Python入门系列（八）日期时间、数学、json

日期时间

Python中的日期本身不是数据类型，但我们可以导入一个名为datetime的模块，将日期作为日期对象使用。

import datetime

x = datetime.datetime.now()
print(x)

日期输出

import datetime

x = datetime.datetime.now()

print(x.year)
print(x.strftime("%A"))

创建日期对象

import datetime

x = datetime.datetime(2020, 5, 17)

print(x)

strftime（）方法

import datetime

x = datetime.datetime(2018, 6, 1)

print(x.strftime("%B"))

Directive	Description	Example
%a	Weekday, short version	Wed
%A	Weekday, full version	Wednesday
%w	Weekday as a number 0-6, 0 is Sunday	3
%d	Day of month 01-31	31
%b	Month name, short version	Dec
%B	Month name, full version	December
%m	Month as a number 01-12	12
%y	Year, short version, without century	18
%Y	Year, full version	2018
%H	Hour 00-23	17
%I	Hour 00-12	05
%p	AM/PM	PM
%M	Minute 00-59	41
%S	Second 00-59	08
%f	Microsecond 000000-999999	548513
%z	UTC offset	+0100
%Z	Timezone	CST
%j	Day number of year 001-366	365
%U	Week number of year, Sunday as the first day of week, 00-53	52
%W	Week number of year, Monday as the first day of week, 00-53	52
%c	Local version of date and time	Mon Dec 31 17:41:00 2018
%C	Century	20
%x	Local version of date	12/31/18
%X	Local version of time	17:41:00
%%	A % character	%
%G	ISO 8601 year	2018
%u	ISO 8601 weekday (1-7)	1

数学

min（）和max（）函数可用于查找可迭代中的最低或最高值

x = min(5, 10, 25)
y = max(5, 10, 25)

print(x)
print(y)

函数的作用是：返回指定数字的绝对（正）值

x = abs(-7.25)

print(x)

pow（x，y）函数将x的值返回到y（xy）的幂。

# Return the value of 4 to the power of 3 (same as 4 * 4 * 4)
x = pow(4, 3)

print(x)

数学模块

import math

x = math.sqrt(64)

print(x)

ceil（）方法将一个数字向上舍入到其最接近的整数，然后进行数学运算。floor（）方法将数字向下舍入到最接近的整数，并返回结果

import math

x = math.ceil(1.4)
y = math.floor(1.4)

print(x) # returns 2
print(y) # returns 1

pi常量，返回pi的值（3.14…）

import math

x = math.pi

print(x)

JSON

从JSON转换为Python

import json

# some JSON:
x =  '{ "name":"John", "age":30, "city":"New York"}'

# parse x:
y = json.loads(x)

# the result is a Python dictionary:
print(y["age"])

从Python转换为JSON

import json

# a Python object (dict):
x = {
  "name": "John",
  "age": 30,
  "city": "New York"
}

# convert into JSON:
y = json.dumps(x)

# the result is a JSON string:
print(y)

您可以将以下类型的Python对象转换为JSON字符串.

当您从Python转换为JSON时，Python对象将转换成JSON（JavaScript）等价物

Python	JSON
dict	Object
list	Array
tuple	Array
str	String
int	Number
float	Number
True	true
False	false
None	null

格式化结果

使用缩进参数定义缩进的数量

json.dumps(x, indent=4)

您还可以定义分隔符，默认值为（“，”，“：”，这意味着使用逗号和空格分隔每个对象，使用冒号和空格分隔键和值

json.dumps(x, indent=4, separators=(". ", " = "))

json_dumps（）方法有参数来对resu中的键进行排序

json.dumps(x, indent=4, sort_keys=True)

正则表达式

Python有一个名为re的内置包，可用于处理正则表达式。

import re

正则表达式函数

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

元字符是具有特殊含义的字符

Character	Description	Example
[]	A set of characters	"[a-m]"
\	Signals a special sequence (can also be used to escape special characters)	"\d"
.	Any character (except newline character)	"he..o"
^	Starts with	"^hello"
$	Ends with	"planet$"
*	Zero or more occurrences	"he.*o"
+	One or more occurrences	"he.+o"
?	Zero or one occurrences	"he.?o"
{}	Exactly the specified number of occurrences	"he.{2}o"
|	Either or	"falls|stays"
()	Capture and group

特殊序列

Character	Description	Example
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"
\b	Returns a match where the specified characters are at the beginning or at the end of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\bain" r"ain\b"
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")	r"\Bain" r"ain\B"
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"
\D	Returns a match where the string DOES NOT contain digits	"\D"
\s	Returns a match where the string contains a white space character	"\s"
\S	Returns a match where the string DOES NOT contain a white space character	"\S"
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	"\w"
\W	Returns a match where the string DOES NOT contain any word characters	"\W"
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"

集合是一对方括号[]内的一组字符，具有特殊含义

Set	Description
[arn]	Returns a match where one of the specified characters (a, r, or n) is present
[a-n]	Returns a match for any lower case character, alphabetically between a and n
[^arn]	Returns a match for any character EXCEPT a, r, and n
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9]	Returns a match for any digit between 0 and 9
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case
[+]	In sets, +, *, ., `

findall()函数的作用是：返回一个包含所有匹配项的列表。

import re

txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)

该列表按找到的顺序包含匹配项。
如果未找到匹配项，则返回空列表

import re

txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)

search()函数的作用是：在字符串中搜索匹配项，如果存在匹配项，则返回匹配对象。

import re

txt = "The rain in Spain"
x = re.search("\s", txt)

print("The first white-space character is located in position:", x.start())

split()函数的作用是：返回一个列表，其中字符串在每次匹配时被拆分

import re

txt = "The rain in Spain"
x = re.split("\s", txt)
print(x)

可以通过指定maxsplit参数来控制出现次数

import re

txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)

sub()函数的作用是：用您选择的文本替换匹配项

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt)
print(x) # The9rain9in9Spain

您可以通过指定count参数来控制替换的数量

import re

txt = "The rain in Spain"
x = re.sub("\s", "9", txt, 2)
print(x)

匹配对象是包含有关搜索和结果的信息的对象。

注意：如果没有匹配，将返回值None，而不是match对象。

.span()返回包含匹配的开始位置和结束位置的元组。

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.span()) # (12, 17)

.string 返回传递到函数中的字符串

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.string) # The rain in Spain

.group() 返回字符串中存在匹配项的部分

import re

txt = "The rain in Spain"
x = re.search(r"\bS\w+", txt)
print(x.group()) # Spain

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