为什么 dict({4:None}) 返回 {4: None} 而 dict([{4,None}]) 在 Python 中返回 {None: 4} 3.11

Question

I was fiddling with dict() and found out that when I do

>>> dict({4:None}) it returns {4: None} which is expected but,

when I try converting a list containing a nested set [{4,None}]

>>> dict([{4,None}]) returns {None: 4} which genuinely unexpected.

Then I checked that it only happens with few integers. That is crazy.

eg dict([{5,None}]) returns {None: 5}

dict([{6,None}]) returns {None: 6}

dict([{7,None}]) returns {None: 7}

although

dict([{8,None}]) returns {8: None}

dict([{9,None}]) returns {9: None}

Can anyone answer this? I am using Python 3.11

Answer 1

The dict built-in supports three ways of creating a dict:

class dict(**kwargs)
class dict(mapping, **kwargs)
class dict(iterable, **kwargs)

The first way is like dict(a=0, b=1) . The second way is similar like dict({4:None}) used in the question. The third way is the one of interest here, the normal usage is providing an iterable of key and value pairs, eg like this:

>>> data = [("a", 0), ("b", 1), ("c", 2)]
>>> dict(data)
{'a': 0, 'b': 1, 'c': 2}

By using a list containing a set of two elements, you're accidentally triggering that code path. Sets are not ordered, so whether you get back {None: 4} or {4: None} here is arbitrary.