An identifier occurring as an atom is a name.

Private name mangling

When an identifier that textually occurs in a class definition begins with two or more underscore characters and does not end in two or more underscores, it is considered a private name of that class. Private names are transformed to a longer form before code is generated for them. The transformation inserts the class name, with leading underscores removed and a single underscore inserted, in front of the name.
For example, the identifier __spam occurring in a class named Ham will be transformed to _Ham__spam. This transformation is independent of the syntactical context in which the identifier is used.
!Note:

• If the transformed name is extremely long (longer than 255 characters), implementation defined truncation may happen.
• If the class name consists only of underscores, no transformation is done.

for example

• 1

class A(object):
        def __init__(self):
                self.__name = 'Apple'

>>> a = A()
>>> a.__name
...
AttributeError: 'A' object has no attribute '__name'

>>> a._A__name
Apple

• 2

class A(object):
       def __init__(self):
              self.__private()
              self.public()
       def __private(self):
              print 'A.__private()'
       def public(self):
              print 'A.public()'
class B(A):
       def __private(self):
              print 'B.__private()'
       def public(self):
              print 'B.public()'

>>> b = B()
A.__private()
B.public()

• 3

class C(A):
       def __init__(self):          
              self.__private()       
              self.public()
       def __private(self):
              print 'C.__private()'
       def public(self):
              print 'C.public()'

>>> c = C()
C.__private()
C.public()

• 4

>>> class A(object):
       def __init__(self):
              self._A__private() 
              self.public()
       def __private(self):
              print 'A.__private()'
       def public(self):
              print 'A.public()'

>>>a = A()
A.__private()
A.public()

• 5

class __(object):
        def __init__(self):
                self.__name = 'Noooo'

>>> a = __()
>>> a.__name
Noooo

• read more
  从菜鸟到菜菜鸟 同学的 理解Python的双下划线命名

UnboundLocalError

When a name is not found at all, a NameError exception is raised. If the current scope is a function scope, and the name refers to a local variable that has not yet been bound to a value at the point where the name is used, an UnboundLocalError exception is raised. UnboundLocalError is a subclass of NameError.

for example

• 1

>>> x = 10
def bar():
  print x
...
>>> bar()
10

• 2

>>> x = 10
def foo():
    print x
    x += 1
...
>>> foo()
...
UnboundLocalError: local variable 'x' referenced before assignment

This is because when you make an assignment to a variable in a scope, that variable becomes local to that scope and shadows any similarly named variable in the outer scope. Since the last statement in foo assigns a new value to x, the compiler recognizes it as a local variable. Consequently when the earlier print x attempts to print the uninitialized local variable and an error results.

• 3

>>> x = 10
def foobar():
    global x
    print x
    x += 1
...
>>> foobar()
10
>>> x
11

If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

• read more
  unboundlocalerror

free variable

If a variable is used in a code block but not defined there, it is a free variable.

>>> i = 10
def f():
    print(i)
...
>>> i = 42
>>> f()
42

Name resolution of free variables occurs at runtime, not at compile time.

• read more
  Naming and binding¶