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Module 11: Study Guide

Review key concepts from File Handling

Key Terms

File object: The Python object returned by open() that provides methods to read or write file data.

File mode: A string like "r", "w", "a" that tells Python how to open the file.

Context manager: An object used with the with statement that automatically manages setup and cleanup (like opening and closing files).

CSV: Comma-Separated Values - a plain text format for tabular data where commas separate values in each row.

Newline character: The invisible \n character that marks the end of a line in a text file.

Lesson 1: Reading Files

open(filename, "r") opens a file for reading
read() - entire file as one string
readline() - one line at a time
readlines() - all lines as a list
Iterating directly: for line in file: (most efficient)
Use strip() to remove trailing newline characters
FileNotFoundError occurs if the file does not exist

Lesson 2: Writing Files

"w" mode creates or overwrites a file (erases existing content)
"a" mode appends to the end (keeps existing content)
write() does NOT add newlines - you must include \n
writelines() writes a list of strings
"x" mode creates a file but raises an error if it already exists

Lesson 3: The with Statement

with open() as file: automatically closes the file when the block ends
Files are closed even if an error occurs inside the block
Variables created inside the block remain available after it ends
You can nest with statements for multiple files
The with statement is the recommended approach for all file operations

Lesson 4: CSV Data

csv.reader reads rows as lists (access by index)
csv.DictReader reads rows as dictionaries (access by column name)
csv.writer writes lists with writerow() and writerows()
csv.DictWriter writes dictionaries with writeheader() and writerows()
Use newline="" in open() when writing CSV files
CSV values are always strings - convert with int() or float() as needed
Use next(reader) to skip the header row with csv.reader

Review Questions

1. What is the difference between "w" and "a" file modes?

2. Why is the with statement preferred over manual open()/close()?

3. What is the difference between read(), readline(), and readlines()?

4. Why do we need newline="" when writing CSV files?

5. What is the advantage of DictReader over csv.reader?

6. What happens if you forget to close a file?

Common Mistakes to Avoid

Forgetting to add \n when writing lines
Using "w" mode when you meant to append (data loss!)
Forgetting to convert CSV values from strings to numbers
Not using strip() when reading lines (extra whitespace in output)
Forgetting newline="" when writing CSV on Windows
Trying to read a file that does not exist without handling the error
Not using the with statement (risk of unclosed files)