Python：字典中的键也被视为变量吗？

Question

I've just recently started learning programming in python as a beginner. I was wondering: are keys in dictionary also treated as variables?

students = {"Jake": 12, "Rachel":12, "Ross":15}

the code contains the student names and their age. For example: Is "Jake" a variable that contains the value 12? or is it treated as a variable?

Answer 1

While you can use a named value (or what you might think of as a 'variable') to construct a dictionary:

>>> x=22
>>> di={x:x}
>>> di
{22: 22}

You can also demonstrate that the value of the named value (if that value is immutable) is used at the time of construction and not dynamic:

>>> x=5
>>> di
{22: 22}

The keys of a dict must be hashable (ie, unchanging; immutable) which would preclude the use of a list, set, or other named value that can change:

>>> changeable_list=[22]
>>> di={changeable_list:changeable_list}
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

But the value of a dict can be dynamic and mutable:

>>> di={tuple(changeable_list):changeable_list}
>>> di
{(22,): [22]}

And that list can be modified:

>>> changeable_list.append(44)
>>> changeable_list
[22, 44]
>>> di
{(22,): [22, 44]}

Notice how the value of the dict changes as the list changes because they are the same object.

Answer 2

As has been said, No, the keys of a dictionary are not variables. However, variables can sometimes key thought of as keys:

students = {"Jake": 12, "Rachel":12, "Ross":15}

class Students:
    pass

s = Students()
s.Jake = 12
print(s.Jake, students['Jake'])

print(getattr(s, 'Jake'))

Output:

12 12
12

There is no syntax that I'm aware of that would allow you to access the value 12 from students in this form: students.Jake

However, modify the above code:

class Students:
    def __getitem__(self, key):
        return getattr(self, key)

... # as before
print(getattr(s, 'Jake'), s['Jake'])

Output:

12 12

Now students has an operator[] a little like a dictionary (Other operators might also be necessary.)

So, an object can become like a dictionary because they have an underlying dictionary to make them work as objects.

Answer 3

TL;DR - it depends on what you mean by a "variable". dicts act like python namespaces but allow more types of variable names.

Python objects have no inherent name. They can be referenced by one or more other objects and when their reference count goes to zero, they are deleted. When you assign an object to a variable, some data structure adds a reference to the the object and associates that object with the name. That data structure is the variable's namespace (that is, the context where the variable name is valid). And for most objects, that data structure is a dict .

Lets look at two examples:

class Students:
    pass

student_obj = Students()

and

student_dct = {}

I could treat Jake as aa variable

>>> student_obj.Jake = 12
>>> student_obj.Jake
12
>>> student_obj.__dict__
{'Jake': 12}

Or add it to the dict

>>> student_dct["Jake"] = 12
>>> student_dct["Jake"]
12
>>> student_dct
{'Jake': 12}

That's really close to the first example! The advantage to a variable is that it is parsed by python and python does the lookup for you. Python turns student_obj.Jake into student_obj.__getattribute__("Jake") . For normal class objects, python will check the object __dict__ for the name then fall back to containing namespaces. Classes that use __slots__ or are implemented in C follow different rules.

But variable assignment is a disadvantage if you want to use names that don't fit python's sytax rules.

>>> student_obj.Jim Bob = 12
  File "<stdin>", line 1
    student_obj.Jim Bob = 12
                      ^
SyntaxError: invalid syntax

Here, you want "Jim Bob" to be a variable but it can't be used directly as a variable because it breaks python. So, you put it into a dict

>>> student_dct["Jim Bob"] = 12

So, dictionary items are "variables" (the value can be referenced and reassigned) but are not "python variables" because python doesn't implement the lookup for you.