How to limit decimal numbers in Python?

Question

How do I format the number of decimals of a decimal number in Python?

For example, I want to display only two decimal places of the following number:

numero_decimal = 3.141592653589793

How could I turn that number to 3.14?

Answer 1

Rounding

If round:

round(3.141592653589793, 2)

Which is what happens when you do something like this.

"%.2f" % 3.141592653589793

Truncating

In this case it already needs more care, because the lack of a dedicated function requires composing a manual solution.

This simple function meets well in everyday life:

def trunc(num, digits):
   sp = str(num).split('.')
   return '.'.join([sp[0], sp[:digits]])

This one works in 2 and 3 and takes into account exponential notation, for more complex scenarios:

def truncate(f, n):
    '''Truncates/pads a float f to n decimal places without rounding'''
    s = '{}'.format(f)
    if 'e' in s or 'E' in s:
        return '{0:.{1}f}'.format(f, n)
    i, p, d = s.partition('.')
    return '.'.join([i, (d+'0'*n)[:n]])

Codes harnessed from here:

https://stackoverflow.com/q/783897/916193

Note: for those who don’t know the difference:

• Rounding up 3.19999 to 2 decimals the result is 3.20;

• By truncating3.19999 to 2 decimals the result is 3.19.

Answer 2

In Python 3 has a new feature that makes this task much easier, is the .format() that you will use when printing the result on the screen.

To print your comado with only 2 decimal places just do the following:

pi = 3.141592653589793
print('O valor de pi formatado é {:.2f}'.format(pi))

I first assigned pi to a float variable and then printed with the .format saying he wanted 2 decimal places after the comma.

Another way is to use the f strings that allow you to do everything necessary inside the print itself and without the format (I prefer this way).

pi = 3.141592653589793
print(f'O valor de pi formatado é {pi:.2f}')

Answer 3

Another way to format in a very similar way is with f-Strings:

pi = 3.141592653589793
print(f'O resultado de PI é {pi:.2f}')

Note the use of 'f' before the message that will come and also the variable 'pi' that goes directly inside the keys along with the formatting of 2 decimal places after the comma

Answer 4

I did using the round.

Example:

A = 3/9
valor = round(A,3)
print (valor)

Or you can do straight:

A = round(3/9,3)
print (A)

Being the 3 the amount of decimal places you want to keep.

Answer 5

To make truncation a simple way:

n1= 1.9999999999999

t =3 # Numero de casas

d = int(n1 * 10**t)/10**t

print('Truncado', d)

print('Arredondado', round(n1,t))