我正在创建一个字典，但我真的想要一个列表，这种情况下的语法是什么？

Question

In a for loop I have the following:

aContract = {'name': 'abc', 'type': 'xyz'}
if 'contracts' in alist:
    alist["contracts"].append(aContract)
else:
    alist["contracts"] = aContract

So json is being returned to me and inside the json we have bool's, dict's, list's etc. 'contracts' is in this json as a list. I can append things to it just fine when my code is in the if statement. However, when 'contracts' does not exist (we are in the else statement), I'm trying to create it (unsuccessfully) and add aContract to it. So the else statement makes it a dict and the next time through when I try to run

alist["contracts"].append(aContract)

I get the error:

AttributeError: 'dict' object has no attribute 'append'

I created a dict when I wanted a list. So how do I make 'contracts' exist as a list and then add 'aContract' to it?

Answer 1

Put the dictionary in a literal list:

aContract = {'name': 'abc', 'type': 'xyz'}
if 'contracts' in alist:
    alist["contracts"].append(aContract)
else:
    alist["contracts"] = [aContract]

Better still, use dict.setdefault() here to skip having to test for the key:

aContract = {'name': 'abc', 'type': 'xyz'}
alist.setdefault("contracts", []).append(aContract)

dict.setdefault() will set the key to the default value given if not yet present, then returns the value. So, if alist has no key 'contracts' , it'll set the value to an empty list.