Chapter 6 Parsing files line by line

6.1 Learning objectives

Understand how to read files in Python
Learn how to iterate through lines in a file using a for loop

6.2 Create a Text File

Let’s create a simple text file called sample.txt using the Jupyter text editor

The file should contain the following lines:
```
apple
banana
cherry
date
```

6.3 Open A File Stream

A file stream is like a pipeline that lets you read data from a file one piece at a time. The most common way to open a file is using the open() function.

Create a new Python script called 03-parse-text-file.py

#!/usr/bin/env python3

import sys

my_file = open( sys.argv[1] )
print(my_file)

Save it and make it executable
```
chmod +x 03-parse-text-file.py
```

Run the script with the file name as an argument

./03-parse-text-file.py sample.txt

This will print something like

<_io.TextIOWrapper name='sample1.txt' mode='r' encoding='UTF-8'>

As you can see, the print() function can’t print the file content directly. This output just indicates the file sample1.txt is opened in read mode ('r') with UTF-8 encoding.

6.4 Add a `for` loop

To read the file and print each line, we can use a for loop.

Update 03-parse-text-file.py:

#!/usr/bin/env python3

import sys

my_file = open(sys.argv[1])

# for iterates through files
for my_line in my_file:
    # objects have methods
    my_line = my_line.rstrip("\n")
    print( my_line )

my_file.close()

We use my_file.close() to close the file after we’re done reading it. This is important to free up system resources.

Save it and run the script again
```
./03-parse-text-file.py sample1.txt
```
This will print each line of the file without extra spaces or newlines
```
apple
banana
cherry
date
```

6.5 Summary

Congratulations! You have just:

Created a Python script that reads a file
Used a for loop to iterate through each line in the file
Printed each line without extra spaces or newlines