伯克利 CS61A 课堂笔记 11 —— Mutability

本系列为加州伯克利大学著名 Python 基础课程 CS61A 的课堂笔记整理，全英文内容，文末附词汇解释。

目录

01 Objects

02 Example: Strings

Ⅰ Representing Strings: the ASCII Standard

Ⅱ Representing Strings: the Unicode Standard

03 Mutation Operations

Ⅰ Some Objects Can Change

Ⅱ Mutation Can Happen Within a Function Call

04 Tuples

05 Mutation

Ⅰ Sameness and Change

Ⅱ Identity Operators

Ⅲ Mutable Default Arguments are Dangerous

06 Mutable Functions

Ⅰ A Function with Behavior That Varies Over Time

Ⅱ Mutable Values & Persistent Local State

附：词汇解释

---

01 Objects

>> from datetime import date
>>> date
<class datetime.date>>>> today = date(2025, 2, 20)
>>> today
datetimme.date(2025, 2, 20)
>>> freedom = date(2025, 5, 12)
>>> freedom
datetime.date(2025, 5, 12)>>> str(freedom - today)
'81 days, 0:00:00'
>>> today.year
2025
>>> today.month
2
>>> today.strftime('%A %B %d')
'Thursday February 20'

Objects represent information, which consist of data and behavior, bundled together to create abstractions.

Objects represent things, but also properties, interactions, & processes.

A type of objects is called a class and classes are first-class values in Python.

Object-oriented programming: A metaphor for organizing large programs; Special syntax that can improve the composition of programs.

In Python, every value is an object: All objects have attributes; A lot of data manipulation happens through object methods; Functions do one thing, while objects do many related things.

02 Example: Strings

>>> s = 'Hello'#String特有的函数
>>> s.upper()
'HELLO'
>>> s.lower()
'hello'
>>> s.swapcase()
'hELLO'

Ⅰ Representing Strings: the ASCII Standard

American Standard Code for Information Interchange.

​

Layout was chosen to support sorting by character code.

Row indexed 2-5 are a useful 6-bit (64 element) subset.

Control characters were designed for transmission.

>>> a = 'A'
>>> ord(a)
65
>>> hex(ord(a))
'0x41'>>> print('\n\n\n') #空三格>>> print('\a\a\a') #响三声

Ⅱ Representing Strings: the Unicode Standard

109, 000 characters.

93 scripts (organized).

Enumeration of character properties, such as case.

A canonical name for every character.

U+0058 LATIN CAPITAL LETTER X

U+263a WHITE SMILING FACE

U+2639 WHITE FROWNING FACE

>>> from unicodedate import name, lookup
>>> name('A')
'LATIN CAPITAL LETTER A'
>>> name('a')
'LATIN SMALL LETTER A'>>> lookup('WHITE SMILING FACE')
😊
>>> lookup('SNOWMAN')
⛄
>>> lookup('SOCCER BELL')
⚽
>>> lookup('BABY')
👶
>>> lookup('BABY').encode()
b'\xf0\x9f\x91\9b6

03 Mutation Operations

>>> suits = ['coin', 'string', 'myriad']
>>> original_suits = suits#删
>>> suits.pop()
>>> suits.remove('string')
>>> suits
['coin']#增
>>> suits.append('cup')
>>> suits.expend(['sword', 'club'])
>>> suits
['coin', 'cup', 'sword', 'club']#改
>>> suits[2] = 'spade'
>>> suits[0:2] = ['heart', 'diamond']
>>> suits
['heart', 'diamond', 'spade', 'club']>>> original_suits
['heart', 'diamond', 'spade', 'club']

Ⅰ Some Objects Can Change

The same object can change in value throughout the course of computation.

​

All names that refer to the same object are affected by a mutation.

Only objects of mutable types can change: lists & dictionaries.

>>> numerals = {'I': 1, 'v': 5, 'X': 10}
>>> numerals
{'V': 5, 'X': 10; 'I':1}#查改
>>> numerals['X']
10
>>> numerals.get('X')
10
>>> numerals['X'] = 11
>>> numerals['X']
11
>>> numerals
{'V': 5, 'X': 11, 'I':1}#增删
>>> numerals['L'] = 50
>>> numerals.pop('X')
11
>>> numerals
{'V': 5, 'I':1, 'L': 50}

Ⅱ Mutation Can Happen Within a Function Call

A function can change the value of any object in its scope.

