Working with strings in depth

The introductory strings lesson covered indexing, slicing, and the basics of f-strings. Now you'll meet the full method toolbox — the one you'll reach for constantly when processing text from files, APIs, or user input.

Cleaning and normalising

Real-world strings rarely arrive in a tidy form. These three methods handle the most common clean-up tasks:

"  hello world  ".strip()      # "hello world"  — removes surrounding whitespace
"  hello world  ".lstrip()     # "hello world  " — left side only
"  hello world  ".rstrip()     # "  hello world" — right side only

"Hello World".lower()          # "hello world"
"Hello World".upper()          # "HELLO WORLD"

strip() is almost always the first thing you do with a line read from a file — the trailing newline and any leading spaces vanish in one call.

Splitting and joining

split() converts a string into a list; join() is the inverse:

"one,two,three".split(",")      # ["one", "two", "three"]
"hello world".split()           # ["hello", "world"]  — splits on any whitespace

",".join(["one", "two", "three"])  # "one,two,three"
" | ".join(["a", "b", "c"])        # "a | b | c"

The join pattern is particularly clean: call it on the separator string and pass the list as the argument. Building strings with + in a loop is both slower and harder to read.

Searching and replacing

"hello world".replace("world", "Python")   # "hello Python"
"hello world".find("world")                # 6  — index of first match
"hello world".find("xyz")                  # -1 — not found
"hello world".count("l")                   # 3
"hello world".startswith("hello")          # True
"hello world".endswith("world")            # True

find() returns -1 when the substring isn't present. If you only need to know whether it's there at all, "needle" in haystack is cleaner:

"world" in "hello world"    # True
"xyz"   in "hello world"    # False

String formatting: f-strings and .format()

You already know f-strings from the previous lesson. They're the preferred style in modern Python (3.6+):

name, score = "Ada", 98
f"{name} scored {score}/100"          # "Ada scored 98/100"
f"{score:.1f}%"                        # "98.0%"  — format spec after the colon
f"{name!r}"                            # "'Ada'"  — repr() of the value

.format() is the older style you'll see in existing codebases:

"{} scored {}".format(name, score)          # positional
"{name} scored {score}".format(name=name, score=score)  # keyword

Both work; f-strings are shorter and catch name errors at definition time.

Multi-line strings

Triple quotes (""" or ''') create strings that can span multiple lines:

message = """
Dear Ada,

Your submission has been received.
Thank you.
"""

The leading newline after """ is often unwanted; you can strip it with .strip() or start the string content on the same line as the opening quotes. Multi-line strings are handy for SQL queries, HTML templates, and docstrings.

String immutability

Strings are immutable — no method ever changes the original. Every method returns a new string:

name = "ada"
name.upper()   # returns "ADA" but name is still "ada"
print(name)    # "ada"

# Correct: reassign if you want to keep the result
name = name.upper()
print(name)    # "ADA"

This is why chaining methods works:

"  Ada Lovelace  ".strip().lower().replace(" ", "_")
# "ada_lovelace"

Each method call receives the return value of the previous one.

Where to go next

You're now fluent with the string toolbox. The lab: filter and report puts boolean logic, file I/O, and string methods together in a practical data- processing task.