How Can I Reliably Preserve Two Decimal Places in Floating-Point Numbers in Python?

Preserving Two Decimal Points in Floating-Point Numbers

When working with floating-point numbers, it's often desirable to limit them to a specific number of decimal points. However, simply using the round() function can lead to unexpected results as witnessed with the following behavior:

>>> a
13.949999999999999
>>> round(a, 2)
13.949999999999999

This occurs because floating-point numbers have limitations in representing all numbers accurately. The values stored in memory are not always the same as those displayed.

Understanding Floating-Point Precision

Double precision floating-point numbers, used in Python, have approximately 16 digits of precision. This means that numbers like 13.95 are represented in a way similar to:

125650429603636838/(2**53)

When applying the round() function, the result is still the same floating-point value, internally represented with the same limitations.

>>> 125650429603636838/(2**53)
13.949999999999999

Alternative Methods for Limiting Decimal Points

If displaying only two decimal places is essential, consider these alternatives:

• Using Integers and Scaling: Represent values in cents instead of dollars and divide by 100 to convert.
• Using Fixed-Point Decimal: Utilize modules like decimal to store and operate on fixed-point numbers with specified precision.

Examples:

>>> a = 13.946
>>> print("%.2f" % a)
13.95
>>> print("{:.2f}".format(a))
13.95

# Using decimal module
>>> import decimal
>>> a = decimal.Decimal('13.946')
>>> a.quantize(decimal.Decimal('.01'))
Decimal('13.95')

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