Introduction to Working with CSV Files

CSV (Comma-Separated Values) files are a common format for storing tabular data. Python provides the csv module to read, write, and manipulate CSV files efficiently.

In this lesson, you’ll learn how to handle CSV files using Python’s built-in csv module.

What Are CSV Files?

CSV files are text files that use a delimiter (usually a comma) to separate values. Each line represents a row, and each value in the line represents a column.

Reading CSV Files

To read a CSV file, use the csv.reader function. This function reads each line as a list of values:

# Reading a CSV file
import csv

with open("data.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)  # Each row is a list of values

Writing CSV Files

To write to a CSV file, use the csv.writer function. This function allows you to write rows to a CSV file:

# Writing to a CSV file
import csv

with open("output.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerow(["Name", "Age", "City"])
    writer.writerow(["Alice", 30, "New York"])
    writer.writerow(["Bob", 25, "London"])

Reading CSV Files with Headers

Use csv.DictReader to read CSV files into dictionaries, where the keys are the column headers:

# Reading a CSV file with headers
import csv

with open("data.csv", "r") as file:
    reader = csv.DictReader(file)
    for row in reader:
        print(row)  # Each row is a dictionary with column headers as keys

Writing CSV Files with Headers

Use csv.DictWriter to write dictionaries into CSV files with headers:

# Writing a CSV file with headers
import csv

with open("output.csv", "w", newline="") as file:
    fieldnames = ["Name", "Age", "City"]
    writer = csv.DictWriter(file, fieldnames=fieldnames)

    writer.writeheader()  # Write the header row
    writer.writerow({"Name": "Alice", "Age": 30, "City": "New York"})
    writer.writerow({"Name": "Bob", "Age": 25, "City": "London"})

Custom Delimiters in CSV Files

You can specify a custom delimiter when reading or writing CSV files. For example, tab-delimited files use a tab (\t) as the delimiter:

# Using a custom delimiter
import csv

with open("data.tsv", "r") as file:
    reader = csv.reader(file, delimiter="\t")
    for row in reader:
        print(row)  # Each row is split by tab

Handling Large CSV Files

For large CSV files, use a loop to process rows incrementally, avoiding loading the entire file into memory:

# Reading a large CSV file efficiently
import csv

with open("large_data.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        # Process each row
        print(row)

Common Mistakes with CSV Files

Here are some mistakes to avoid:

• Forgetting to use newline="" when writing CSV files on Windows.
• Using incorrect delimiters for non-standard CSV files.
• Not handling missing or malformed data in the file.

What Did We Learn?

In this lesson, you learned:

• How to read and write CSV files using Python’s csv module.
• How to work with CSV files that have headers using DictReader and DictWriter.
• How to handle large CSV files efficiently.

Great job! Let’s move to the next topic.