Python 3.9部分教程 计算 列表 字符串 切片等

>>> 2 + 2

4

>>> 50 - 5*6

20

>>> (50 - 5*6) / 4

5.0

>>> 8 / 5

1.6 # division always returns a floating point number 除法总是返回一个浮点数

>>> 17 / 3

5.666666666666667 # classic division returns a float 经典除法返回一个浮点数

>>>

>>> 17 // 3

5 # floor division discards the fractional part 向下取整丢弃小数部分

>>> 17 % 3

2 # the % operator returns the remainder of the

division %操作符返回除法的余数

>>> 5 * 3 + 2

17 # floored quotient * divisor + remainder 商*除数+余数

>>> 5 ** 2 # 5 squared 5的平方

25

>>> 2 ** 7 # 2 to the power of 7 2的7次方

128

>>> width = 20

>>> height = 5 * 9

>>> width * height

900

>>> n # try to access an undefined variable 未定义变量

Traceback (most recent call last):

File "<stdin>", line 1, in <module> 第一行

NameError: name 'n' is not defined 未定义n

>>> 4 * 3.75 - 1 浮点运算

14.0

>>> tax = 12.5 / 100

>>> price = 100.50

>>> price * tax

12.5625

>>> price + _

113.0625

>>> round(_, 4)

113.0625

>>> round(_,3)

113.062

>>>'spam eggs' # single quotes 单引号

'spam eggs'

>>> 'doesn\'t'

"doesn't" # use \' to escape the single quote... 使用 \' 转义符 把单引号变双引号 …

>>> "doesn't"

"doesn't" # ...or use double quotes instead 或者使用双引号

>>> '"Yes," they said.' 放在单引号内

'"Yes," they said.'

>>> "\"Yes,\" they said." 使用\' 转义符 把双引号变单引号

'"Yes," they said.'

>>> '"Isn\'t," they said.'

'"Isn\'t," they said.'

>>> print('"Isn\'t," they said.') #输出单引号内的内容

"Isn't," they said.

# \n means newline \ n表示换行符

>>> s = 'First line.\nSecond line.'

>>> s

'First line.\nSecond line.' # without print(), \n is included in the output 没有print() \n包含在输出中

>>> print(s)

First line.

Second line # with print(), \n produces a new line 使用print()， \n产生换行作用

>>> print('C:\some\name')

C:\some

ame # here \n means newline! 这里\n默认会表示换行

>>> print(r'C:\some\name')

C:\some\name # note the r before the quote 注意前面加 r 可以让特殊符号失去作用

''' ''' 或 """ """ \ 可以跨越多行输入

print(''' ''') 或者 print(""" """)

>>> # 3 times 'un', followed by 'ium' 3乘以un，然后加ium

>>> 3 * 'un' + 'ium'

'unununium'

>>> 'Py' 'thon'

'Python'

>>> '大''家''好'

'大家好'

>>> text = ('Put several strings within parentheses '

'to have them joined together.')

>>> text

'Put several strings within parentheses to have them joined together.

如果您要将变量或变量与文字串联，请使用 +

>>> prefix = 'Py'

>>> prefix + 'thon'

'Python'

>>> word = 'Python'

>>> word[0]

'P' # character in position 0 索引第一个字符默认是 0

>>> word[5]

'n' # character in position 5 位置5的字符是n 因第一个字符是0

>>> word[-1]

'n' # last character 最后一个字符 由于 -0 与 0 相同 负指数从 -1 开始

>>> word[-2] # second-last character 倒数第二字符

'o'

>>> word[-6]

'P

>>> word[0:2]

'Py' # characters from position 0 (included) to 2 (excluded) 还可以切片 从0到2但不包括2的所有字符

>>> word[2:5]

'tho # characters from position 2 (included) to 5 (excluded) 从位置2到5但不包括5的所有字符

>>> word[:2]

'Py' # character from the beginning to position 2 (excluded) :前没数索引位置默认是0也就是最开头 从开始到位置2但不包括位置2全部字符

>>> word[4:]

'on' # characters from position 4 (included) to the end :后没数默认是从......一直到最后 从位置4到最后的全部字符

>>> word[-2:]

'on' # characters from the second-last (included) to the end 从倒数第2到最后的全部字符

>>> word[:2] + word[2:]

'Python' 开始到第2位置但不包含2 + 从第2位置到最后

>>> word[:4] + word[4:]

