Building Abstractions with Data
2.1 Introduction
2.1.1 Native Data Types
Python includes three native numeric types: integers (int), real numbers (float), and complex numbers (complex).
>>> type(2)
<class 'int'>
>>> type(1.5)
<class 'float'>
>>> type(1+1j)
<class 'complex'>
2.2 Data Abstraction
2.3 Sequences
2.3.1 Lists
>>> digits = [1, 8, 2, 7]
>>> len(digits) # 返回list长度
4
>>> digits[3]
8
>>> digits[-1] # 最后一个元素
7
# 这样也可以取list中的值
>>> digits = [1, 2, 3]
>>> x, y, z = digits # 注意这边必须和list中元素的数量对应上才能unpack
>>> x
1
>>> y
2
>>> z
3
List 的加法和乘法：
>>> [2, 7] + digits * 2
[2, 7, 1, 8, 2, 8, 1, 8, 2, 8]
List 还可以嵌套：
>>> pairs = [[10, 20], [30, 40]]
>>> pairs[1]
[30, 40]
>>> pairs[1][0]
30
2.3.2 Sequence Iteration
python for 循环
for <name> in <expression>:
<suite>
可以方便的遍历 List 中的元素
>>> def count(s, value):
"""Count the number of occurrences of value in sequence s."""
total = 0
for elem in s:
if elem == value:
total = total + 1
return total
>>> count(digits, 8)
2
Sequence unpacking:
for 循环中的<name>可以是多个元素
>>> pairs = [[1, 2], [2, 2], [2, 3], [4, 4]]
>>> same_count = 0
>>> for x, y in pairs:
if x == y:
same_count = same_count + 1
>>> same_count
2
Ranges:
>>> range(1, 10) # Includes 1, but not 10
range(1, 10)
>>> list(range(5, 8))
[5, 6, 7]
>>> list(range(4)) # 如果只给出一个参数，那么就是从0开始
[0, 1, 2, 3]
如果 for 循环中的<name>在循环中没有用到，那么可以使用_代替
>>> for _ in range(3):
print('Go Bears!')
2.3.3 Sequence Processing
List Comprehensions：
格式为：
[<map expression> for <name> in <sequence expression> if <filter expression>]
比如对 list 中每个元素加 1，可以用这种简单的方法：
>>> odds = [1, 3, 5, 7, 9]
>>> [x+1 for x in odds]
[2, 4, 6, 8, 10]
提取某些元素：
>>> [x for x in odds if 25 % x == 0]
[1, 5]
Aggregation：
将序列中的所有值聚合为单个值
>>> def divisors(n):
return [1] + [x for x in range(2, n) if n % x == 0]
>>> divisors(4)
[1, 2]
>>> divisors(12)
[1, 2, 3, 4, 6]
Higher-Order Functions:
>>> def reduce(reduce_fn, s, initial):
reduced = initial
for x in s:
reduced = reduce_fn(reduced, x)
return reduced
>>> reduce(mul, [2, 4, 8], 1)
64
2.3.4 Sequence Abstraction
membership:
提供了 in 和 not in 来判断 value 是否存在于 sequence。
>>> digits
[1, 8, 2, 8]
>>> 2 in digits
True
>>> 1828 not in digits
True
slicing:
>>> digits[0:2]
[1, 8]
>>> digits[1:]
[8, 2, 8]
python 切片还有更多的用法。
2.3.5 Strings
字符串用单引号或双引号包裹:
>>> 'I am string!'
'I am string!'
>>> "I've got an apostrophe"
"I've got an apostrophe"
>>> '您好'
'您好'
String 支持 len()函数和索引:
>>> city = 'Berkeley'
>>> len(city)
8
>>> city[3]
'k'
String 也支持加法和乘法操作：
>>> 'Berkeley' + ', CA'
'Berkeley, CA'
>>> 'Shabu ' * 2
'Shabu Shabu '
membership：
匹配子字符串返回 True, 否则返回 False
>>> 'here' in "Where's Waldo?"
