intermediatePython Framework • Core Concepts

File Handling

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Opening Files & Modes

Use `open(filename, mode)` to access files. Common modes: 'r' (read), 'w' (write, overwrites!), 'a' (append), 'x' (create). Add 'b' for binary files (e.g., 'rb' for images).

Python
# Read mode (default)
f = open("data.txt", "r")

# Write mode (careful, clears file!)
f = open("output.txt", "w")

# Append mode (adds to end)
f = open("log.txt", "a")

File Modes

Start
↓
if
Select Mode
↓
'r': Read Only
↓
'w': Overwrite
↓
'a': Append
Start/End
Condition
Action

Reading Data

Use `.read()` for the whole file, `.readline()` for a single line, or `.readlines()` for a list of lines. Iterating over the file object is memory efficient.

Python
with open("story.txt", "r") as f:
    # Read entire content
    # content = f.read()
    
    # Best practice: Iterate line by line
    for line in f:
        print(line.strip())  # strip() removes \n

Writing & Appending

Use `.write()` to save string data. It does NOT add a newline automatically, so add `\n` manually. `.writelines()` writes a list of strings.

Python
lines = ["Entry 1\n", "Entry 2\n"]

# 'w' clears existing content!
with open("notes.txt", "w") as f:
    f.write("Title\n")
    f.writelines(lines)

# 'a' adds to the end
with open("notes.txt", "a") as f:
    f.write("Entry 3\n")

The 'with' Statement

Always use `with` (context manager) to open files. It acts as a safety net that automatically closes the file, even if your code crashes inside the block.

Python
try:
    with open("data.txt", "r") as f:
        data = f.read()
        # Auto-closed after this block
except FileNotFoundError:
    print("File missing!")

# No need for f.close() here

With Statement Magic

Start
↓
Enter 'with'
↓
Open File
↓
Execute Block
↓
Auto Close
Start/End
Condition
Action

Paths & Error Handling

Use `os.path` for handling file paths universally. Wrap operations in `try-except` to handle missing files (`FileNotFoundError`) or permission issues (`PermissionError`).

Python
import os

filename = "config.ini"

if os.path.exists(filename):
    try:
        with open(filename, "r") as f:
            print(f.read())
    except PermissionError:
        print("Access denied!")
else:
    print("File not found.")

Real-World: Logging

Applications use files to store logs. Appending ('a') is key here so you don't lose old history.

Python
import datetime

def log_event(message):
    timestamp = datetime.datetime.now()
    entry = f"[{timestamp}] {message}\n"
    
    with open("app.log", "a") as log_file:
        log_file.write(entry)

log_event("User logged in")
log_event("Data export failed")

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Visual Lab

File Lifecycle

Start
↓
open(file, mode)
↓
if
Mode?
'r'
↓
Read (r)
↓
file.close()
'w', 'a'
↓
Write (w/a)
↓
file.close()
Start/End
Condition
Action

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