Rounding and Display Precision

The round function returns a number rounded to a stated number of decimal places. You give the value first, then the number of decimal places: round(8.6742, 2) gives 8.67. The result is a real number, so you can print it, store it in a variable or use it in a later calculation. Rounding does not pad a number with zeros, so 6.5 rounded to two decimal places is still 6.5 and prints as 6.5 rather than 6.50: the trailing zero is not part of the number.

Rounding for later use

Because round hands back a number rather than text, it suits a plain print where no layout is needed, and arithmetic that carries on afterwards. This program finds an average lap time in a kart race and shows both the tidy version and the precise one.

total_time = 47.368   # seconds for three laps
laps = 3
average_lap = total_time / laps
print('Average lap to one decimal place: ' + str(round(average_lap, 1)))
print('Average lap kept in full: ' + str(average_lap))

Output:

Average lap to one decimal place: 15.8
Average lap kept in full: 15.789333333333333

The variable still holds the full result, so the second print can show it exactly, and adding to, multiplying or comparing the rounded value works normally.

Fixed decimal places on screen

A format specification does a similar job for display: placed inside a placeholder of a template string, it shows a value to a stated number of decimal places and keeps trailing zeros, so 6.5 appears as 6.50. It produces text for the screen only, so it cannot be used in a calculation, and the variable itself is untouched.

Keeping the full value

Formatting only changes the text that is displayed, and round only returns a new number; neither alters the value already stored in a variable. Storing a rounded or formatted result back into the variable does change it, and that loses precision, making later output inaccurate.

mass = 7.4562   # kilograms
mass = round(mass, 1)
print('Stored mass: ' + str(mass))
print('Twice the stored mass: ' + str(mass * 2))

Output:

Stored mass: 7.5
Twice the stored mass: 15.0

Doubling the exact mass would give 14.9124, but the rounded figure was stored back into the variable, so the last print works from a value that is already too rough.