True
Multiline Literals:
多行字符串使用三重引号
字符串中的\n 换行符属于 1 个元素。
String Coercion:
str()强制转换为字符串。
>>> str(2) + ' is an element of ' + str(digits)
'2 is an element of [1, 8, 2, 8]'
2.3.6 Trees
2.3.7 Linked Lists
2.4 Mutable Data
Python 中的一些对象，如列表和字典，是 Mutable 可变的，这意味着它们的内容或状态可以改变。其他对象，如数字类型、元组和字符串，是 Immutable 不可变的，这意味着它们一旦被创建就不能被更改。
2.4.2 Sequence Objects
List 支持一系列操作,包括 pop,remove,append,extend,insert 等。
>>> chinese = ['coin', 'string', 'myriad'] # A list literal
>>> suits = chinese # Two names refer to the same list
>>> suits.pop() # Remove and return the final element
'myriad'
>>> suits.remove('string') # Remove the first element that equals the argument
>>> suits.append('cup') # Add an element to the end
>>> suits.extend(['sword', 'club']) # Add all elements of a sequence to the end
>>> suits[2] = 'spade' # Replace an element
>>> suits
['coin', 'cup', 'spade', 'club']
Sharing and Identity
is 判断, 类似于指针判断，指向同一个 object 返回 true, 和==判断内容不同。
The former checks for identity, while the latter checks for the equality of contents.
suits = ['heart', 'diamond', 'spade', 'club']
nest[0] = suits
>>> suits is nest[0]
True
>>> suits is ['heart', 'diamond', 'spade', 'club']
False
>>> suits == ['heart', 'diamond', 'spade', 'club']
True
List comprehensions
List 是 mutable data。
A list comprehension always creates a new list.
>>> from unicodedata import lookup
>>> [lookup('WHITE ' + s.upper() + ' SUIT') for s in suits]
['♡', '♢', '♤', '♧']
This resulting list does not share any of its contents with suits
Tuples
元组。immutable data。只支持 count, index 操作，改变内容的操作不支持。
>>> code = ("up", "up", "down", "down") + ("left", "right") * 2
>>> len(code)
8
>>> code[3]
'down'
>>> code.count("down")
2
>>> code.index("left")
4
2.4.3 Dictionaries
key-value 对。key 一般是字符串也可以是任何其他的，可用来索引 value。
一个 key 只能对应一个 value。
>>> numerals = {'I': 1.0, 'V': 5, 'X': 10}
>>> numerals['X']
10
>>> numerals['I'] = 1 # 修改key对应的value
>>> numerals['L'] = 50
>>> numerals
{'I': 1, 'X': 10, 'L': 50, 'V': 5}
字典支持.keys(), values(), items()操作：
>>> d = {2: 4, 'two': ['four'], (1, 1): 4}
>>> for k in d.keys():
... print(k)
...
2
two
(1, 1)
>>> for v in d.values():
... print(v)
...
4
['four']
4
>>> for k, v in d.items():
... print(k, v)
...
2 4
two ['four']
(1, 1) 4
字典还支持 get 操作，传入 key 和 default value, 如果没找到对应的 value, 则返回传入的 default value。
>>> numerals.get('A', 0)
0
>>> numerals.get('V', 0)
5
in 用来判断 key 是否在字典内：
>>> 'two' in d
True
字典也支持 comprehension 操作来创建, 使用大括号。
>>> {x: x*x for x in range(3,6)}
{3: 9, 4: 16, 5: 25}
2.4.4 Local State
nonlocal 关键字，表示该变量是上一层嵌套函数中的变量。
>>> def make_withdraw(balance):
"""Return a withdraw function that draws down balance with each call."""
def withdraw(amount):
nonlocal balance # Declare the name "balance" nonlocal
if amount > balance:
return 'Insufficient funds'
balance = balance - amount # Re-bind the existing balance name
return balance
return withdraw
可以看到 balance 是上一层的变量，如果要对其修改，则必须加上 nonlocal 关键字，否则会报错。如果只是打印之类的操作，不对变量修改的话可以直接使用。
2.5 Object-Oriented Programming
2.5.1 Objects and Classes
Attributes are the data associated with an object.
