Printing and f-strings

To communicate something to the user (or to see what is happening inside our program) we use the built-in print(...) function. To compose text from multiple values, the modern and preferred syntax is the f-string.

print(...)

print("Ciao, mondo!")           # Ciao, mondo!
print("a", "b", "c")            # a b c       (separati da spazio)
print(2 + 2)                    # 4
print("riga 1", "riga 2")       # riga 1 riga 2

print can take as many arguments as you want: it joins them with a space (by default) and appends a newline at the end. You can change both with the sep and end parameters:

print("a", "b", "c", sep="-")   # a-b-c
print("senza newline", end="")  # niente \n dopo

f-string: modern interpolation

An f-string is a string prefixed by f: inside the braces {...} you can insert any Python expression, which will be evaluated and inserted into the result:

nome = "Ada"
anni = 30
print(f"Mi chiamo {nome} e ho {anni} anni.")
# Mi chiamo Ada e ho 30 anni.

Inside the braces you can put calls, operations, object accesses — it is Python code in all respects:

prezzo = 12.5
quantita = 3
print(f"Totale: {prezzo * quantita:.2f} €")
# Totale: 37.50 €

The :.2f after the colon is a format spec: in this case "two decimal digits, fixed-point". The most useful format specs:

• :.Nf — N decimal digits (f"{pi:.4f}" → 3.1416).
• :N / :>N / :<N / :^N — width N, right/left/center alignment.
• :_ or :, — thousands separator (f"{1000000:,}" → 1,000,000).

Alternative syntaxes (for reference)

Before f-strings (introduced in Python 3.6) two forms were used that you'll still encounter in older code:

# stile .format() — verboso ma esplicito
"{} ha {} anni".format("Ada", 30)

# stile %-formatting — molto vecchio
"%s ha %d anni" % ("Ada", 30)

For new code, always use f-strings: shorter, more readable, faster.

Advanced formatting with f-strings

F-strings support rich formatting specifications directly inside the curly braces. For instance, to format a decimal number with exactly two decimal places, you can write:

price = 19.99
print(f"Price: {price:.2f} $")

The :.2f specifier indicates that we want to represent the float with exactly 2 decimal places.

Try it yourself

Review exercise

Additional challenge