为什么运行python解释器和python代码之间的结果不同？

Question

I made a simple code on python interpreter and run it.

Python 3.5.3 (v3.5.3:1880cb95a742, Jan 16 2017, 16:02:32) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> x=np.array([0,1])
>>> w=np.array([0.5,0.5])
>>> b=-0.7
>>> np.sum(w*x)+b
-0.19999999999999996

the result -0.19999999999999996 is weird. I think.... it is caused by IEEE 754 rule. But when I try to run almost same code by file, result is a lot different.

import numpy as np
x = np.array([0,1])
w = np.array([0.5,0.5])
b = -0.7
print(np.sum(w * x) + b)

the result is "-0.2". IEEE 754 rule does not affect the result.

what is the difference between file based running and interpreter based running?

Answer 1

The difference is due to how the interpreter displays output.

The print function will try to use an object's __str__ method, but the interpreter will use an object's __repr__ .

If, in the interpreter you wrote:

...
z = np.sum(w*x)+b
print(z)

(which is what you're doing in your code) you'd see -0.2 .

Similarly, if in your code you wrote:

print(repr(np.sum(w * x) + b))

(which is what you're doing in the interpreter) you'd see -0.19999999999999996

Answer 2

我认为不同之处在于，您使用print()作为基于文件的代码（转换数字），而在解释器的情况下，您不使用print() ，而是要求解释器显示结果。