Topic 07: Files & String Processing

🎯 Learning Objectives

Open, read, write, and close files using open()
Use the with statement to safely handle files
Use common string methods: upper(), lower(), strip(), split(), join(), replace()
Format strings using f-strings and .format()
Read files line by line and process text data

📚 Key Concepts

Opening Files — open()

Mode	Meaning
`'r'`	Read (default). File must exist.
`'w'`	Write. Creates new file or overwrites existing.
`'a'`	Append. Adds to end of file. Creates if not exists.
`'r+'`	Read and write.

f = open("data.txt", "r")   # open for reading
content = f.read()           # read all as one string
f.close()                    # always close!

The with Statement (Recommended)

Automatically closes the file when the block ends — even if an error occurs. Always use with.

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

Reading Files

# Read entire file as string
with open("data.txt") as f:
    text = f.read()

# Read one line at a time
with open("data.txt") as f:
    line = f.readline()       # reads one line (includes \n)

# Read all lines into a list
with open("data.txt") as f:
    lines = f.readlines()     # list of strings with \n

# Iterate line by line (most efficient)
with open("data.txt") as f:
    for line in f:
        print(line.strip())   # strip() removes \n

Writing Files

# Write (overwrites if file exists)
with open("output.txt", "w") as f:
    f.write("Hello, World!\n")
    f.write("Second line\n")

# Append (adds to existing file)
with open("output.txt", "a") as f:
    f.write("Appended line\n")

Note: write() does not add newlines automatically — you must add \n.

String Methods

Method	What it does	Example
`s.upper()`	All uppercase	`"hello".upper()` → `"HELLO"`
`s.lower()`	All lowercase	`"HELLO".lower()` → `"hello"`
`s.strip()`	Remove leading/trailing whitespace	`" hi ".strip()` → `"hi"`
`s.split(sep)`	Split into list by separator	`"a,b,c".split(",")` → `['a','b','c']`
`sep.join(list)`	Join list into string	`",".join(['a','b'])` → `"a,b"`
`s.replace(old, new)`	Replace occurrences	`"abba".replace("b","x")` → `"axxa"`
`s.find(sub)`	Index of first occurrence (-1 if not found)	`"hello".find("ll")` → `2`
`s.startswith(pre)`	Check if starts with prefix	`"hello".startswith("he")` → `True`
`s.endswith(suf)`	Check if ends with suffix	`"file.py".endswith(".py")` → `True`
`len(s)`	Length of string	`len("hello")` → `5`

String Formatting — f-strings (Recommended)

f-strings (formatted string literals) are the modern way to embed variables in strings.

name  = "Alice"
score = 85.5

# f-string (Python 3.6+)
print(f"Name: {name}, Score: {score}")
# Output: Name: Alice, Score: 85.5

# Format numbers
print(f"Score: {score:.2f}")    # 2 decimal places: 85.50
print(f"Pi: {3.14159:.3f}")     # Pi: 3.142

# Expressions inside
print(f"Double: {score * 2}")   # Double: 171.0

String Indexing and Immutability

Strings are sequences — you can index and slice them like lists. But strings are immutable — you cannot change individual characters.

s = "Python"
print(s[0])       # P
print(s[-1])      # n
print(s[2:5])     # tho
print(s[::-1])    # nohtyP (reversed)
s[0] = "J"        # TypeError! Strings are immutable

💻 Code Examples

Example 1 — Count words in a file

with open("story.txt", "r") as f:
    text = f.read()
words = text.split()
print(f"Word count: {len(words)}")

Example 2 — CSV-style file reading

# students.txt content: "Alice,85\nBob,72\n"
with open("students.txt") as f:
    for line in f:
        name, score = line.strip().split(",")
        print(f"{name}: {score}")

Example 3 — String processing

sentence = "  The Quick Brown Fox  "
clean = sentence.strip().lower()
words = clean.split()
print(words)              # ['the', 'quick', 'brown', 'fox']
print(len(words))         # 4
print(" ".join(words))    # the quick brown fox

⚠️ Exam Focus

Know file modes: 'r' read, 'w' write (overwrites!), 'a' append.
The with statement automatically closes the file — always preferred.
Strings are immutable — s.upper() returns a new string, it doesn't change s.
Know split() and join(): "a b c".split() → ['a','b','c']; " ".join(['a','b','c']) → "a b c".
f-string formatting: f"{value:.2f}" for 2 decimal places.

❌ Common Mistakes

Opening with 'w' when you want to append — 'w' erases existing content!
Forgetting f.close() — use with to avoid this.
Not stripping \n from lines — line.strip() removes newline and spaces.
Trying to reassign string characters: s[0] = 'x' is a TypeError.

⚡ Quick Recap

open(file, mode): 'r' read, 'w' write/overwrite, 'a' append.
Always use with open(...) as f: — auto-closes the file.
Read: f.read(), f.readline(), f.readlines(), or iterate with for line in f.
String methods return new strings — strings are immutable.
f-strings: f"Hello {name}" — cleaner than concatenation.
Key methods: strip(), split(), join(), replace(), upper(), lower().

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