead. Use the 'a' flag as the second argument to open(). If a file doesn’t exist, it will be created for append mode.

with open('shopping.txt', 'a') as shop:
  shop.write('Tomatoes, cucumbers, celery\n')

By default, a file when opened with open() is only for reading. A second argument 'r' is passed to it by default. To write to a file, first open the file with write permission via the 'w' argument. Then use the .write() method to write to the file. If the file already exists, all prior content will be overwritten.

with open('diary.txt','w') as diary:
  diary.write('Special events for today')

Instead of reading the entire content of a file, you can read a single line at a time. Instead of .read() which returns a string, call .readlines() to return a list of strings, each representing an individual line in the file. Calling this code:

with open('lines.txt') as file_object:
  file_data = file_object.readlines()
print(file_data)

returns a list of strings in file_data:

['1. Learn Python.\n', '2. Work hard.\n', '3. Graduate.']

Iterating over the list, file_data, and printing it:

for line in file_data:
  print(line)

outputs:

1. Learn Python.

2. Work hard.

3. Graduate.

In Python, the csv module implements classes to read and write tabular data in CSV format. It has a class DictWriter which operates like a regular writer but maps a dictionary onto output rows. The keys of the dictionary are column names while values are actual data.

The csv.DictWriter constructor takes two arguments. The first is the open file handler that the CSV is being written to. The second named parameter, fieldnames, is a list of field names that the CSV is going to handle.

After a file is opened with open() returning a file object, call the .read() method of the file object to return the entire file content as a Python string. Executing the following Python code:

with open('mystery.txt') as text_file:
  text_data = text_file.read()
print(text_data)

will produce a string containing the entire content of the read file:

Mystery solved. 
Congratulations!