Numerical

1, declare variables

age = 20
f = 3.14 #浮点型
f = 0.3
f = .3

2, expression

Mainly to do some simple arithmetic operations, direct results

1.1 + 2.2
2.2 + 3.0
3.14 * 2
5.0 / 2

3.3000000000000003
5.2
6.28
2.5

In the example above, mainly to see the simple '1.1 + 2.2' output in python is not as simple as we thought in 3.3, but there is a long decimals '3.3000000000000003'.

3, the display (using .format () to replace characters)

I: '{0: .2f}'. format (3.3333)

'{0}'.format(20)
'3 + 2 ={0}'.format(5)
f = 3.333
'f = {0}'.format(f)
'f={0:.2f}'.format(f)
d = {'name':'Tom', 'age':20, 'job':'dev'}
'姓名:{0},年龄:{1}'.format(d.get('name'), d.get('age'))

The above code is output as follows:

'20'
'3 + 2 =5'
'f = 3.333'
'f=3.33'
'姓名:Tom,年龄:20'

In the example above we see .format () of power of, dynamically replacing the existing string of characters, and we can also added in {0} ': .2f' need to be added to a predetermined value data types and retention of decimal places type object.

4, comparison, returns a Boolean result: True, False

Common operators>, <,> =, <=, ==,! =

5, divided

10 / 4
10 // 4
10 // 4.0

Output is as follows:

2.5
2
2.0

The first one is above normal peacetime use of the division, behind a common "floor except", also known as divisible, is derived quotient (without remainder).
The third addition result of the formula to obtain the float.

6, three methods of rounding

floor()、trunc()、round()

floor () rounding

import math
math.floor(3.4)  # 取整的函数,直接截取整数
math.floor(3.9)
math.floor(-3.4)
math.floor(-3.9)  # 假如把数值放在坐标轴上，那么floor是取数值左边的整数，往左移动坐标轴

3
3
-4
-4

the trunc () rounding

math.trunc(3.14) # tuanc函数是往坐标轴0中间移动
math.trunc(3.94)
math.trunc(-3.14)
math.trunc(-3.94)

3
3
-3
-3

round () rounding

round(3.14) #常规意义上的四舍五入
round(3.94)
round(-3.14)

3
4
-3
-4

7, integer: infinite precision, limited only by memory and computer precision

Hex 0o, octal, 0x, binary 0b

Octal 0o

0o3  # 0o开头表示八进制，但是八进制只能表示0-7
0o7
0o8 # 报错

3
7

Hex 0x

0x3 #十六进制
0x9
0xa
0xf

Binary 0b

0b1 # 二进制
0b0
0b11
0b1011

Several functions convert hexadecimal

oct(64) # 转换64进制的写法
hex(64) # 十六进制的写法
bin(64) # 二进制的写法

'0o100'
'0x40'
'0b1000000'

Other trivial

math.pi # math里内置的一些函数
math.sqrt(144)

3.141592653589793
12.0

About decimal

import decimal
decimal.Decimal('3.14')
decimal.Decimal('1.1')+decimal.Decimal('2.2')  # 得到需要的精度

Decimal('3.14')
Decimal('3.3')

String

1, the statement

''
""
"" "##" "" explanatory document
to deal with the problem quoted strings can reference methods

word = "What's your name?"
word
'what\'s your name?'

"What's your name?"
"what's your name?"

2, escapes

Escapes

    \ 
    \'    
    \"

Newline \ n
Backspace \ b
Form four spaces \ t
Ignore escapes r '...'
directly indicate the path will be given:

path = 'C:\abc\xyz.txt'

SyntaxError: (unicode error) 'unicodeescape' codec can't decode bytes in position 6-7: truncated \xXX escape

You can use the following methods to transfer ""

path = 'C:\\abc\\xyz,txt'
path

'C:\\abc\\xyz,txt'

The best escape character is ignored:

path = r'c:\abc\xyz.txt'
path

'c:\\abc\\xyz.txt'

You can use "" "##" "" comment in the code:

def avg():
    """计算平均分"""
    pass
# 这是一行注释

3, the basic operation

name = 'tom'
len(name)
'abc' + 'xyz'
'ok' * 5
print('-' * 30)

3
'abcxyz'
'okokokokok'
------------------------------

If I wanted to literally print it, it can be:

s = 'hello'
for c in s:
    print(c)

h
e
l
l
o

Want to add something literally in the middle? :

for c in s:
    print(c, end = ' ')

h e l l o

I want to call a character in a field:

s[0]
s = 'sfdsfsfsdf'
s[0:4] # 后面的数字坐标是不包括的，不会引用出来

'h'
'sfds'

Call two methods last field

s[-1]
s[len(s)-1]

'f'
'f'

