time.time() and datetime.datetime.now() does not get current time when used in a descriptor

Question

When coding in Python, the common way to get the current time is time.time() or datetime.datetime.now(). But I never doubted their result until this code:

import time

class A(object):
    def __get__(self, instance, owner):
        print '__get__ is run'
        return time.time()
    def __set__(self, instance, value):
        print('let me set, but i will not', value)

class C(object):
    a = A()

c1 = C()
c2 = C()
print(c1.a)
c1.a = time.time()
print(c1.a)
print('-' * 20)
print(c2.a)

The result of this code is:

__get__ is run
1512115138.19
('let me set, but i will not', 1512115138.188)
__get__ is run
1512115138.19
--------------------
__get__ is run
1512115138.19
[Finished in 0.2s]

What happened on earth? Why do these three time.time() calls get the same result? The same result is for datetime.datetime.now(). And the same result in python 2 and python 3. I can't find some python technique to explain it.

This should help you https://stackoverflow.com/questions/415511/how-to-get-current-time-in-python — Krzysztof, Dec 01 '17 at 08:17

score 2 · Answer 1 · answered Dec 01 '17 at 08:13

2

Actually, it is a problem about precision, check this link. You can see a more precise output:

__get__ is run
1512115904.6300762
let me set, but i will not 1512115904.631677
__get__ is run
1512115904.6346197
--------------------
__get__ is run
1512115904.6372328

answered Dec 01 '17 at 08:13

Sraw

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So it is:) Thank you! – 张笑凝 Dec 01 '17 at 08:20

time.time() and datetime.datetime.now() does not get current time when used in a descriptor

1 Answers1