>>> four = [1, 2, 3, 4]
>>> len(four)
4
>>> mustery(four)
>>> len(four)
2def mystery(s):s.pop()s.pop()def mystery(s):s[2:] = []

>>> four = [1, 2, 3, 4]
>>> len(four)
4
>>> another_mystery()
>>> len(four)
2def another_mystery(s):four.pop()four.pop()

​

04 Tuples

#Tuple的精髓在于逗号，而不是小括号
>>> (3, 4, 5, 6)
(3, 4, 5, 6)
>>> 3, 4, 5, 6
(3, 4, 5, 6)>>> ()
()
>>> tuple()
()
>>> tuple([3, 4, 5, 6])
(3, 4, 5, 6)>>> 2,
(2,)
>>> (2,)
(2,)
>>> 2
2>>> (3, 4) + (5, 6)
(3, 4, 5, 6)
>>> 5 in (3, 4, 5)
True

#Dictionary的key中严禁出现list
>>> {(1, 2): 3}
{(1, 2): 3}
>>> {[1, 2]: 3}
Traceback (most recent call last):File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'
>>> {(1, [2]): 3}
Traceback (most recent call last):File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

Tuples are Immutable Sequences:

Immutable values are protected from mutation.

>>> turtle = (1, 2, 3)
>>> ooze()
>>> turtle
(1, 2, 3)>>> turtle = [1, 2, 3]
>>> ooze()
>>> turtle
['Anything could be inside!']

​The value of an expression can change because of changes in names or objects.

​An immutable sequence may still change if it contains a mutable values as an element.

​05 Mutation

Ⅰ Sameness and Change

As long as we never modify objects, a compound object is just the totality of its pieces and a rational number is just its numerator and denominator. But, this view is no longer valid in the presence of change.

A compound data object has an "identity" in addition to the pieces of which it is composed.

A list is still "the same" list even if we change its contents. Conversely, we could have two lists that happen to have the same contents, but are different.

​

Ⅱ Identity Operators

>>> [10] == [10]
True>>> a = [10]
>>> b = [10]
>>> a == b
True
>>> a is b
False>>> a.extend([20, 30])
>>> a
[10, 20, 30]
>>> b
[10]>>> c = b
>>> c is b
True
>>> c.pop()
>>> c
[]
>>> b
[]
>>> a
[10, 20, 30]

Ⅲ Mutable Default Arguments are Dangerous

A default argument is part of a function value, not generated by a call.

>>> def f(s = []):
...    s.append(5)
...    return len(s)
>>> f()
1
>>> f()
2
>>> f()
3

​

06 Mutable Functions

Ⅰ A Function with Behavior That Varies Over Time

Let's model a bank account that has a balance of $100.

​

Ⅱ Mutable Values & Persistent Local State

def make_withdraw_list(balance):b = [balance]def withdraw(amount):if amount < b[0]:return 'Insufficient funds'b[0] = b[0] - amountreturn b[0]withdraw = make_withdraw_list(100)
withdraw(25)

附：词汇解释

bundle / ˈbʌnd(ə)l / 捆绑、property 属性、interaction / ˌɪntərˈækʃ(ə)n / 交互、object-oriented /ˈɑːbdʒekt ɔːrientɪd / 面向对象的、metaphor / ˈmetəfər / 象征，比喻、syntax / ˈsɪntæks / 语法、data manipulation / məˌnɪpjuˈleɪʃn / 数据操作、first-class 优秀的，一流的、interchange 信息交换、row 行、column / ˈkɑːləm / 列、tilde / ˈtɪldə / 波浪号、subset / ˈsʌbset / 子集、transmission / trænzˈmɪʃ(ə)n / 传播、bell 钟，铃、upper 上层的、lower 下层的、unicode / ˈjuːnɪˌkoʊd / 统一码、script / skrɪpt /（一种语言的）字母系统，字母表、enumeration / ɪˌnuːməˈreɪʃn / 枚举、canonical / kəˈnɑːnɪkl / 标准的，规范的、frown / fraʊn / 皱眉、Latin / ˈlætn / 拉丁语、mutation / mjuːˈteɪʃ(ə)n / 改变、coin 硬币、myriad / ˈmɪriəd / 一万、line feed [计]换行、suit / suːt / 套装、sword / sɔːrd / 剑、spade / speɪd / 锹，铲、diamond /ˈdaɪəmənd / 钻石、mutable 可变的、scope / skoʊp / 范围，领域、mystery / ˈmɪstəri / 神秘的、numerator / ˈnuːməreɪtər / 分子、denominator / dɪˈnɑːmɪneɪtər / 分母、identity / aɪˈdentəti / 个体、default 默认的、generate 产生，引起、withdraw 提，取、persistent 持续的，反复出现的、insufficient / ˌɪnsəˈfɪʃ(ə)nt / 不充分的、assignment 赋值、balance 余款