Fancy call:

s[:]
s[::2] # 以2作为间隔获取
h = 'hello'
h[::-1]
int('42')+1 # 使得字符串与数字相加

'sfdsfsfsdf'
'sdffd'
'olleh'
43

About ASCII table

ord('c') # 查看ascii表里位置
chr(99) #查看相应位置对应的字符
chr(75)

99
'c'
'K'

Use replace () a field replacement string

h = 'hello'
h = h.replace('e', 'a')
h

'hallo'

4, Boolean

True、False

type(True)  # 区分大小写
type(3)
isinstance(True, int) # 这个函数是判断某个字符是否是某个类型
True == 1
False == 0
result = True + 3
result

bool
int
True
True
True
4

5, the basic operation of Sao

As an intermediary in the string is changed by one field list

Becomes first string list

s = 'sdhishd.com'
l = list(s)
l

['s', 'd', 'h', 'i', 's', 'h', 'd', '.', 'c', 'o', 'm']

Replace fields in the list

l[len(l)-1] = 'c'  # 或者可以用 l[-1] = 'c'
l

['s', 'd', 'h', 'i', 's', 'h', 'd', '.', 'c', 'o', 'c']

Then into a string

s = ''.join(l) #通过一个空的字符串
s

'sdhishd.coc'

s = '|'.join(l) #将字符串中的每一个字段都用符号隔开
s
url = 'sadfhsk.com,sdfjl.cc,sfhioho.com'  # 使用逗号隔开字符串中的字段，前提是知道字符串中存在的分隔符
l = url.split(',')
l

's|d|h|i|s|h|d|.|c|o|c'
['sadfhsk.com', 'sdfjl.cc', 'sfhioho.com']

len (l) # This is actually a function, not attached to the object
s.split () # This is a method, when the python is defined string type is set a good method.

url = 'sdfi.cc'
url.startswith('url') # 判断字符串的开头方法
url.endswith('cc') # 判断字符串结尾的方法
url.find('sdf') # 返回字段在字符串中的位置，返回寻找字段首字符位置
url.find('.')

False
True
True
0
4

About Assignment

a,b = 1,2
a
b

1
2

a,b = b,a #交换对象
a
b

2
1

- 交换过程的展示

'{0} = > {1}'.format(a, b) #交换的过程表示
'{} = > {}'.format(a, b)  #也可以不写索引
'{name}=>{salary}'.format(salary = 9000.00,name = 'Tom' ) 
#可以不按顺序，但是要指定变量与对象
url.upper() #大写变量

'2 = > 1'
'2 = > 1'
'Tom=>9000.0'
'SDFI.CC'

List

Ordered set of arbitrary objects
Access the elements by index index
Variable length
Belonging to the variable sequence
Common operations:

len([3, 4, 5]) #求列表长度的时候，会计算列表中的对象数量
[1,2,3]+[3,4,5]
l = list('dfasihdi') #这里的list是个函数，要用小括号
l
'k' in l

3
[1, 2, 3, 3, 4, 5]
['d', 'f', 'a', 's', 'i', 'h', 'd', 'i']
False

for c in l:
    print(c)
for c in l:
    print(c,end = '|')

d
f
a
s
i
h
d
i
d|f|a|s|i|h|d|i|

append()

l = [1,2,3,4,5,6]
res = []
for i in l:
    res.append(i**2) #append方法就是扩展
    
res
l1 = [i**2 for i in l] # 外侧的中括号决定返回的是一个列表
l1

[1, 4, 9, 16, 25, 36]
[1, 4, 9, 16, 25, 36]

[c*3 for c in 'code']
l = [1,2,3]
l[1:2] = [4,5] #替换列表
l

['ccc', 'ooo', 'ddd', 'eee']
[1, 4, 5, 3]

extend()

l.extend([9,8,5]) # 扩展一个列表，连接起来
l

[1, 4, 5, 3, 7, 9, 8, 5]

Sequence

l.sort() #正向排序
l
l.reverse() #反向排序
l

[1, 3, 4, 5]
[5, 4, 3, 1]

Removing elements

l.pop() #删除最后一个元素
l
del(l[0])
l

[5, 4, 3]
[4,3]

l.index(5) #返回列表中元素的索引
l.count(5) #统计列表中元素5的个数

2
1

Note: reference types, support for in-situ change. Copies can be used: [:] ,. copy ()

l1 = [1,2,3,4,5]
l2 = l1
l2
l1[1] = 9 #改变l1中的元素，然后l2中相应的元素也会改变的
l1
l2

[1, 2, 3, 4, 5]
[1, 9, 3, 4, 5]
[1, 9, 3, 4, 5] #l2也跟着改变了

Avoid changing the method of

l3 = l1[:] #第一种避免l3改变的方法
l3
l4 = l1.copy() #使用.copy()方法
l4

[1, 9, 3, 4, 5]
[1, 9, 3, 4, 5]

The next day: values and strings, lists, list