这个python嵌套循环的工作方式是什么？

Question

Can anyone explain why the output of the following nested loop is {1:6, 2:6, 3:6} ?

>>> {x:y for x in [1, 2, 3] for y in [4, 5, 6]}
{1:6, 2:6, 3:6}

Answer 1

my_dict = {x:y for x in [1,2,3] for y in [4,5,6]}

is the same is creating it as follows

my_dict = {}
for x in [1,2,3]:
    for y in [4,5,6]:
        my_dict[x] = y

Which would look like this if you unroll the loops:

my_dict = {}
my_dict[1] = 4
my_dict[1] = 5
my_dict[1] = 6
my_dict[2] = 4
my_dict[2] = 5
my_dict[2] = 6
my_dict[3] = 4
my_dict[3] = 5
my_dict[3] = 6

You are effectively inserting nine key value pairs into the dictionary. However, each time you insert a pair with a key that already exists it overwrites the previous value. Thus you only ended up with the last insert for each key where the value was six.

Answer 2

The difference is you are making a dictionary vs a list. In your own example, you are effectively constructing a dictionary and because you set a different value for the same key 3 times, the last value sticks.

You are effectively doing:

dict[1] = 4
dict[1] = 5
dict[1] = 6
...
dict[3] = 4
dict[3] = 5
dict[3] = 6

So the last value sticks.

Answer 3

如果期望创建{1:4, 2:5, 3:6} ，请尝试以下操作：

{x[0]:x[1] for x in zip([1,2,3], [4,5,6])}