Methods are the functions associated with an object.
2.5.2 Defining Classes
__init__ 是构造函数, 用于初始化对象的属性。
class Account:
def __init__(self, account_holder):
self.balance = 0
self.holder = account_holder
def deposit(self, amount):
self.balance = self.balance + amount
return self.balance
def withdraw(self, amount):
if amount > self.balance:
return 'Insufficient funds'
self.balance = self.balance - amount
return self.balance
2.5.3 Message Passing and Dot Expressions
两种调用方法. 在前一种情况下，必须显式地为 self 形参提供实参。在后一种情况下，self 参数被自动绑定。
>>> spock_account = Account('Spock')
>>> Account.deposit(spock_account, 1001) # The deposit function takes 2 arguments
1011
>>> spock_account.deposit(1000) # The deposit method takes 1 argument
2011
以下划线开头和结尾的属性是私有的, 不能被外部访问。
2.5.4 Class Attributes
Class attributes are attributes that are shared by all instances of a class.
>>> class Account:
interest = 0.02 # A class attribute
def __init__(self, account_holder):
self.balance = 0
self.holder = account_holder
# Additional methods would be defined here
改变 class 的 class attribute 会影响所有实例的属性。
>>> spock_account = Account('Spock')
>>> kirk_account = Account('Kirk')
>>> spock_account.interest
0.02
>>> kirk_account.interest
0.02
>>> Account.interest = 0.04
>>> spock_account.interest
0.04
>>> kirk_account.interest
0.04
如果有同名的 instance attribute, 则 instance attribute 比 class attribute 优先级更高。
改变某个 instance 的 class attribute, 不会影响其他 instance 的 class attribute。
>>> kirk_account.interest = 0.08
>>> kirk_account.interest
0.08
>>> spock_account.interest
0.04
>>> Account.interest = 0.05 # changing the class attribute
>>> spock_account.interest # changes instances without like-named instance attributes
0.05
>>> kirk_account.interest # but the existing instance attribute is unaffected
0.08
因为 kirk_account 在前面赋值了 instance attribute, 所以改变 class 的 class attribute 不会影响 kirk_account 的 instance attribute。
2.5.5 Inheritance
子类继承其基类的 attributes, 但可以覆盖某些 attributes, 包括某些 methods。
在子类中未指定的任何内容都会自动假定为与基类一样的行为。
2.5.6 Using Inheritance
定义子类时，在类名后加括号，括号内写基类名。
>>> class CheckingAccount(Account):
"""A bank account that charges for withdrawals."""
withdraw_charge = 1
interest = 0.01
def withdraw(self, amount):
return Account.withdraw(self, amount + self.withdraw_charge)
覆盖了基类的 withdraw 方法。
2.5.7 Multiple Inheritance
Python 支持子类从多个基类继承属性的概念, 称为多重继承.
如上图的菱形继承, 从左到右, 从下到上检索 attributes.
2.5.8 The Role of Objects
2.6 Implementing Classes and Objects
2.6.1 Instances
2.6.2 Classes
2.6.3 Using Implemented Objects
2.7 Object Abstraction
2.7.1 String Conversion
2.7.2 Special Methods
介绍一些 object 的特殊 Methods.
__init__ : 在 object 创建时自动调用.
__str__: 定义 str(object) 的行为, 返回 human readable 的字符串.
__repr__: 定义repr(object) 的行为, 返回 python interpreter readable 的字符串.
__bool__: 定义 bool(object) 的行为.
__len__: 定义 len(object) 的行为.
__getitem__: 定义 object[i]的行为.
__call__: 构造 object 可以返回一个函数.
>>> class Adder(object):
def __init__(self, n):
self.n = n
def __call__(self, k):
return self.n + k
>>> add_three_obj = Adder(3)
>>> add_three_obj(4)
7