Reading and Writing Files

13. Reading and Writing Files#

Files can generally be categorized into binary files and text files. Binary files consist of bit patterns and allow efficient data storage, often used in well-known formats like images, videos, or audio files. Such files usually require specialized tools or Python libraries for import. Text files, on the other hand, are readable with a standard text editor and include formats like .txt, .csv, .yml, or .py. These are the files we’ll primarily focus on.

13.1. Reading Files#

Files can be read in Python using the open() function. For example, consider a file named testfile.txt located in the same directory:

data = open("testfile.txt", "r")
print(data.readline())  # => 'Here is some sample text for testing.\n'

Here, the "r" argument specifies the mode in which the file is opened. The following table summarizes the most common modes:

Mode

Meaning

Explanation

r

Read mode, text

Cursor starts at the beginning of the file. Does not create new files.

rb

Read mode, binary

w

Write mode, text

Overwrites existing files or creates a new file.

wb

Write mode, binary

a

Append mode, text

Writes to the end of the file without overwriting the existing content.

ab

Append mode, binary

x

Exclusive creation, text

Fails if the file exists.

xb

Exclusive creation, binary

13.1.1. Reading Line by Line#

You can also read a file line by line:

data = open("testfile.txt", "r")
for line in data:
    print(line)

If the file isn’t in the same folder, you need to provide the file path.

  • Windows: Use "C:/User/my_folder/testfile.txt" or escape backslashes like "C:\\User\\my_folder\\testfile.txt".

  • macOS/Linux: Use "/home/user/my_folder/testfile.txt".

You can also use relative paths:

data = open("my_data/testfile2.txt", "r")

For advanced path handling, consider using libraries like os.

13.2. Writing Files#

To write to a file, use the write() method. First, open a file in write mode:

output = open("output_file.txt", "w")
text = "Now for something completely different."
output.write(text + "\n")
output.close()

With mode "w", a new file is created each time. To append to an existing file instead, use "a":

output = open("output_file.txt", "a")
output.write("Yes. Exactly.\n")
output.write("Yes " * 5 + "\n")
output.close()

13.2.1. Using with for File Management#

A common and recommended approach is to use the with statement for file handling. This ensures the file is properly closed, even if an error occurs.

with open("output_file.txt", "w") as file:
    for i in range(1, 6):
        file.write(f"Line {i} \n")

Reading a file with with:

with open("output_file.txt", "r") as file:
    for line in file:
        if "3" in line:
            print(line)

13.3. Encoding#

Computers don’t inherently understand letters or characters—only bytes, which are numbers from 0 to 255. To map these numbers to characters, encoding systems are used.

13.3.1. ASCII#

ASCII (American Standard Code for Information Interchange) can represent 256 characters. However, it lacks many special characters.

13.3.2. Unicode#

Unicode has become the standard, especially for the web (HTML 4.0), supporting over 100,000 characters. The most commonly used Unicode encoding in Python is UTF-8, as it efficiently represents characters while saving space.

For more on encoding, refer to the SelfHTML Wiki on character encoding.

13.3.3. Handling Encoding in Python#

If you encounter issues with special characters (e.g., umlauts), it’s often due to improper conversion between ASCII and UTF-8. To avoid such problems, explicitly specify the encoding:

with open("output_file.txt", "w", encoding="utf-8") as file:
    for i in range(1, 6):
        file.write(f"Line {i} \n")
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