Python · Lesson 3 of 15

Working with Strings

Indexing, slicing, the everyday string methods and clean formatting with f-strings.

• Beginner
• 14 min read
• 3 objectives

Before this lessonLesson 2: Variables and Types

Text is everywhere in real programs: names, emails, log lines, file paths, API responses. A Python str is an immutable sequence of characters. Sequence means you can index and slice it like a list. Immutable means no operation changes a string in place; every method returns a new string and leaves the original alone.

Creating strings

Use single or double quotes, whichever avoids escaping. Triple quotes span several lines. A backslash starts an escape sequence such as \n (new line) or \t (tab). Prefix a string with r to keep backslashes literal, which is handy for Windows paths and regular expressions.

single = 'It is fine'
double = "She said \"hi\""
poem = """Roses are red,
Violets are blue"""
path = r"C:\new\table"

print(single)
print(double)
print(poem)
print(path)

It is fine
She said "hi"
Roses are red,
Violets are blue
C:\new\table

Indexing and slicing

Each character has a position starting at 0. Negative positions count from the end, so -1 is the last character. A slice text[start:stop] takes characters from start up to but not including stop. Leave either side empty to go to that end, and add a third number as the step.

word = "stackcone"
print(word[0], word[-1])
print(word[0:5])
print(word[5:])
print(word[::-1])
print(word[::2])
print(len(word))

s e
stack
cone
enockcats
sakoe
9

Everyday string methods

Methods are functions attached to a value, called with a dot. The ones you will use constantly clean up text (strip, lower), search it (startswith, find, in) and rewrite it (replace).

raw = "   Hello, World!  "
clean = raw.strip()

print(clean)
print(clean.lower(), clean.upper())
print(clean.replace("World", "Python"))
print(clean.startswith("Hello"), clean.endswith("?"))
print("World" in clean)
print(clean.find("o"), clean.count("l"))
print(raw)

Hello, World!
hello, world! HELLO, WORLD!
Hello, Python!
True False
True
4 3
   Hello, World!  

Notice the last line: raw is unchanged, because strip() returned a new string.

Splitting and joining

split() turns a string into a list of pieces, and join() glues a list back into one string. Together they handle most text-processing chores, such as reading comma-separated values or building a sentence. Note the odd-looking order of join: you call it on the separator.

line = "ada,grace,linus"
names = line.split(",")
print(names)

print(" & ".join(names))
print("one two  three".split())
print("-".join(["2026", "01", "15"]))

['ada', 'grace', 'linus']
ada & grace & linus
['one', 'two', 'three']
2026-01-15

Formatting with f-strings

An f-string embeds expressions inside braces. After a colon you can control the format: number of decimals, thousands separators, alignment and padding. This is the modern, readable replacement for + concatenation and % formatting.

name = "Ada"
score = 93.456
big = 1234567

print(f"{name} scored {score:.1f}")
print(f"Population: {big:,}")
print(f"{name:>8}|{name:<8}|{name:^8}|")
print(f"{7:03d}")
print(f"{0.256:.0%}")
print(f"{name!r}")

Ada scored 93.5
Population: 1,234,567
     Ada|Ada     |  Ada   |
007
26%
'Ada'

Checking what a string contains

Methods such as isdigit(), isalpha() and isspace() answer yes/no questions about the characters, which is a cheap way to validate input before converting it.

for text in ["2026", "abc", "12a", ""]:
    print(repr(text), text.isdigit(), text.isalpha())

'2026' True False
'abc' False True
'12a' False False
'' False False

Common mistakes

• Expecting in-place changes: s.upper() on its own does nothing useful. Assign the result: s = s.upper().
• Assigning to an index: s[0] = "X" raises TypeError because strings are immutable. Build a new string with slicing or replace.
• Joining non-strings: ",".join([1, 2]) fails. Convert first: ",".join(str(n) for n in nums).
• Off-by-one slices: the stop index is excluded, so s[0:3] has three characters, not four.

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