Construct code to perform file-processing operations

What is a File?

A file is a named collection of data stored on secondary storage (e.g., hard drive, SSD). Files allow data to persist after a program ends — unlike variables in RAM, which are lost when the program exits.

File Paths

Term	Meaning
Absolute path	Full path from the root of the file system — e.g., C:\Users\Michael\data.txt
Relative path	Path relative to the current working directory — e.g., data/sample.txt

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File Modes

Before opening a file, you must specify the mode — what operation you intend to perform.

Mode	Character	Description	Creates file?	Overwrites?
Read	r	Read from file (default)	❌ No	❌ No
Write	w	Write to file	✅ Yes	✅ Yes (truncates)
Append	a	Append to end of file	✅ Yes	❌ No
Read+Write	r+	Read and write	❌ No	❌ No
Write+Read	w+	Write and read	✅ Yes	✅ Yes
Append+Read	a+	Append and read	✅ Yes	❌ No

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Opening and Closing Files

The Old Way (Manual)

file = open("data.txt", "r")
content = file.read()
file.close()

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The with Statement (Context Manager) — Recommended

The with statement automatically closes the file when the block exits, even if an exception occurs:

with open("data.txt", "r") as file:
    content = file.read()
# File is automatically closed here

with open("data.txt", "w") as file:
    file.write("Hello, World!")
# File is automatically closed here

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Reading Files

Read Entire File as a String

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

Read Specific Number of Characters

with open("data.txt", "r") as file:
    first_10 = file.read(10)   # Read first 10 characters

Read One Line at a Time

with open("data.txt", "r") as file:
    line1 = file.readline()    # Read the first line (includes '\n')
    line2 = file.readline()    # Read the second line

Read All Lines into a List

with open("data.txt", "r") as file:
    lines = file.readlines()   # Returns a list of all lines

for line in lines:
    print(line, end="")

Iterate Line by Line (Memory Efficient)

with open("data.txt", "r") as file:
    for line in file:
        print(line, end="")

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Writing Files

Write a String (Overwrites)

with open("output.txt", "w") as file:
    file.write("First line\n")
    file.write("Second line\n")

Write Multiple Lines (writelines)

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]

with open("output.txt", "w") as file:
    file.writelines(lines)

Append to a File

with open("output.txt", "a") as file:
    file.write("Appended line\n")

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Common Exceptions in File Processing

Exception	Cause
FileNotFoundError	Trying to read a file that doesn’t exist
PermissionError	No permission to read/write the file
IsADirectoryError	Path points to a directory, not a file
UnicodeDecodeError	File cannot be decoded with the given encoding

Handling File Exceptions

try:
    with open("missing.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("File does not exist.")
except PermissionError:
    print("You do not have permission to access this file.")
else:
    print("File read successfully.")

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Best Practices Summary

Practice	Why it matters
Always use with statement	Guarantees file closure, even on exceptions
Use encoding="utf-8"	Handles international characters correctly
Check if file exists before reading	Prevents FileNotFoundError crashes
Avoid mode w when preserving data	Use a to append, not overwrite