Interview

interview questions

wtfpython

  • 传参数不能传 list / dict / set 等对象，为什么？（mutable / immutable)
  • pass-by-value / pass-by-reference, all objects
  • 哪些是 immutable 的 (bool, number, str, tuple)，修改会发生什么？(TypeError, does not support item assignment)
  • list / tuple 的区别 (mutable / immutable)
  • super
  • is / == 区别
  • descriptor
  • 单下划线和双下划线: __foo__内部用, _foo 私有变量，不能 from module import *; __foo => _classname__foo

iterable / iterator / generator / yield

  • 将列表生成式中[]改成() 之后数据结构是否改变？(从列表变为生成器)
  • Everything you can use “for... in...” on is an iterable
  • Generators are iterators, a kind of iterable you can only iterate over once. Generators do not store all the values in memory, they generate the values on the fly
  • Stop
  • yield

__metaclass__ / __new__ / __init__

  • singleton

1 使用__new__方法 # 

class Singleton(object):
    def __new__(cls, *args, **kw):
        if not hasattr(cls, '_instance'):
            orig = super(Singleton, cls)
            cls._instance = orig.__new__(cls, *args, **kw)
        return cls._instance

class MyClass(Singleton):
    a = 1

2 共享属性 #

创建实例时把所有实例的__dict__指向同一个字典,这样它们具有相同的属性和方法.

class Borg(object):
    _state = {}
    def __new__(cls, *args, **kw):
        ob = super(Borg, cls).__new__(cls, *args, **kw)
        ob.__dict__ = cls._state
        return ob

class MyClass2(Borg):
    a = 1

3 装饰器版本 # 

def singleton(cls):
    instances = {}
    def getinstance(*args, **kw):
        if cls not in instances:
            instances[cls] = cls(*args, **kw)
        return instances[cls]
    return getinstance

@singleton
class MyClass:
  ...

4 import方法 #

作为python的模块是天然的单例模式

# mysingleton.py
class My_Singleton(object):
    def foo(self):
        pass

my_singleton = My_Singleton()

# to use
from mysingleton import my_singleton

my_singleton.foo()

scope: local => enclosing local => global => builtin

GIL: byte interpreter 上的锁

coroutine

copy

GC: reference counting, mark and sweep,

  • generation collection
    • 默认定义了三代对象集合，索引数越大，对象存活时间越长