在Python中将项目添加到aa字典中

Question

I have a dictionary that store the values as lists for each key. For example:

dict1={}
dict1["A"]=[]

I want to append numbers into this list now, but how do I do this correctly? I tried dict1["A"]=dict1["A"].append(1)

this only appended "None" . How do I do this?

Answer 1

You just need to call append()

dict1["A"].append(1)

Since the return value of append() itself is None, your version just replaced the old value (a list) with None after you successfully added the item to the list.

A quick demo:

>>> dict1 = {'A': []}
>>> dict1['A'].append(1)
>>> dict1
{'A': [1]}

In Python, in-place operations such as appending to the list, return None :

>>> alist = []
>>> alist.append(1) is None
True
>>> alist
[1]

but as you can see, the list itself was changed.

Answer 2

No need to reassign. Just do dict1["A"].append(1) .

The mistake you have done is dict1["A"].append(1) returns None and that you were assigning it back to dict1 . Which explains the None you were getting...

Answer 3

That's because append() changes the list in-place and returns None . In your code you assigned that returned Value to dict1["A"]

In [25]: dict1={}

In [26]: dict1["A"]=[]

In [27]: dict1["A"].append(1)  #try print dict1["A"].append(1) here

In [28]: dict1
Out[28]: {'A': [1]}