Python : Identifer

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菜鳥から菜鳥同級生への理解Pythonの二重下線命名

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¶