f-strings in Python: format numbers the way people read them

Rounding changes a number; it doesn't control how it prints. round(0.8, 2) is still displayed as 0.8. A format spec sets the display directly:

cheap_per_wear = 8 / 10
print(round(cheap_per_wear, 2))
print(f"${cheap_per_wear:.2f} per wear")

0.8
$0.80 per wear

:.2f means "fixed-point, two decimal places," so 0.8 is shown as 0.80. Rounding removes digits; it never adds them back.

Other specs handle other shapes of number. :, inserts thousands separators and :.1% multiplies by 100 and adds a percent sign:

margin = (38 - 14) / 38
year_one = 331398
launch_costs = 12500.0

print(f"Margin: {margin:.1%}")
print(f"Year one revenue: ${year_one:,}")
print(f"Launch costs: ${launch_costs:,.0f}")

Margin: 63.2%
Year one revenue: $331,398
Launch costs: $12,500

Specs combine: :,.0f means thousands separators and zero decimal places. The underlying values are unchanged; only their presentation is.

A number after the colon sets a field width, with < for left-aligned and > for right-aligned. That's enough to print a tidy table:

names = ["Organic tee", "Everyday pants", "Wool socks"]
prices = [38, 78, 14]
costs = [14, 29, 4]

print(f"{'Product':<16}{'Price':>7}{'Margin':>9}")
for i in range(len(names)):
    margin = (prices[i] - costs[i]) / prices[i]
    print(f"{names[i]:<16}{prices[i]:>7.2f}{margin:>9.1%}")

Product           Price   Margin
Organic tee       38.00    63.2%
Everyday pants    78.00    62.8%
Wool socks        14.00    71.4%

Every row uses the same widths, so the columns line up whatever the lengths of the names. Text reads best left-aligned and numbers right-aligned.