File system interaction basics in Python

Introduction

We use file system interaction to save, read, and manage data on your computer. It helps programs remember information even after they stop running.

Saving a list of your favorite movies to a file.

Reading a text file with instructions or data.

Creating a new folder to organize your photos.

Checking if a file exists before opening it.

Deleting old files you no longer need.

Syntax

Python

open('filename', 'mode')

# Modes:
# 'r' - read
# 'w' - write (overwrite)
# 'a' - append (add to end)
# 'x' - create new file
# 'b' - binary mode
# 't' - text mode (default)

Always close the file after using it, or use with to handle it automatically.

File modes control how you interact with the file (read, write, append).

Examples

Open a file named 'notes.txt' to read its content safely.

Python

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

Add a new line to the end of 'log.txt' without erasing existing content.

Python

with open('log.txt', 'a') as file:
    file.write('New entry\n')

Open 'data.txt' to write text, then close the file to save changes.

Python

file = open('data.txt', 'w')
file.write('Hello world!')
file.close()

Sample Program

This program creates a file called 'example.txt', writes two lines of text, then reads and prints the content.

Python

filename = 'example.txt'

# Write some text to the file
with open(filename, 'w') as file:
    file.write('Hello, file system!\n')
    file.write('This is a test file.')

# Read the text back from the file
with open(filename, 'r') as file:
    content = file.read()

print('File content:')
print(content)

Important Notes

Using with open(...) is safer because it closes the file automatically.

Writing with mode 'w' erases the old content, so be careful.

Reading a file that does not exist will cause an error.

Summary

Files let your program save and load information on your computer.

Use open() with modes like 'r', 'w', and 'a' to read, write, or add to files.

Always close files or use with to avoid problems.

Practice

1. What does the mode 'r' mean when opening a file with open() in Python?

easy

A. Open the file for reading only

B. Open the file for writing only

C. Open the file for appending data

D. Create a new file or overwrite existing

Solution

1. Step 1: Understand file modes in Python

   The mode 'r' stands for reading the file only, meaning you can read data but not change it.

2. Step 2: Compare with other modes

   Modes like 'w' are for writing (which overwrites), and 'a' is for appending. 'r' does not allow writing.

3. Final Answer:

   Open the file for reading only -> Option A

4. Quick Check:

   Mode 'r' = read only [OK]

Hint: Remember 'r' means read, 'w' means write, 'a' means append [OK]

Common Mistakes:

• Confusing 'r' with 'w' or 'a'
• Thinking 'r' creates a new file
• Trying to write to a file opened with 'r'

2. Which of the following is the correct syntax to open a file named 'data.txt' for writing in Python?

easy

A. open('data.txt', 'r')

B. open('data.txt', 'w')

C. open('data.txt', 'rw')

D. open('data.txt', 'a+')

Solution

1. Step 1: Identify the mode for writing

   The mode 'w' opens a file for writing and creates it if it doesn't exist or overwrites if it does.

2. Step 2: Check syntax correctness

   open('data.txt', 'w') is the correct syntax. 'r' is for reading, 'rw' is invalid, 'a+' is for appending and reading.

3. Final Answer:

   open('data.txt', 'w') -> Option B

4. Quick Check:

   Write mode = 'w' [OK]

Hint: Use 'w' to write or overwrite files [OK]

Common Mistakes:

• Using 'r' when intending to write
• Using invalid mode 'rw'
• Confusing 'a+' with 'w'

3. What will be the output of this code?

with open('test.txt', 'w') as f:
    f.write('Hello')

with open('test.txt', 'a') as f:
    f.write(' World')

with open('test.txt', 'r') as f:
    print(f.read())

medium

A. Error: file not found

B. Hello

C. Hello World

D. World

Solution

1. Step 1: Write 'Hello' to the file

   The first block opens 'test.txt' in write mode, which creates or clears the file, then writes 'Hello'.

2. Step 2: Append ' World' to the file

   The second block opens the file in append mode and adds ' World' after 'Hello'.

3. Step 3: Read and print the file content

   The last block reads the full content, which is 'Hello World', and prints it.

4. Final Answer:

   Hello World -> Option C

5. Quick Check:

   Write + append = 'Hello World' [OK]

Hint: Write clears file, append adds to end [OK]

Common Mistakes:

• Expecting append to overwrite
• Not closing files before reading
• Confusing write and append modes

4. What is wrong with this code snippet?

f = open('log.txt', 'r')
print(f.read())
f.write('New entry')
f.close()

medium

A. File is opened in read mode but write is attempted

B. File is not closed properly

C. Missing mode argument in open()

D. File path is incorrect

Solution

1. Step 1: Check file mode and operations

   The file is opened with mode 'r' which allows reading only.

2. Step 2: Identify invalid operation

   Calling f.write() on a file opened in read mode causes an error because writing is not allowed.

3. Final Answer:

   File is opened in read mode but write is attempted -> Option A

4. Quick Check:

   Write not allowed in 'r' mode [OK]

Hint: Don't write to files opened with 'r' mode [OK]

Common Mistakes:

• Trying to write without 'w' or 'a' mode
• Forgetting to close files
• Assuming 'r' mode allows writing

5. You want to read a file line by line and print only lines that contain the word 'error'. Which is the best way to do this in Python?

hard

A. open('log.txt', 'r').read().split('error')

B. f = open('log.txt', 'r')
lines = f.readlines()
for line in lines:
if line == 'error':
print(line)
f.close()

C. with open('log.txt', 'w') as f:
for line in f:
if 'error' in line:
print(line)

D. with open('log.txt', 'r') as f:
for line in f:
if 'error' in line:
print(line.strip())

Solution

1. Step 1: Use 'with' and read line by line

   with open('log.txt', 'r') as f: for line in f: if 'error' in line: print(line.strip()) uses 'with' to open the file safely and iterates line by line, which is memory efficient.

2. Step 2: Check condition and print matching lines

   It checks if 'error' is in each line and prints the line without extra spaces using strip().

3. Final Answer:

   with open('log.txt', 'r') as f: for line in f: if 'error' in line: print(line.strip()) -> Option D

4. Quick Check:

   Use 'with' + for line in file + condition [OK]

Hint: Use 'with' and loop lines to filter content [OK]

Common Mistakes:

• Opening file in 'w' mode when reading
• Comparing whole line to 'error' instead of substring
• Not closing file properly
• Using split incorrectly for line filtering