清单不是清单时的Python吗？ 列表对象没有属性拆分

Question

I have a list of user ids being sent to my API:

Users = ['x1','x2']

In my serializer create method I try to iterate over them:

    users = validated_data.get('users', None)
    for user in users:
        print(user)
        print("===")

The output I receive is:

> x1,x2
> ===

It instead of iterating over the list, it outputs as one line! This suggests to me it is not a list, but a string, so I checked the type(users) which gave a blank ie no type.

So I tried splitting users up users.split() Then I get a contradiction" " list object has no attribute split "!

What is wrong here, confused?

Answer 1

It seems that users is a list that looks like this: ['x1,x2'] . Instead of what you expected: ['x1','x2']

You can use ast.literal_eval to make a list first, or you can just split that one element:

users = ['x1,x2']
for user in users[0].split(','):
    print(user)
    print('===')

Output:

>>> users = ['x1,x2']
>>> for user in users[0].split(','):
...    print(user)
...    print('===')


x1
===
x2
===

A per your comment:

from ast import literal_eval
users = ['[1,2]']

users = literal_eval(users[0])
for user in users:
    print(user)
    print('===')

Output:

1
===
2
===

For users = ['[x1,x1]'] :

users = ['[x1,x1]']
users = users[0][1:-1]

for user in users.split(','):
    print(user)
    print('===')

Answer 2

It looks as is users is a list with a single item x1,x2 :

>>> users = ['x1,x2']
>>> for user in users:
...     print(user)
...     print("===")
... 
x1,x2
===

Answer 3

Others have pointed out that you seem to have a list containing a single string, ['x1, x2'] .

I took a look at the Django REST Framework internals, and it's definitely returning a list.

ListField inherits from Field , which defines a few methods, including run_validation , to_internal_value , and to_representation .

Two of those methods are abstract, and one of them, run_validation actually invokes the validation by calling self.to_internal_value(data) .

So to see what the validator is doing, we have to look at the ListField's implementation of to_internal_value .

The comment inside of to_internal_value says this:

"""
    List of dicts of native values <- List of dicts of primitive datatypes.
"""

Then it checks for invalid input types and finally, calls run_validation .

According to my IDE, there are 5 implementations of run_validation in Django REST Framework. The most relevant one is probably ListSerializer .

The comments above ListSerializer tell us that we're probably in the right place:

# There's some replication of `ListField` here,
# but that's probably better than obfuscating the call hierarchy.

The ListSerializer class validates each item (source) and then appends it to a list called ret . So we should be returning a list.

The unsolved piece of the puzzle here is what your input is that is causing the output to be incorrect, but tracing through the call stack, the code appears to be working as intended.

EDIT:

Could it be that to_representation is flattening your list because it thinks it's a dictionary?