Strings

A string is a sequence of characters

s = "Hi there!"
print(len(s))     # 9 (the space and ! count)
print(type(s))    # <class 'str'>
print(s[0])       # H  (a string of length 1)

Strings are everywhere

Names, passwords, messages, file names, web addresses, and everything a user types with input() are strings. Learning to pull strings apart and build new ones is one of the most useful skills in programming.

Digits in a string are just characters

"2026" is four characters: "2", "0", "2", "6". It is not a number until you convert it with int(). So len("2026") is 4, and "2026"[0] is the string "2".

Ways to build a string

Choosing the right way to build

• Fixed text: just use quotes.
• Joining a few strings: +. Remember to add your own spaces.
• Mixing text and numbers: an f-string is the easiest. Anything in { } is calculated and turned into text for you: f"{name} has {score} points". No str() needed.
• A repeated pattern: *, like "-" * 20 for a line.

f-strings can format numbers too: f"{3.14159:.2f}" gives '3.14'. The :.2f means "2 digits after the decimal point."

Escape characters and raw strings

Why do we need escapes?

Some characters cannot be typed directly inside a string. Pressing Enter in the middle of a normal string ends the line of code, and a quote of the same kind ends the string. The backslash tells Python "the next character is special, treat it differently."

Counting escapes

An escape sequence is two characters in your code but one character in the string. len("a\tb") is 3: a, a tab, b. When you index, the escape takes one position: "a\tb"[1] is the tab.

When to use raw strings: mostly for Windows file paths and special search patterns that have lots of backslashes. For everyday text, normal strings are fine.

Indexing: one character at a time

word = "Python"
#       P  y  t  h  o  n
#       0  1  2  3  4  5
#      -6 -5 -4 -3 -2 -1
print(word[0])     # P
print(word[3])     # h
print(word[-1])    # n
print(word[6])     # IndexError: string index out of range

How negative indexes work

Python turns a negative index into a positive one by adding the length. For "Python" (length 6), word[-2] is the same as word[6 - 2], which is word[4], the letter o.

That is why [-1] is always the last character, no matter how long the string is. You do not need to know the length.

An index gives one character

The result of word[0] is a string of length 1. You can use it anywhere a string works: word[0].upper(), word[0] == "P", or word[0] + "!".

Looping through characters, and in

for ch in "hey":          # one character at a time
    print(ch)

for i, ch in enumerate("hey"):
    print(i, ch)          # index and character

"gram" in "programming"   # True
"G" in "programming"      # False (case sensitive)
"x" not in "programming"  # True

Value loop or index loop?

• for ch in s: when you only need each character.
• for i, ch in enumerate(s): when you also need its position, for example "find where the first space is."
• for i in range(len(s)): also works and gives you just the index; use s[i] to get the character.

A loop that builds a new string

word = "banana"
result = ""
for ch in word:
    if ch != "a":
        result += ch
print(result)     # bnn

Start with an empty string and add only the characters you want. This is the string version of the accumulator pattern.

Slicing: extracting a substring

Think of indexes as the gaps between letters

 | P | y | t | h | o | n |
 0   1   2   3   4   5   6

If you picture each index as the edge before a character, a slice s[1:4] is everything between edge 1 and edge 4: yth. This picture also explains why s[6] is an error (there is no character after edge 6) but s[6:] is fine (it is just empty).

Slices never crash. Python quietly trims a slice to fit the string. Indexes are strict; slices are forgiving. So s[10] is an IndexError, but s[10:11] is just ''.

Quick length check: s[2:5] has 5 − 2 = 3 characters.

Steps and reversing

How a negative step reads

With a negative step, the slice walks backward, so the start should be the larger index. s[4:0:-1] starts at index 4 (o) and goes down to, but not including, index 0: o, h, t, y.

Leaving both ends blank with [::-1] means "start at the end and go all the way to the beginning," which reverses the whole string.

Palindromes with spaces and capitals

"Race car" fails the simple check because of the capital R and the space. Clean it first:

clean = "Race car".lower().replace(" ", "")   # 'racecar'
print(clean == clean[::-1])                    # True

String Slicer

Type any text, pick a start, stop, and step, and watch which characters end up in the slice. The small numbers show the order. Nothing is recorded.

