# python下划线\_命名

refer:<https://blog.csdn.net/tcx1992/article/details/80105645>

python的几种下划线意义：

* 单前 `_var`：约定、非强制，起提示作用
* 单后 `var_`：约定、非强制，避免关键字
* 双前 `__var`：特殊意义，会触发修饰名称
* 前后 `__var__`：特殊方法定义

## 单前 `_var`：

```python
# 变量中使用，你看，他能直接访问里面的变量
class Test:
   def __init__(self):
       self.foo = 11
       self._bar = 23

>>> t = Test()
>>> t.foo
11
>>> t._bar
23


# 函数使用，你看，前下划线的时候，从模块中*通配符*导入，既然报错
# my_module.py:
def external_func():
   return 23

def _internal_func():
   return 42

>>> from my_module import *
>>> external_func()
23
>>> _internal_func()
NameError: "name '_internal_func' is not defined"

# 函数使用，你看，前下划线的时候，从模块中指名道姓，才不会出问题
>>> import my_module
>>> my_module.external_func()
23
>>> my_module._internal_func()
42
```

## 单后 `var_`

主要（约定）用于避免关键字

```python
>>> def make_object(name, class):
SyntaxError: "invalid syntax"

>>> def make_object(name, class_):
...    pass
```

## 双前 `__var`：

特殊意义，会触发修饰名称，不能在外面直接访问

```python
class Test:
   def __init__(self):
       self.foo = 11
       self._bar = 23
       self.__baz = 23

# 会自动修饰 双前下划线变量 名称，变成 _Test__baz
>>> t = Test()
>>> dir(t)
['_Test__baz', '__class__', '__delattr__', '__dict__', '__dir__',
'__doc__', '__eq__', '__format__', '__ge__', '__getattribute__',
'__gt__', '__hash__', '__init__', '__le__', '__lt__', '__module__',
'__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__',
'__setattr__', '__sizeof__', '__str__', '__subclasshook__',
'__weakref__', '_bar', 'foo']

# 访问定义的 双前下划线变量，无法访问（变成了 _Test__baz）
>>> t2 = ExtendedTest()
>>> t2.foo
'overridden'
>>> t2._bar
'overridden'
>>> t2.__baz
AttributeError: "'ExtendedTest' object has no attribute '__baz'"

# 可以这样访问
>>> t2._Test__baz
42
```

```python
# 继承之前的Test，全部重写变量定义
class ExtendedTest(Test):
   def __init__(self):
       super().__init__()
       self.foo = 'overridden'
       self._bar = 'overridden'
       self.__baz = 'overridden'

# 双前下划线变量 的无法访问，其它可以访问且被重写了
>>> t2 = ExtendedTest()
>>> t2.foo
'overridden'
>>> t2._bar
'overridden'
>>> t2.__baz
AttributeError: "'ExtendedTest' object has no attribute '__baz'"

# 查看变量，多了 _ExtendedTest__baz, _Test__baz
>>> dir(t2)
['_ExtendedTest__baz', '_Test__baz', '__class__', '__delattr__',
'__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__',
'__getattribute__', '__gt__', '__hash__', '__init__', '__le__',
'__lt__', '__module__', '__ne__', '__new__', '__reduce__',
'__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__',
'__subclasshook__', '__weakref__', '_bar', 'foo', 'get_vars']

# 可以这样访问
>>> t2._ExtendedTest__baz
'overridden'
>>> t2._Test__baz
42
```

在程序内部编程时，`__`定义的是可以直接调用的

```python
# 定义的 __变量
class ManglingTest:
   def __init__(self):
       self.__mangled = 'hello'

   def get_mangled(self):#直接.__mangled调用
       return self.__mangled

>>> ManglingTest().get_mangled()
'hello'
# 但是类外面就是之前的样子
>>> ManglingTest().__mangled
AttributeError: "'ManglingTest' object has no attribute '__mangled'"


# 定义的 __函数 
class MangledMethod:
   def __method(self):
       return 42

   def call_it(self):
       return self.__method()

>>> MangledMethod().__method()
AttributeError: "'MangledMethod' object has no attribute '__method'"
>>> MangledMethod().call_it()
42
```

直接定义

```python
_MangledGlobal__mangled = 23

class MangledGlobal:
   def test(self):
       return __mangled

>>> MangledGlobal().test()
23
```

## 前后 `__var__`

不会修饰变量名称，用于特殊方法定义，如python的魔术方法，`__init`，这类命名风格最好不要自己定义，避免混乱

```python
class PrefixPostfixTest:
   def __init__(self):
       self.__bam__ = 42

>>> PrefixPostfixTest().__bam__
42
```

PS：\_ 单下划线，用于不考虑的填充变量

```python
# 不想使用的充当占位
a,_ =call(a,b)
for _ in range(32):
    print('Hello, World.')


# 未定义的会赋值给_
>>> 20 + 3
23
>>> _
23
>>> print(_)
23

>>> list()
[]
>>> _.append(1)
>>> _.append(2)
>>> _.append(3)
>>> _
[1, 2, 3]
```


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