Python - Reading from and writing to files

Reading From and Writing to Files in Python

In Python programming, reading from and writing to files are frequent operations. It entails starting a file, working with it, and then shutting it. Python has straightforward means for handling these chores effectively and neatly; the with statement is frequently used to ensure that files are closed correctly when their suite is completed.

Reading From a File

In programming, especially in Python, reading from a file refers to retrieving the contents of a file stored on a disk and retrieving that data for use in your program.

The following steps involve reading from files

1. Open the File: You use the open() function to specify the file name and the mode (e.g., read mode 'r').
2. Read the Contents: You can read the entire file at once, read it line by line, or read specific portions, depending on your needs.
3. Process the Data: Once you have the data, you can manipulate it, display it, or use it in calculations.
4. Close the File: It’s essential to close the file after reading to free up system resources. However, using a with statement automatically closes the file for you.

Let’s discuss code examples of reading from a file.

Using open() and read():

with open('file.txt', 'r') as file:
    content = file.read()
print(content)

Reading Line by Line:

with open('file.txt', 'r') as file:
    for line in file:
        print(line.strip()) # strip() removes the newline character

Reading All Lines into a List:

with open('file.txt', 'r') as file:
    lines = file.readlines()
print(lines) # This will print a list of lines

Writing to a File

In programming, especially in Python, file writing refers to the process of creating or modifying a file by appending it to data. This can include creating a new file, overwriting an existing file, or appending data to the end of an existing file.

The following steps involve Writing a File

1. Open the File: You use the open() function to specify the file name and the mode (e.g., write mode 'w' or append mode 'a').
2. Write the Data: You can write a string, a list of strings, or formatted data to the file.
3. Close the File: It’s important to close the file after writing to ensure that all data is saved properly and system resources are freed. Using a with statement automatically handles closing the file for you.

Let’s discuss code examples of writing to a file.

Using open() with write():

with open('file.txt', 'w') as file:
    file.write("Hello, World!\n")

Appending to a File:

with open('file.txt', 'a') as file:
    file.write("Appending a new line.\n")

Writing Multiple Lines:

lines = ["First line.\n", "Second line.\n", "Third line.\n"]
with open('file.txt', 'w') as file:
    file.writelines(lines)

Important Point

File Modes: When opening a file, the mode determines how the file can be used:

1. 'r': Read (default mode).
2. 'w': Write (truncates the file if it exists).
3. 'a': Append (adds to the end of the file).
4. 'r+': Read and write mode (allows both reading and writing without truncating the file).

Reading Methods:

1. file.read(): Reads the entire file.
2. file.readline(): Reads the next line.
3. file.readlines(): Reads all lines and returns them as a list.
4. file.write(string): Writes a string to the file. It does not automatically add a newline.
5. file.writelines(list): Writes a list of strings to the file.

Error Handling: It’s often a good practice to handle errors that might occur when trying to read a file, such as file not found errors.

Example

Let’s demonstrate the full example that reads from one file and writes the content to another:

# Read from 'input.txt' and write to 'output.txt'
try:
    with open('input.txt', 'r') as infile:
        content = infile.read()

    with open('output.txt', 'w') as outfile:
        outfile.write(content)
    print("Content copied successfully!")

except FileNotFoundError:
    print("The file was not found.")

except Exception as e:
    print(f"An error occurred: {e}")