Python Strings

1. Indexing & Length (Recap)

• Access characters by index: s[0] → first character
• Negative indexes work too: s[-1] → last character
• Get length: len(s)

2. String Methods

.count()

Counts how many times a character/substring appears.

s = "hello"
s.count("l")  # → 2

.find()

Returns the index of the first occurrence of a character. Returns -1 if not found (does not raise an error).

s.find("l")  # → 2
s.find("a")  # → -1

Guide: Unlike .index() on lists (which raises an error if not found), .find() safely returns -1.

.upper() / .lower()

Convert a string to all uppercase or all lowercase.

s.upper()  # → "HELLO"
s.lower()  # → "hello"

Practical use: Normalize user input before comparing.

if answer.lower() == "tim":
    print("Correct!")

.capitalize()

Capitalizes only the first letter of the string.

"algo expert".capitalize()  # → "Algo expert"

.isdigit()

Returns True if the string represents a valid integer, False otherwise.

"19".isdigit()    # → True
"19h".isdigit()   # → False
"19.4".isdigit()  # → False  (floats return False)

Practical use: Validate user input before converting to int.

num = input("Number: ")
if num.isdigit():
    num = int(num)
    print(num + 5)
else:
    print("Not an int")

.split(delimiter)

Splits a string into a list of substrings using a delimiter. Default delimiter is a space.

"hello my name is Tim".split()       # → ['hello', 'my', 'name', 'is', 'Tim']
"hello,my,name".split(",")           # → ['hello', 'my', 'name']
"hello,,name".split(",")             # → ['hello', '', 'name']  ← empty string between commas

.replace(old, new)

Replaces all occurrences of a substring with a new one. Returns a new string (original is unchanged).

s = "a,b,c"
s2 = s.replace(",", "|")  # → "a|b|c"
# s is still "a,b,c"

.join(iterable)

Builds a string from a list/tuple by joining elements with a separator.

lst = ["T", "i", "m"]
"".join(lst)    # → "Tim"
"-".join(lst)   # → "T-i-m"
"|".join(lst)   # → "T|i|m"

3. The in Operator

Check whether a character or substring exists in a string.

"h" in "hello"  # → True
"z" in "hello"  # → False

4. F-Strings (Python 3.6+)

Embed variables or expressions directly inside a string using {}.

name = "Tim"
s = f"Hello, {name}!"         # → "Hello, Tim!"
s = f"1 + 4 = {1 + 4}"       # → "1 + 4 = 5"

• Start with f or F before the opening quote.
• Use {} to embed any Python expression — no manual conversion needed.
• Can be used directly inside print():

print(f"Hello, {name}!")

5. String Multiplication

Repeat a string by multiplying it with an integer.

"Tim" * 3  # → "TimTimTim"

6. Multi-line Strings

Use triple quotes (""" or ''') to span a string across multiple lines.

s = """Hello,
my name is Tim,
and this is a multi-line string!"""

Guide: Without assignment, triple quotes act as a multi-line comment.

7. Escape Characters

Use a backslash \ to include a quote character inside a string that uses the same quote type.

s = 'Tim\'s code'   # → Tim's code
s = "She said \"hi\""

Tip: Avoid escaping by mixing quote types:

s = "Tim's code"    # single quote inside double-quoted string — no escape needed

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