python.md

title	description
Floating-point cheat sheet for Python	Tips for using floating-point and decimal numbers in 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

---

title: 为 Python 准备的浮点小抄 description: Tips for using floating-point and decimal numbers in Python

浮点类型

几乎所有平台把 Python 的浮点映射成 IEEE 754 双精度。

	f = 0.1

小数类型

Python 有 arbitrary-precision 小数类型，称为 Decimal，这个类型在 decimal 模块中，它同样允许选择 取整模式.

	a = Decimal('0.1')
	b = Decimal('0.2')
	c = a + b # 返回 Decimal 表示的精确的 0.3

如何取整

得到字符串：

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

打印到标准输出：

	print "%.2f" % 1.2399 # 只是使用打印和字符串格式化

特殊的 取整模式 和其它参数可以在一个 Context 对象中定义：

	getcontext().prec = 7

资源

• 浮点代数：问题和限制
• 小数模块
  • Context 对象
• Python 中字符串的定义