Python: Referencing and assignment in inner functions

这一篇的内容在 Digest of Fluent Python: 7.4 Variable Scope Rules 和 Python: Variable scope in if-statement 都有讲到，但是 Effective Python 的 Item 15: Know How Closures Interact with Variable Scope 讲得特别好，特摘录如下。

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When you reference a variable in an expression, the Python interpreter will traverse the scope to resolve the reference in this order:

1. The current function’s scope
2. Any enclosing scopes (like other containing functions)
3. The scope of the module that contains the code (also called the global scope)
4. The built-in scope (that contains functions like len and str)

If none of these places have a defined variable with the referenced name, then a NameError exception is raised.

Assigning a value to a variable works differently. If the variable is already defined in the current scope, then it will just take on the new value. If the variable doesn’t exist in the current scope, then Python treats the assignment as a variable definition. The scope of the newly defined variable is the function that contains the assignment.

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看三个例子：

def foo_1():
    found = False
    
    def bar():
        found = True
        
    bar()
    print(found)
    
foo_1()

# Output: False

def foo_2():
    found = False
    
    def bar():
        nonlocal found
        found = True
        
    bar()
    print(found)
    
foo_2()

# Output: True

def foo_3():
    found = [False]
    
    def bar():
        found[0] = True
        
    bar()
    print(found[0])
    
foo_3()

# Output: True

• foo_1 里的 bar 的 found = True 是一句 assignment，但是 found 在 bar 的 scope 下找不到，所以变成了 variable definition，相当于内部外部各有一个 found，你内部的 found 的赋值不会影响到外部的值。
• foo_2 这是最常见的解决 foo_1 问题的写法，用 nonlocal，告诉 interpreter 不要在 bar 的 scope 下去找 found。这么一来内外 reference 到的都是同一个 found。
• foo_3 是 python 2 里 nonlocal 的 workaround，因为 python 2 里并没有 nonlocal。它的巧妙之处在于 found[0] = True 这句并不是对 found 的赋值。所以这里内外都是 reference 到同一个 found，相当于是改变了 found 的内部状态，但是并没有 assign 一个新对象给 found。
  • 同理可知，你在 inner function 里修改外部对象的状态也不算是 assignment，比如下面的 foo_4。

class Found:
    def __init__(self, boolean):
        self.value = boolean
        
def foo_4():
    found = Found(False)
    
    def bar():
        found.value = True
        
    bar()
    print(found.value)
    
foo_4()

# Output: True