Things to try in the slicer

• Leave start and stop blank and set step to 2. Which characters are picked?
• Set a start that is bigger than the stop with a step of 1. Why is the result empty?
• Now keep the same start and stop but make the step −1. What changed?
• Use negative numbers for start and stop, like −4 and −1. Which characters do you get?

Strings are immutable

What "reassigning" really does

s = "J" + s[1:] does not change the old string. Step by step:

1. s[1:] makes a new string "ython".
2. "J" + "ython" makes another new string, "Jython".
3. s = ... moves the name s to point at "Jython". The old "Python" is untouched; if nothing else uses it, Python cleans it up.

So the variable can change what it refers to, even though the string itself never changes.

Why are strings immutable?

A simple way to see it

Think of a string like a printed word on a sticker. You cannot erase one letter, but you can print a new sticker and put the label (your variable name) on it instead. Lists are more like a whiteboard: you can erase and rewrite parts.

"Changing" a string means making a new one

name = "ava"
name.upper()            # makes 'AVA' ... then throws it away
print(name)             # ava   (unchanged!)
name = name.upper()     # keep the new string
print(name)             # AVA

Useful methods that return new strings

And some that give back information instead of a string: "banana".count("a") is 3, "Python".startswith("Py") is True, and "a,b,c".split(",") gives the list ['a', 'b', 'c'].

Cleaning user input: answer = input("Yes or no? ").strip().lower() removes extra spaces and capital letters in one line, so " YES " becomes "yes".

Real-world slicing: extracting pieces

full = "Ava Lee"
space = full.find(" ")       # 3
first = full[:space]         # 'Ava'
last = full[space + 1:]      # 'Lee'

email = "ava@school.org"
at = email.find("@")
user = email[:at]            # 'ava'
domain = email[at + 1:]      # 'school.org'

Watch out when find() fails

If the text is not found, find() returns -1, and -1 is a valid slice position (the last character). So with email = "avaschool" (no @), email[:at] becomes email[:-1], which is 'avaschoo'. No error, just a wrong answer.

at = email.find("@")
if at == -1:
    print("That is not an email address.")
else:
    print("User:", email[:at])

Why space + 1? The slice for the last name should start after the space, not on it. Without the + 1, the result would be ' Lee' with a leading space.

Substring challenge

Type an index or slice in brackets (like [0] or [0:5]) that gets each target from s, then click Check. Practice only; nothing is recorded.

Strategy

1. Write the string with the index number under every character, including spaces and punctuation.
2. Find the index of the first character you want. That is your start.
3. Find the index of the last character you want and add 1. That is your stop.
4. For pieces at the end, a negative start with no stop is often easiest: [-4:].

Common mistakes

The two sneakiest bugs

• Forgetting to save a method's result. name.upper() on its own line runs, makes a new string, and immediately throws it away. If a string "didn't change," check for a missing name = ....
• Off-by-one in a slice. If a slice is one character short, the stop is probably one too small. Remember: stop is the first index you do not want.

Error type tells you where to look: TypeError ... item assignment means you tried to change a string. IndexError means an index is past the end. AttributeError means you used a method strings do not have (like a list method).

Check yourself

Every question uses s = "Python". Choose an answer, then click Check. Practice only; nothing is graded or recorded.

 P  y  t  h  o  n
 0  1  2  3  4  5
-6 -5 -4 -3 -2 -1

Cheat sheet

"Hi"  'Hi'  ""
"It's"   'Say "hi"'
"a\nb"   r"C:\new"
str(42)          # '42'
"ab" + "cd"      # 'abcd'
"ha" * 3         # 'hahaha'
f"{n} apples"

s = "Python"
s[0]      # 'P'
s[-1]     # 'n'
s[1:4]    # 'yth'
s[:3]     # 'Pyt'
s[-3:]    # 'hon'
s[::-1]   # 'nohtyP'
len(s)    # 6

Remember:

• Indexes start at 0; the last character is [-1]
• Slices: start is in, stop is out, and a slice never raises an IndexError
• Spaces and punctuation are characters with their own indexes
• You never change a string; you build a new one and reassign the name

Key words to know

string · character · escape sequence · raw string · f-string · index (positive and negative) · slice · step · substring · immutable · method