函数式编程

Python最佳实践模式

　　　组合方法composed method

将程序功能切分到每个方法，每个方法执行一个单独的任务，保证方法中所有操作都是在同一个抽象层次上。

不好的代码：

class Boiler:
    def safety_check(self):
        # Convert fixed-point floating point
        temperature = self.modbus.read_holding()
        pressure_psi = self.abb_f100.register / F100_FACTOR
        if psi_to_pascal(pressure_psi) > MAX_PRESSURE:
            if temperature > MAX_TEMPERATURE:
                # Shutdown!
                self.pnoz.relay[15] &= MASK_POWER_COIL
                self.pnoz.port.write('$PL,15\0')
                sleep(RELAY_RESPONSE_DELAY)
                # Successful shutdown?
                if self.pnoz.relay[16] & MASK_POWER_OFF:
                    # Play alarm
                    with open(BUZZER_MP3_FILE) as f:
                        play_sound(f.read())
      

下面好的代码展示不同层次的抽象：bitmaps, 文件系统操作 播放声音：

# Better
class Boiler:
    def safety_check(self):
        if any([self.temperature > MAX_TEMPERATURE,
                self.pressure > MAX_PRESSURE]):
            if not self.shutdown():
                self.alarm()
 
    def alarm(self):
        with open(BUZZER_MP3_FILE) as f:
            play_sound(f.read())
 
    @property
    def pressure(self):
        pressure_psi = abb_f100.register / F100_FACTOR
        return psi_to_pascal(pressure_psi)
    ...
      

• safety_check处理温度和压力
• alarm 处理文件和声音
• pressure 处理字节bit并转换它们

构造器方法Constructor method

提供构造器方法用来创建美观的实例，将所有参数都要传给这个方法：

# At initiation `point` is not well-formed
point = Point()
point.x = 12
point.y = 5
 
# Better
point = Point(x=12, y=5)
      

最后一行是推荐的好的实现方法。

使用类方法@classmethod制造多个构造器：

class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
 
    @classmethod
    def polar(cls, r, theta):
        return cls(r * cos(theta),
                   r * sin(theta))
 
point = Point.polar(r=13, theta=22.6)
      

围绕方法执行(in Python: Context manager)

f = open('file.txt', 'w')
f.write('hi')
f.close()
 
# Better好的代码如下
with open('file.txt', 'w') as f:
    f.write('hi')   
 
with pytest.raises(ValueError):
    int('hi')
 
with SomeProtocol(host, port) as protocol:
    protocol.send(['get', signal])
    result = protocol.receive() 
 
class SomeProtocol:
    def __init__(self, host, port):
        self.host, self.port = host, port
 
    def __enter__(self):
        self._client = socket()
        self._client.connect((self.host,
                                self.port))
 
    def __exit__(self, exception, value, traceback):
        self._client.close()
 
    def send(self, payload): ...
 
    def receive(self): ...
      

方法注解

一些小方法比注解（如下面 # Am I visible?）更有效。

if self.flags & 0b1000:  # Am I visible?
    ...
 
# Better
...
@property
def is_visible(self):
    return self.flags & 0b1000
 
if self.is_visible:
    ...
      

选择消息

# Bad
if type(entry) is Film:
    responsible = entry.producer
else:
    responsible = entry.author
 
# Shouldn't use type() or isinstance() in conditional --> smelly
 
# Better
class Film:
    ...
    @property
    def responsible(self):
        return self.producer
 
entry.responsible
      

意图显露消息

如何在intent 意图之间简单实现联系？

class ParagraphEditor:
    ...
    def highlight(self, rectangle):
        self.reverse(rectangle)
 
# Better
class ParagraphEditor:
    ...
    highlight = reverse  # More readable, more composable
      

不变性常量方法

_DEFAULT_PORT = 1234
 
class SomeProtocol:
    ...
    def __enter__(self):
        self._client = socket()
        self._client.connect(
            (self.host,
            self.port or _DEFAULT_PORT)
        )
        return self  
 
# If you want to subclass SomeProtocol, you would have to overwrite every method!
 
# Better
class SomeProtocol:
    _default_port = 1234
    ...
    def __enter__(self):
        self._client = socket()
        self._client.connect(
            (self.host,
            self.port or self._default_port))
      

直接或间接的变量访问

直接是不需要getters和 setters

class Point:
    def __init__(self, x, y):
        self.x, self.y = x, y
 
# Sometimes need more flexibility --> use properties
 
class Point:
    def __init__(self, x, y):
        self._x, self._y = x, y
 
    @property
    def x(self):
        return self._x
 
    @x.setter
    def x(self, value):
        self._x = value
      

Enumeration (iteration) 枚举方法

# Bad
# Can't change type of collection
#     e.g. can't change employees from a list to a set
class Department:
    def __init__(self, *employees):
        self.employees = employees
 
for employee in department.employees:
    ...
    
# Better
class Department:
    def __init__(self, *employees):
        self._employees = employees
 
    def __iter__(self):
        return iter(self._employees)
 
for employee in department:  # More readable, more composable
    ...
    
      

临时变量

# Meh
class Rectangle:
    def bottom_right(self):
        return Point(self.left + self.width,
                    self.top + self.height)
 
# Better to use temporary variables for readability
class Rectangle:
    ...
    def bottom_right(self):
        right = self.left + self.width
        bottom = self.top + self.height
        return Point(right, bottom)
      

Sets

item in a_set
item not in a_set
 
# a_set <= other
a_set.is_subset(other)
 
# a_set | other
a_set.union(other)
 
# a_set & other
a_set.intersection(other)
 
# a_set - other
a_set.difference(other)
      

等同方法

obj == obj2
obj1 is obj2
 
class Book:
    ...
    def __eq__(self, other):
        if not isinstance(other, self.__class__):
            return NotImplemented
        return (self.author == other.author and 
                self.title == other.title)
      

HASH哈希方法

class Book:
    ...
    def __hash__(self):
        return hash(self.author) ^ hash(self.other)
      

排序集合 collection

class Book:
    ...
    def __lt__(self, other):
        return (self.author, self.title) < (other.author, other.title)
      

级联Concatenation

class Config:
    def __init__(self, **entries):
        self.entries = entries
 
    def __add__(self, other):
        entries = (self.entries.items() + 
                    other.entries.items())
        return Config(**entries)
default_config = Config(color=False, port=8080)
config = default_config + Config(color=True)
      

简单枚举参数

# Awkward
for options_shortcut in self.options_shortcuts:
    options_shortcut.this()
    options_shortcut.that()
 
# Better
for each in self.options_shortcuts:
    each.this()
    each.that()   
      

Cascades级联

不要让方法返回值，而是返回它们自己：

# Instead of this
self.release_water()
self.shutdown()
self.alarm()
 
class Reactor:
    ...
    def release_water(self):
        self.valve.open()
        return self
 
self.release_water().shutdown().alarm()   
      

显式好于隐式

# Meh
def match(self, items):
    for each in items:
        if each.match(self):
            return each
# Is this supposed to reach the end? Is this a bug?
 
# Better
def match(self, items):
    for each in items:
        if each.match(self):
            return each
    return None
      

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