String Manipulation and File Handling

String Manipulation and File Handling

This note covers essential string operations and file handling techniques in Python that appear frequently in the AQA exam.

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String Manipulation

A string is a sequence of characters. In Python, strings are immutable (they cannot be changed in place — operations return a new string).

Accessing Characters

Each character has an index starting from 0:

word = "COMPUTER"
# Index:  01234567
print(word[0])      # C
print(word[3])      # P
print(word[-1])     # R (last character)

String Length

word = "Hello"
print(len(word))    # 5

Slicing (Substrings)

Slicing extracts part of a string using [start:stop]. The stop index is not included.

word = "COMPUTER"
print(word[0:3])    # COM
print(word[3:6])    # PUT
print(word[:4])     # COMP (from start)
print(word[4:])     # UTER (to end)

Common String Methods

Method	Purpose	Example	Result
.upper()	Convert to uppercase	"hello".upper()	"HELLO"
.lower()	Convert to lowercase	"HELLO".lower()	"hello"
.strip()	Remove leading/trailing whitespace	" hi ".strip()	"hi"
.find(x)	Find index of substring	"hello".find("ll")	2
.replace(a,b)	Replace occurrences	"cat".replace("c","b")	"bat"
.count(x)	Count occurrences	"hello".count("l")	2
.split(x)	Split into list	"a,b,c".split(",")	["a","b","c"]

Concatenation and Conversion

first = "Bright"
last = "Revision"
full = first + " " + last    # "Bright Revision"

age = 16
message = "Age: " + str(age)  # Must convert int to str

Traversing a String

word = "Hello"
for char in word:
    print(char)

# Or by index:
for i in range(len(word)):
    print(word[i])

ASCII Conversion

print(ord("A"))     # 65 — character to ASCII code
print(chr(65))      # A  — ASCII code to character

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String Manipulation Exam Examples

Example 1: Extract initials

name = "Alan Turing"
parts = name.split(" ")
initials = parts[0][0] + parts[1][0]
print(initials)     # AT

Example 2: Reverse a string

word = "Python"
reversed_word = word[::-1]
print(reversed_word)  # nohtyP

Example 3: Check for a palindrome

word = input("Enter a word: ").lower()
if word == word[::-1]:
    print("Palindrome")
else:
    print("Not a palindrome")

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File Handling

Programs need to read from and write to files to store data persistently (data that remains after the program closes).

Opening Files

Python uses the open() function with a mode:

Mode	Purpose	Creates file?
"r"	Read only	No (error if missing)
"w"	Write (overwrites existing content)	Yes
"a"	Append (adds to end)	Yes

Writing to a File

file = open("scores.txt", "w")
file.write("Alice,85\n")
file.write("Bob,92\n")
file.close()

Using with (recommended) — automatically closes the file:

with open("scores.txt", "w") as file:
    file.write("Alice,85\n")
    file.write("Bob,92\n")

Reading from a File

# Read entire file
with open("scores.txt", "r") as file:
    content = file.read()
    print(content)

# Read line by line
with open("scores.txt", "r") as file:
    for line in file:
        print(line.strip())

# Read all lines into a list
with open("scores.txt", "r") as file:
    lines = file.readlines()

Appending to a File

with open("scores.txt", "a") as file:
    file.write("Charlie,78\n")

Working with CSV-Style Data

# Reading and processing
with open("scores.txt", "r") as file:
    for line in file:
        parts = line.strip().split(",")
        name = parts[0]
        score = int(parts[1])
        print(f"{name} scored {score}")

Common File Operations

# Count lines in a file
with open("data.txt", "r") as file:
    count = 0
    for line in file:
        count += 1
    print(f"The file has {count} lines")

# Search for a record
with open("scores.txt", "r") as file:
    for line in file:
        if "Alice" in line:
            print("Found:", line.strip())

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Exam Tips

• The with statement is preferred — examiners accept both styles but with shows good practice
• Remember that "w" mode overwrites the entire file — use "a" to add data
• \n creates a new line when writing; .strip() removes it when reading
• String indexing starts at 0 — this catches many students out
• When tracing string operations, write out the index numbers above each character
• Know the difference between .read() (whole file as one string), .readline() (one line), and .readlines() (list of all lines)