python 中的字典理解问题

Question

A=[1,2,3,4,5]
B=['a','b','c']

I want output:

{1: 'a', 2: 'b', 3: 'c', 4: None, 5: None}

We can have any number of keys.

I want to create a dictionary with these two lists using dictionary comprehension in python and list A will become keys and other is value for key 4 and 5 we have to print None and other keys with corresponding values how can we do it using dict comprehension. Thanks in advance.

Answer 1

[None]*(len(a)-len(b)) will create n (Here, it is len(a)-len(b) )) time None and you can use += to add them in to list:

b += [None]*(len(a)-len(b))

And you can make a dictionary by using dict() function:

dict(zip(a,b))

Full code:

a = [1, 2, 3, 4, 5]
b = ['a', 'b', 'c']

b += [None]*(len(a)-len(b))

dic = dict(zip(a, b))
print(dic)

---

Note :
If you don't know which is the longest list then you can use:

 if len(a) > len(b): b += [None]*(len(a)-len(b)) elif len(a) < len(b): a += [None]*(len(b)-len(a))

In len(a) < len(b) case None will be overwrite and value of None and it will be last item in b. To avoid that you can use dict(zip(b, a)) instead of dict(zip(a, b)) . Then you should change the full code as follow, but it changes your output (Ex: {'a': 1, 'b': 2, 'c': None} ):

 a = [1, 2] b = ['a', 'b', 'c'] if len(a) > len(b): b += [None]*(len(a)-len(b)) dic = dict(zip(a, b)) elif len(a) < len(b): a += [None]*(len(b)-len(a)) dic = dict(zip(b, a)) else: dic = dict(zip(a, b)) print(dic)

Answer 2

def dict_maker(keys, values):
    dict = {}
    for i, key in enumerate(keys):
        try:
            dict[key] = values[i]
        except IndexError:
            dict[key] = None
    return dict

Answer 3

The way to go is to use itertools.zip_longest , as already suggested in the comments. zip_longest uses None as default for the missing values in the shortest list, which is exactly what you want:

from itertools import zip_longest

out = dict(zip_longest(A, B))

. You can also achieve this without importing anything, by using zip , which will only consume the common start of the two list, then update this dict with another one which has the remaining values from the second list as keys, and None as values. dict_fromkeys builds exactly that by default:

A = [1, 2, 3, 4, 5]
B = ['a', 'b', 'c']

out = dict(zip(A, B))
out.update(dict.fromkeys(A[len(B):]))

print(out)
# {1: 'a', 2: 'b', 3: 'c', 4: None, 5: None}