对于python中的循环，在每次迭代时初始化一个外部变量

Question

I can't understand why the following codes gives different results:

from datetime import datetime

def foo():
    return {datetime.now() : True}

a = {}
a.update(foo())
a.update(foo())
a.update(foo())

And:

a = {}
for i in xrange(3):
    a.update(foo())

In the former a ends up with three elements, while in the later a ends up with just one element (from the last iteration).

Answer 1

Just your luck. The second code is likely to execute in a single millisecond (or microsecond, depending on your OS), and give three identical datetimes, first is less likely to do so (on your system/your way of executing code). Keep trying, you might end up with two elements. Don't write code that depends on luck.

Answer 2

The number of elements you end up with will depend on how many calls to foo Python executes within the granularity of datetime.now() .

If you're running this in an interactive console then between statements executed at the prompt the console will perform housekeeping (for example, displaying the >>> prompt) that will significantly delay the next statement, whereas for the for loop the loop will be executed in its entirety before the console does anything.