TopolearnAlmost every program reads, builds or prints text, and f-strings are the modern way to format it.
A str is an immutable sequence of Unicode characters. Single and double quotes are equivalent; triple quotes ('''...''') span lines. A backslash starts an escape like \n or \t; prefix with r for a raw string where backslashes are literal, which is handy for regexes and Windows paths.
Strings support sequence operations: len(s), indexing s[0], s[-1] for the last character, in for substring tests, + to concatenate and * to repeat. Because they are immutable, every method returns a new string:
s.upper(),s.lower(),s.strip()(remove surrounding whitespace)s.split(",")returns a list;", ".join(items)is the reverses.replace(old, new),s.startswith(prefix),s.find(sub)(-1 if missing)
An f-string (f"...") evaluates expressions inside braces and inserts the result. After a colon you can add a format spec:
{price:.2f}two decimal places,{n:,}thousands separators,{ratio:.1%}percent{name:>10}right-align in 10 characters,{name:<10}left,{name:^10}centre{x!r}usesrepr();{x=}printsx=followed by the value, great for debugging
Since Python 3.12 you can even reuse the same quote type inside the braces. Build big strings by collecting parts in a list and calling "".join(parts), not by += in a loop.
Example
name = "Ada Lovelace"
print(name.upper(), len(name)) # ADA LOVELACE 12
print(name.split()) # ['Ada', 'Lovelace']
print("-".join(["2026", "10", "16"])) # 2026-10-16
print("love" in name.lower()) # True
price, qty = 4.5, 1234
print(f"{qty:,} items at ${price:.2f}") # 1,234 items at $4.50
print(f"[{'left':<6}][{'right':>6}]") # [left ][ right]
print(f"{0.256:.1%}") # 25.6%
total = price * 3
print(f"{total=}") # total=13.5
print(r"C:\new\table") # C:\new\tableRemember
- Strings are immutable; methods return new strings
- Use f-strings with format specs like
:.2f,:,,:>10 sep.join(list)ands.split(sep)convert between strings and lists
Watch out for
- Calling
s.upper()and expectingsitself to change. - Concatenating in a loop instead of collecting parts and joining once.