双星号 (**) 运算符给出不同的结果

Question

I have the following code

from werkzeug.datastructures import ImmutableMultiDict
f = ImmutableMultiDict([('name', ''), ('desc', 'example'), ('species', '1747')])
def print_data(name, desc, species):
    print(desc)

print_data(**f)

On python 3.6.1 the result is

['example']

And on python 3.7.2 the result is

'example'

I couldn't find any information about this change. And is seems strange that it is the result of the python version (BUT everything else is the same (werkzeug v1.0.1))

Am I right and this is a python thing? Can anyone point me to the release notes (Yes, I searched and didn't find...)

Answer 1

Yes, CPython has changed behaviour in 3.7.1. Paraphrasing this bug report on Werkzeug, which also contains the solution:

CPython [..] changed [..] the implementation of **kwargs to unpack using data.items() instead of dict.items(data) . MultiDict.items() always returned the first value, so CPython [was] previously [..] using dict.items(data) [..]

 m = MultiDict({"x": [1, 2]}) m.items() [("x", 1)] dict.items(m) [("x", [1, 2])]

There's not really anything Werkzeug can do about this, we're not going to change how MultiDict.items() works.

I'm not even sure if **MultiDict was ever intended to work. Use data.as_dict(flat=False) to get a dict to unpack.

 m.as_dict(flat=False) {"x": [1, 2]} fn(**m.as_dict(flat=False)) {"x": [1, 2]}