float deviation in python list [duplicate]

问题

This question already has answers here:

Closed 8 years ago.

Possible Duplicate:
Python float - str - float weirdness

I run the following code in python on codepad.org:

num = 1.6
print num
list = [num]
print list
num2 = list[0]
print num2

And I get the following output:

1.6
[1.6000000000000001]
1.6

Why the tiny deviation in the list?

回答1:

list.__str__ calls repr on its elements, where as print calls str:

>>> str(1.6)
'1.6'
>>> repr(1.6)
'1.6000000000000001'

Since floating point numbers are not guaranteed to be exact (and cannot be exact for values that can't be expressed as a * 2^b for integers a,b), both representations are correct, or, in other words:

>>> 1.6 == 1.6000000000000001
True

来源：https://stackoverflow.com/questions/7374836/float-deviation-in-python-list