Round Function does not work in v2.6.6, but works in v3.4.2

问题

My round function does not work in linux python 2.6.6, whereas it works fine in Windows 3.4.2 after using the following type of code:

Array[i] = round(math.e ** AnotherArray[i], 4)

v.3.4.2:  0.0025999999999999999 => 0.0026
v.2.6.6:  0.0025999999999999999 =>  0.0025999999999999999

回答1:

They both work the same, but Python 2.7 and up will round floating point numbers when printing their representations, in order to not confuse users by the (language- and machine-independent) limitations of floating point arithmetic.

The decimal number 0.0026 can't be represented exactly as a binary float, so there will always be some rounding error.

If you want less confusion, just print the numbers:

>>> a = 0.0025999999999999999
>>> b = round(a,5)
>>> b                  # this calls repr(b) 
0.0025999999999999999
>>> print b            # this calls str(b)
0.0026

In practice, those rounding errors rarely matter, although you need to be aware of them, especially when comparing for equality.

The following loop doesn't stop at 0:

x = 1.0
while x != 0:
    print x
    x -= 0.1

Why? Let's take a look:

>>> x = 1.0
>>> while x != 0:
...     print repr(x)
...     x -= 0.1
...     if x<0: break
...
1.0
0.90000000000000002
0.80000000000000004
0.70000000000000007
0.60000000000000009
0.50000000000000011
0.40000000000000013
0.30000000000000016
0.20000000000000015
0.10000000000000014
1.3877787807814457e-16

Therefore, always take sys.float_info.epsilon into account:

>>> x = 1.0
>>> while abs(x) > sys.float_info.epsilon:
...     print x
...     x -= 0.1
...
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1

来源：https://stackoverflow.com/questions/27214184/round-function-does-not-work-in-v2-6-6-but-works-in-v3-4-2