03-strings

03 · Strings

Part 1 — Fundamentals · Estimated time: 25–35 min · Prerequisite: 02 · Data Types & Numbers

A string is text — a "string" of characters like "Hello". Strings are everywhere: names, messages, file contents, user input. This lesson shows how to create them, join them, pull pieces out of them, and transform them with built-in methods.

What you'll learn

• Creating strings with single, double, and triple quotes
• Joining strings with +, +=, and (best of all) f-strings
• Measuring length with len() and reading characters by index
• Slicing out a piece of a string
• Handy string methods: .upper(), .lower(), .strip(), .replace(), .split(), .join()
• Why strings are immutable

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1. Creating strings

single = 'Hello'
double = "Hello"          # single and double quotes both work
sentence = "It's fine"    # use double quotes when the text has an apostrophe

paragraph = """This string
spans multiple lines."""  # triple quotes keep line breaks

print(len(single))        # 5  -> number of characters

▶️ Run it: python examples/01_creating_strings.py

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2. Joining strings

first = "Hello"
second = "World"
print(first + " " + second)   # Hello World   (+ glues strings together)

greeting = "Hi"
greeting += "!"               # += appends
print(greeting)               # Hi!

The cleanest way to build text from variables is an f-string:

name = "Ada"
age = 36
print(f"{name} is {age} years old.")   # Ada is 36 years old.

You can't glue text to a number directly ("age: " + 36 is an error) — but an f-string converts for you automatically. If you ever do need it as text, use str(36).

▶️ Run it: python examples/02_concatenation_and_fstrings.py

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3. Indexing and slicing

Each character has a position (an index) starting at 0. Negative indexes count from the end.

word = "Python"
print(word[0])    # P   (first character)
print(word[-1])   # n   (last character)

print(word[0:3])  # Pyt  (slice: from index 0 up to — but not including — 3)
print(word[3:])   # hon  (from index 3 to the end)
print(word[:2])   # Py   (from the start up to index 2)

▶️ Run it: python examples/03_indexing_and_slicing.py

💡 Try it yourself: given word = "Python", can you print just "tho" using a slice?

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4. Useful string methods (and immutability)

A method is a function attached to a value; you call it with a dot.

text = "  Hello World  "
print(text.upper())        # "  HELLO WORLD  "
print(text.lower())        # "  hello world  "
print(text.strip())        # "Hello World"  (trims spaces at both ends)
print(text.replace("o", "0"))   # "  Hell0 W0rld  "
print("a,b,c".split(","))  # ['a', 'b', 'c']  -> a list
print("-".join(["a", "b", "c"]))  # "a-b-c"

Strings are immutable — methods return a new string and never change the original. word[0] = "J" is an error.

▶️ Run it: python examples/04_string_methods.py

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Common mistakes

Mistake	What happens	Fix
"age: " + 36	TypeError	Use an f-string, or str(36).
word[6] on "Python"	IndexError (valid indexes are 0–5)	Remember indexing starts at 0; last is len-1 or -1.
Expecting text.strip() to change text	text is unchanged	Strings are immutable — save the result: text = text.strip().
'It's fine'	SyntaxError (the apostrophe ends the string)	Use double quotes: "It's fine".

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Recap / cheat-sheet

s = "Hello"
len(s)            # 5
s[0]              # 'H'   first character
s[-1]             # 'o'   last character
s[1:4]            # 'ell' slice [start:stop]
s + " there"      # join with +
f"{name} is {n}"  # f-string (preferred)
s.upper() / s.lower() / s.strip()
s.replace("l", "L")
"a,b".split(",")  # -> ['a', 'b']
",".join(parts)   # list -> string

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_full_name.py — build a full name and report its length.
2. exercises/02_initials.py — extract initials using indexing.
3. exercises/03_clean_username.py — tidy a messy username with .strip() and .lower().

Try each yourself before opening the solution.

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Next → 04 · Booleans & Comparison (coming soon)