Article Directory

• 1. Decimals should be kept in designated decimal places
• • 1. %f method
  • 2.format function
  • 3.round() function
  • 4.Direct truncation
• 2. Judgment Data type of variable
• • 1.type():
  • 2.isinstance()
  • 3.__class__.__name__ attribute
• 3. Class method (@classmethod) in Python
• 4. str.format and tab character\tSet Chinese alignment
• 5. Datetime module timedelta Usage of the class
• 6. Obtain the specific information of the thrown exception
• 7. Use the BeautifulSoup library to remove the HTML tags in the string

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1. Decimals should be retained in designated decimal places

1.%f method

f = 1.23456f1 = '%.4f' % f f2 = '%.2f' % fprint(f1,type(f1))print(f2,type(f2))

Print

1.2346 1.23 

It is easy to know that this method will round, but convert the floating point type into a string value , is no longer the original value.

2.format function

f = 1.23456f1 = format(f,'.4f')f2 = format(f,'.2f')print(f1,type(f1))print(f2,type(f2))

Print

1.2346 1.23 

Similarly, this method will also round, but the floating point type will be converted into String values are no longer primitive numeric values.

3.round() function

a = 1.23456b = 2.355c = 3.5d = 2.5a1 = round(a, 3)b1 = round(b, 2)c1 = round(c)d1 = round(d)print(a1,type(a1))print(b1,type(b1))print(c1,type(c1))print(d1,type(d1))

Print

1.235 2.35 4 2 

It can be seen that the round() function finally gets a numerical value (floating point type or integer type), but the rules of "rounding" and "rounding" are not necessarily true:
(1) In the round(x,n) function, whether to carry or round depends on n digits and n 1 decimal digits Value
(2) It is easy to be confused only when the n 1-digit number is 5. If n is an even number, then the n 1-digit number is 5, then carry, for example, round (1.23456, 3) finally becomes 1.235
(3) If n is an odd number, then n 1 bit is the number 5, and there is no carry, such as round (2.355, 2), the final value is 2.35
(4) If n is 0, that is, when n is not filled in, The final result is the opposite of the above, that is, when the integer part is an even number, the decimal place 5 is not carried, for example (round (2.5) becomes 2).
(5) When the integer part is an odd number, the decimal place is rounded up to 5. (round (3.5) becomes 4)

4. Directly truncate

a = int(1.23456 * 1000) / 1000print(a)

and print1.234
This method is simple, crude and direct Remove the following ones, regardless of whether they are greater than 5.

2. Determine the data type of the variable

1.type():

a = 1.23print(type(a))

Print< class 'float'>.

2.isinstance()

The prototype isisinstance(x, A_tuple),

a = 1.23tp = isinstance(a,float)print(tp)

PrintTrue.

3.class.__name__ attribute

Use the__class__.__name__attribute of the variable, which is alsotype()The essence of the method:

num = 1.23print(num.__class__.__name__)

Print:

float

3. Class method (@classmethod) in Python

When doing object-oriented programming in Python , it is often necessary to use theclassmethodclass method, which is not a method in the class.
Class methods are also a practical technique, which can be described simply as: "Class methods make class templates memorable."
Class template is the class we define. Under normal circumstances, if you do not use class methods to instantiate a class, the class itself has no memory. Just when a static template is applied multiple times. If we want the class to record some memory after each instantiation, it will be useful for many operations.

class Man: id = 0 # 类变量 def __init__(self, name): self.name = name self.id = self.id_number() @classmethod def id_number(cls): cls.id += 1 return cls.id a = Man('A')print(a.id)b = Man('B')print(b.id)

Print

12

Instantiate the Man class twice, and the id of each instance is different. This is achieved by relying on class methods: first, use@classmethodto describe the class method, and then use "cls" to represent this class. The processing of class attributes by class methods is memorized.

It should be noted that the variables processed by the class method must be class variables. Because you can't use self to address instance variables in class methods, you need to put the class variables at the front of the description, as shown in "id=0" above. Class variables can be accessed by self, so after the class variables are defined, there is no need to describe the class variables again in the _init_ function. Therefore, self.id is not necessarily needed in the above code.

4. str.format and tab character \tSet Chinese alignment

str.format formats the string,{:means left alignment,{:>x}means right alignment,{:^x}means centering), and less than x digits will be automatically filled in (default Fill in spaces), but it does not support Chinese characters very well, so it will lead to the phenomenon of non-alignment when there are multiple lines of Chinese strings. You need to consider the length of the string and encode the Chinese string before calculate.

#name是包含中文的字符串，22是整个字符串一行的总长度，一般要顾及测试才能得到，\t后的x是一标记字符，可换为别的所需的字符串print('[{string:<{len}}\tx'.format(string=string+']',len=22-len(string.encode('GBK'))+len(string)))

class datetime.timedelta(days=0, seconds=0, microseconds=0, milliseconds=0, hours=0, weeks=0)

>>> import datetime>>>>>> datetime.timedelta(365).total_seconds() # 一年包含的总秒数31536000.0>>> dt = datetime.datetime.now()>>> dt + datetime.timedelta(3) # 3天后datetime.datetime(2020, 1, 22, 11, 17, 0, 214877)>>> dt + datetime.timedelta(-3) # 3天前datetime.datetime(2020, 1, 16, 11, 17, 0, 214877)>>> dt + datetime.timedelta(hours=3) # 3小时后datetime.datetime(2020, 1, 19, 14, 17, 0, 214877)>>> dt + datetime.timedelta(hours=-3) # 3小时前datetime.datetime(2020, 1, 19, 8, 17, 0, 214877)>>> dt + datetime.timedelta(hours=3, seconds=30) # 3小时30秒后 datetime.datetime(2020, 1, 19, 14, 17, 30, 214877)

def get_exception_info(): try: s = 2 / 0 except Exception as e: print('异常内容：', e.args[0]) print('异常行数：', e.__traceback__.tb_lineno) print('异常文件：', e.__traceback__.tb_frame.f_globals["__file__"])get_exception_info()

异常内容： pision by zero 异常行数： 8异常文件： XXX/test.py

text = """
 
Title
 
A long text........ 
  a link  
"""

from bs4 import BeautifulSoup text = """
 
Title
 
A long text........ 
  a link  
"""clean_text = BeautifulSoup(text, "lxml").textprint(clean_text)

Title A long text........ a link