使用for循环将多个值添加到字典中的一个键

Question

I'm using a similar method in a function I'm using. Why do I get a key error when I try to do this?

def trial():
     adict={}
     for x in [1,2,3]:
          for y in [1,2]:
               adict[y] += x
print(adict)

Answer 1

adict starts off empty. You can't add an integer to a value that doesn't already exist.

Answer 2

You did not initialize adict for each key. You can use defaultdict to solve this issue:

from collections import defaultdict
def trial():
    adict=defaultdict(int)
    for x in [1,2,3]:
        for y in [1,2]:
            adict[y] += x
    print(adict)
trial()

The result is defaultdict(<class 'int'>, {1: 6, 2: 6})

Answer 3

There is no value assigned to adict[y] when you first use it.

def trial():
     adict={}
     for x in [1,2,3]:
          for y in [1,2]:
               if y in adict: # Check if we have y in adict or not 
                   adict[y] += x
               else: # If not, set it to x
                   adict[y] = x
     print(adict)

Output:

>>> trial()
{1: 6, 2: 6}

Answer 4

You should modify youre code like this:

def trial():
   adict={0,0,0}
   for x in [1,2,3]:
      for y in [1,2]:
           adict[y] += x
print(adict)

"adict" has no entries. So adict[1] fails because it is accessing a variable that does not exist.