Floating-point cheat sheet for Python

Floating-Point Types

Almost all platforms map Python floats to IEEE 754 double precision.

	f = 0.1 

Decimal Types

Python has an arbitrary-precision decimal type named Decimal in the decimal module, which also allows to choose the rounding mode.

	a = Decimal('0.1')
	b = Decimal('0.2')
	c = a + b # returns a Decimal representing exactly 0.3

How to Round

To get a string:

	"%.2f" % 1.2399 # returns "1.24"
	"%.3f" % 1.2399 # returns "1.240"
	"%.2f" % 1.2 # returns "1.20"

To print to standard output:

	print("%.2f" % 1.2399) # just use print and string formatting

Specific rounding modes and other parameters can be defined in a Context object:

	getcontext().prec = 7

Resources

• Floating Point Arithmetic: Issues and Limitations
• The decimal module
  • Context objects
• String formatting in Python

The Floating-Point Guide

• 
• Home
• Basic Answers
• References
• xkcd

Number Formats

• Binary Fractions
• Floating-Point
• Exact Types
• On Using Integers

Errors

• Rounding
• Comparison
• Propagation

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