如何在Python中引用类中的静态属性？

Question

I have the following python snippet:

class myClass:
    myVar = 'a'

    def __init__(self):
        self.myOtherVar = 'b'
        myVar = 'c'  # Gets assigned but only as a local variable.

    print myVar            # prints 'a' !
    print self.myOtherVar  # says 'self' not found

My question is this; What's the proper way to print the contents of myVar from within myClass and/or re-assign them from init ?

Answer 1

The problem you are facing is because you don't understand how the scoping of class declarations work. A class declaration is executed in its own scope. After the execution is completed a new class object is created and the obtained scope is attached to the class as its __dict__ .

Note : the class scope is not searched from within the methods scopes! This means that you have to reference class attributes as MyClass.attribute when inside a method definition.

For example:

class MyClass:
    var = 1

    # we are executing this code as a single block
    # so you must reference the variable as is usual
    print(var)

    # default values are *not* inside the definition.
    # they are evaluated in the outer scope, so use plain "var" here
    def method(self, a_default=var):
        print(a_default)

    def other_method(self):

        # inside methods you are in a different scope
        print(MyClass.var)

        # equivalent *if* no "var" instance attributes exists
        print(self.var)

Note: since the class doesn't still exist when executing its declaration you cannot refer to MyClass at the "top level" of MyClass declaration:

class MyClass:
    var = 1
    print(MyClass.var)   # error: MyClass still doesn't exist.

---

A side effect of this, is that the following code:

class MyClass:
    x = 1
    results = list(i+x for i in range(10))

Produces:

NameError                                 Traceback (most recent call last)
<ipython-input-6-f1d4417b2e52> in <module>()
----> 1 class MyClass:
      2     x = 1
      3     results = list(i+x for i in range(10))
      4 

<ipython-input-6-f1d4417b2e52> in MyClass()
      1 class MyClass:
      2     x = 1
----> 3     results = list(i+x for i in range(10))
      4 

<ipython-input-6-f1d4417b2e52> in <genexpr>(.0)
      1 class MyClass:
      2     x = 1
----> 3     results = list(i+x for i in range(10))
      4 

NameError: name 'x' is not defined

Because generator expressions (and list-comprehensions in python3) are, in fact, considered functions with their own scope. Since the class scope isn't searched from inner function scopes the x cannot be found.

You can word around this using a function definition and default values:

class MyClass:
    x = 1
    def _make_results(x=x):
        return list(i+x for i in range(10))
    results = _make_results()
    del _make_results    # otherwise it would be added as a method.
    # or:
    results = (lambda x=x: list(i+x for i in range(10)))()

This isn't usually a problem since class definitions rarely contain anything other than method definitions and a few constants.

---

There are already a few questions on SO about class scopes:

• The scope of names defined in class block doesn't extend to the methods' blocks. Why is that?
• Short Description of the Scoping Rules?
• Nested classes' scope?
• Variable scopes in python classes
• Why static binding works differently for class and function?

Answer 2

self.var will:

1. give var in self.__dict__ if present
2. give var in self.__class__.__dict__ if present
3. AttributeError

So use this or self.__class__.var if you want to access the static variable minding inheritance . If you extend myClass , the children instances will access the static variable in the child class.

If you want to access the static variable in myClass even when called from descendants, use myClass.var .

As for reassigning them, this must be done explicitly on the class object, or the assignment will just target the instance.

Answer 3

class myClass:
    myVar = 'a'

    def __init__(self):
        self.myOtherVar = 'b'

    print myVar  # -> 'a'

    class EmptyClass: pass
    s = EmptyClass()
    __init__(s)

    myVar = s.myOtherVar
    print myVar  # -> 'b'

print myClass.myVar  # -> 'b'