'Python' 开始到第4位置但不包含4 + 从第4位置到最后

# the word only has 6 characters 太大的索引将导致错误 word只有6个字符

>>> word[42]

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

IndexError: string index out of rang 索引错误字符串索引超出范围

但是切片就会优雅地处理范围外切片索引

>>> word[4:42]

'on'

>>> word[42:]

''

Python字符串无法更改 它们是不变的。

>>> word[0] = 'J'

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: 'str' object does not support item assignment

但是可以创建一个新的字符串

>>> 'J' + word[1:]

'Jython'

>>> word[:2] + 'py'

'Pypy'

索引位置

+---+---+---+---+---+---+

| P | y | t | h | o | n |

+---+---+---+---+---+---+

0 1 2 3 4 5 6

-6 -5 -4 -3 -2 -1

内置功能 len() 返回字符串长度

>>> s = 'supercalifragilisticexpialidocious'

>>> len(s)

34

列表

>>> squares = [1, 4, 9, 16, 25]

>>> squares

[1, 4, 9, 16, 25]

列表也可以索引和切片

>>> squares[0] # indexing returns the item

1

>>> squares[-1]

25

>>> squares[-3:] # slicing returns a new list

[9, 16, 25]

>>> squares[:]

[1, 4, 9, 16, 25]

列表还支持串联等操作

>>> squares + [36, 49, 64, 81, 100]

[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

与不变的字符串不同，列表是一种可变类型，即可以更改其内容

>>> cubes = [1, 8, 27, 65, 125] # something's wrong here 这里的第四个是错误输入

>>> 4 ** 3

64 # the cube of 4 is 64, not 65! 4的三次方是64不是65

>>> cubes[3] = 64 # replace the wrong value 替换65

>>> cubes

[1, 8, 27, 64, 125]

在列表末尾添加新项目

>>> cubes.append(216) # add the cube of 6 加入6的3次方

>>> cubes.append(7 ** 3) # and the cube of 7 加入7的3次方

>>> cubes

[1, 8, 27, 64, 125, 216, 343]

分配到切片也是可能的，这甚至可以更改列表的大小或完全清除它

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']

>>> letters

['a', 'b', 'c', 'd', 'e', 'f', 'g']

>>> # replace some values 替换一些值

>>> letters[2:5] = ['C', 'D', 'E']

>>> letters

['a', 'b', 'C', 'D', 'E', 'f', 'g']

>>> # now remove them 现在删除它们

>>> letters[2:5] = []

>>> letters

['a', 'b', 'f', 'g']

>>> # clear the list by replacing all the elements with an empty list 通过将所有元素替换为空列表来清除列表

>>> letters[:] = []

>>> letters

[]

内置功能len()也适用于列表

>>> letters = ['a', 'b', 'c', 'd']

>>> len(letters)

4

可以嵌套列表（创建包含其他列表的列表)

>>> a = ['a', 'b', 'c']

>>> n = [1, 2, 3]

>>> x = [a, n]

>>> x

[['a', 'b', 'c'], [1, 2, 3]]

>>> x[0] # x第一个列表a

['a', 'b', 'c']

>>> x[0][1] # x第一个列表 a里的第2个元素

'b'

>>> # Fibonacci series: 斐波纳契数列

... # the sum of two elements defines the next

... a, b = 0, 1 # a b分别赋值0和1

>>> while a < 10:

... print(a) # 注意缩进TAB

... a, b = b, a+b # a赋值等于b ,b赋值等于a+b

...

0

1

1

2

3

5

8

>>> i = 256*256

>>> print('The value of i is', i)

The value of i is 65536

>>> a, b = 0, 1

>>> while a < 1000:

... print(a, end=',')

... a, b = b, a+b

...

0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

任何非零整数值都是真实的 零是假的 任何长度为非零的都是真的， 空序列是假的

（小于）、（大于）、（等于）、（小于或等于）、（大于或等于）和（不等于）

< > == <= >= !=

>>> x = int(input("Please enter an integer: "))

Please enter an integer: 56

if语句

>>> if x < 0: # 如果

... x = 0 # 注意缩进

... print('Negative changed to zero')

... elif x == 0: 否则

... print('Zero')

... elif x == 1:

... print('Single')

... else:

... print('More')

...

More

for语句

>>> # Measure some strings: 测量一些字符串

... words = ['cat', 'window', 'defenestrate']

>>> for w in words:

... print(w, len(w))

...

cat 3

window 6

defenestrate 12