Module 7: Strings in Depth -- Study Guide

Review all key concepts from Module 7. Use this guide to prepare for the quiz or as a reference while coding.

1. Strings Are Immutable

Core Principle: Strings cannot be changed in place. Every string method returns a NEW string -- the original is never modified. This is the fundamental difference from lists.

name = "alice"
upper_name = name.upper()   # Returns "ALICE"
print(name)                 # Still "alice"

# To keep the change, reassign:
name = name.upper()         # Now name is "ALICE"

Common mistake: Calling a string method without saving the result. text.upper() alone does nothing to text. You must write text = text.upper() or result = text.upper().

2. String Methods

Case Conversion

.upper() -- all uppercase
.lower() -- all lowercase
.title() -- capitalize each word
.capitalize() -- capitalize first letter only
.swapcase() -- swap upper and lower

Whitespace Removal

.strip() -- remove whitespace from both ends
.lstrip() -- remove from left only
.rstrip() -- remove from right only
All three accept optional characters to strip: .strip("#")

Other Key Methods

.replace(old, new) -- substitute all occurrences
.count(sub) -- count occurrences of substring
.startswith(sub) / .endswith(sub) -- check boundaries

Tip: startswith() and endswith() accept tuples for multiple checks: filename.endswith((".jpg", ".png"))

3. String Slicing

Same as Lists: String slicing uses identical syntax to list slicing: string[start:stop:step]

Key patterns:

s[:n] -- first n characters
s[-n:] -- last n characters
s[a:b] -- from index a to b-1
s[::2] -- every other character
s[::-1] -- reversed string
s[1:-1] -- everything except first and last

text = "Programming"
print(text[:4])      # "Prog"
print(text[-4:])     # "ming"
print(text[::-1])    # "gnimmargorP"

4. Searching Strings

find() vs index()

find(sub) -- returns index of first match, or -1 if not found
index(sub) -- returns index of first match, or raises ValueError if not found
rfind(sub) -- like find(), but searches from the right (last occurrence)

When to use which: Use find() when the substring might not exist (check for -1). Use index() when not finding it would be a bug.

The `in` Operator

if "python" in text.lower():
    print("Found it!")

Always convert to the same case for case-insensitive searches.

5. Splitting and Joining

split() and join() are inverses: split() turns a string into a list; join() turns a list into a string.

# Split
words = "Hello World".split()     # ["Hello", "World"]
parts = "a,b,c".split(",")        # ["a", "b", "c"]

# Join
result = " ".join(["Hello", "World"])   # "Hello World"
result = ",".join(["a", "b", "c"])     # "a,b,c"

# Clean up spacing
clean = " ".join("  too   many  spaces  ".split())
# "too many spaces"

Remember: join() is called on the separator, not the list: ",".join(my_list) not my_list.join(",").

6. Special String Types

f-strings: f"Hello {name}" -- embed expressions inside strings
Raw strings: r"C:\path\file" -- backslashes are literal
Multiline: """triple quotes""" -- span multiple lines

Use splitlines() to split multiline strings into a list of lines.

7. Common Patterns

# Case-insensitive comparison
if user_input.lower() == "yes":

# Extract email parts
at = email.find("@")
username = email[:at]
domain = email[at+1:]

# Extract file extension
ext = filename[filename.rfind(".")+1:]

# Check palindrome
is_palindrome = word.lower() == word.lower()[::-1]

# Clean phone number
clean = "".join(c for c in phone if c.isdigit())

# Parse CSV
for line in csv_text.split("\n"):
    fields = line.split(",")

8. Strings vs. Lists -- Comparison

Feature	Strings	Lists
Mutable?	No (immutable)	Yes (mutable)
Indexing	Yes	Yes
Slicing	Yes	Yes
`in` operator	Yes	Yes
`len()`	Yes	Yes
Methods modify in place?	No (return new string)	Yes (most return None)
`sort()` / `reverse()`	Not available	Available

9. Review Questions

Q1: Why do string methods return new strings instead of modifying the original?

Q2: What is the difference between find() and index()?

Q3: How do you reverse a string?

Q4: What does split() return, and what does join() do?

Q5: How do you perform a case-insensitive search?

Q6: What is a raw string and when would you use one?

Q7: How do you check if a filename ends with ".pdf" or ".txt"?

Q8: What is the most efficient way to build a string from many pieces?

Take the Module Quiz → Quick Reference Card