Note for Taipei.py 2016/06/30 Meetup

Python 中常遇到的小錯誤 - Tim

Space vs Tab

若是縮排減少，會把堆疊 pop 到縮排與堆疊頂端一致。

Default Argument

def play(song_list=[]):  
    song_list.append('a random song')  
    print(song_list)  

play()  
play()

=> 

['a random song']  
['a random song', 'a random song']

初始化只會初始一次

Closure

def outer():  
    outer_variable = 10  

    def common():  
        print(outer_variable)  

    outer_variable = 100  

    return common

=> 

outer()()  

100
  • 當產生 Closure 的時候，Python 並沒有執行函數內的程式。
  • 裡面的變數還沒有取得 reference，Python 只是記住變數名字而已(Lexical Scope)
  • 被 closure 記住的變數，不會立刻被 GC。 
def outer():  
    outer_variable = 10  

    def common(parameter=outer_variable):  
        print(parameter)  

    outer_variable = 100  

    return common

=> 

outer()()  

10
  • 改成用 class, 並使用 __call__。資源抓取會更清楚，而不是依賴 lexical scope。
  • 或是使用 functool.partial

Global variable

value = 10  

def change():  
    value = 20  

change()  
print(value)  

=> 10  

value = 10  

def change():  
    value = value + 20  

change()  
print(value)  

=> UnboundError  

value = 10  

def change():  
    global value  
    value += 10  

change()  
print(value)  

=> 20

del

class SimpleType(object):  
    def __init__(self):  
        print('simple born')  
    def __del__(self):  
        print('simple born')  

simple = SimpleType()  
del simple  

class FooType(object):  
    def __init__(self, parent):  
        self.parent = parent  
        print('Foo born')  

    def __del__(self):  
        print('Foo died')  

class BarType(object):  
    def __init__(self):  
        self = FooType(self)  
        print('Bar born')  

    def __del__(self):  
        print('Bar died')  

=>  
Foo born  
Bar born
  • del 只會讓名字從 symbol table 消失，並讓 reference count - 1
  • 當 reference count 為 0 的時候，才會執行 GC 的動作(__del__)
  • 可以解決 Circular Reference 的問題
  • 有 circular reference 的時候 Python 不會真的執行 del

Interface

from abc import ABCMeta, abstractmethod  

class Sized:  
    __metaclass__ = ABCMeta  

    @abstractmethod  
    def __len__(self):  
        return 0  

class Missing(Sized):  
    pass  


Missing()  

=> TypeError  
    Can't instantiate abstract class Missing with abstract methods __len__  

class Struggle(object):  
    def __len__():  
        return 23  

isinstance(Struggle, Sized)  

=> True

Duck Typing

List related

# Python 2  
data = range(10)  
for l in data:  
    data.remove(l)  
print(data)  
=> [1, 3, 5, 7, 9]  

# 跟 Python 的 list 實作採用 linked list 實作有關  

# Python 2  
data = range(10)  
for l in data[:]:  
    data.remove(l)  
print(data)  
=> []

Package

  • 不要 pip freeze > requirements.txt
    • 會把 dependency 的 dependency 也寫進去，不知道到底真正要用的是哪些。

2 => 3

Quality

  • flake8
    • install-hook

Inheritance

  • 正確使用 super() 可以避免很多問題 
class Base(object):  
    def __init__(self):  
        print("init Base")  

class Left(Base):  
    def __init__(self):  
        print("init Left")  

class Right(Base):  
    def __init__(self):  
        print("init Right")  

class C(Left, Right):  
    def __init__(self):  
        Left.__init__()  
        Right.__init__()  
        print("init C")  

=>  
init Base  
init Left  
init Base  
init Right

Base.__init__() 有副作用的話就會很恐怖
正確的寫法要用 super()

Related links

Physics of IOT - David Mikolas

pynsist

  • py2exe
  • pip install pynsist

Lightning talks

  • PyCon TW 2017 主席開放報名中

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