Try...except of exception handling in python

Exception handling try...except

When writing python code, sometimes the code may be wrong for some reasons-specific analysis of the specific reasons, we sometimes need to know the reason for the error to be more convenient to correct the error.

The following example illustrates the error in python

1. Syntax error

# 例1
if 2>1
    print(2222)
    
# 例2   
dic = {
    
    "name";"alex"}
tu = (2,3,4''6)

# 错误提示：SyntaxError: invalid syntax

2. Logic error

dic = {
    
    "name":"海狗","age":18}
dic["hobby"]

# 错误提示：KeyError: 'hobby'

Commonly used is if to perform abnormal judgment, but the following will use try as an example:

Single branch:

try:
    num = int(input(">>>")) #出现value错误之后，直接跳转except语句
    print(111)
except ValueError:
    print(666)

# >>>a
# 666

try:
    dic = {
    
    "name":"嘉欣"}
    print(dic["age"])
    num = int(input(">>>"))  # 出现value错误之后，直接跳转except语句
    print(111)
except ValueError:
    print(666)

# KeyError: 'age'

try:
    dic = {
    
    "name":"嘉欣"}
    print(dic["age"])
    num = int(input(">>>"))  # 出现value错误之后，直接跳转except语句
    print(111)
except KeyError:
    print(666)

# 666 

Multiple branches:

try:
    num = int(input(">>>"))  # 出现value错误之后，直接跳转except语句
    dic = {
    
    "name": "嘉欣"}
    #print(dic["age"])
    l1 = [1,2]
    print(l1[100])
except ValueError:
    print("输入的有非数字元素")
except KeyError:
    print("没有此键")
except IndexError:
    print("没有此下标")
print(666)

# >>>1233
# 没有此键
# 666

# >>>asd
# 输入的有非数字元素
# 666

# >>>123
# 没有此下标
# 666

One exception:

try:
    dic = {
    
    "name": "嘉欣"}
    # print(dic["age"])
    l1 = [1,2]
    print(l1[100])
    print(111)
    for i in 123:
        pass
except Exception as e: # Exception 会将错误报出来
    print(e)

# 'age'
# list index out of range
# 'int' object is not iterable

When to use Magnum? When to use multiple branches?

If you don’t care about the error message, but just want to eliminate the error and let the program continue to run, use the universal exception

But generally write code...Don’t add try to every paragraph... Isn’t that worrying?

Multi-branch plus universal

def func():
    pass
def func1():
    pass

dic = {
    
    
    1:func,
    2:func1,
}

try:
    num= int(input("请输入序号"))
    dic[num]()
except ValueError:
    print("请输入数字")
except KeyError:
    print("请输入范围内的序号")
except Exception:
    print("程序出现意料之外的错误....")

# 请输入序号456
# 请输入范围内的序号

# 请输入序号asfd
# 请输入数字

# 请输入序号1

try else finally structure:

try:
    dic = {
    
    "name": "嘉欣"}
    #print(dic["age"])
    l1 = [1,2]
    # print(l1[100])
    print(111)
except ValueError:
    print("输入的有非数字元素")
except KeyError:
    print("没有此键")
else:
    print("如果没有出现异常则执行这里")
finally:
    print("finally 666")

# 没有此键
# finally 666

# finally 666
# IndexError: list index out of range

# 111
# 如果没有出现异常则执行这里
# finally 666

# 如果将except都注释掉
try:
    dic = {
    
    "name": "嘉欣"}
    # print(dic["age"])
    l1 = [1,2]
    # print(l1[100])
    print(111)
# except ValueError:
#     print("输入的有非数字元素")
# except KeyError:
#     print("没有此键")
# else:
#     print("如果没有出现异常则执行这里")
finally:
    print("finally 666")

# 111
# finally 666

except must depend on try else must depend on except and try
finally only depend on try
finally: before the exception occurs, the execution of finally
finally is used to close the database connection, the file handle is closed, the data is saved, etc., the finally is used

with open("teat1",encoding="utf-8",mode="a+") as f1:
    try:
        for i in f1:
            print(i)
        if ....:
    finally:
        f1.close()
# SyntaxError: invalid syntax

def func():
    try:
        print(111)
        return 666
    finally:
        print(22)
print(func())

# 111
# 22
# 666

Before the return end of the function, execute the finally code

while 1:
    try:
        print(1)
        break
    finally:
        print(222)

# 1
# 222

Before break, execute finally code

Actively trigger an exception:

raise ValueError("出现了value错误")

# ValueError: 出现了value错误

Assertion: Show a tough attitude

name = "1"
n1 = input("请输入：")
assert name == n1
print(111)
print(222)

# 请输入：as
# AssertionError

#请输入：1
# 111
# 222

Custom exceptions:
There are many types of errors provided to you in python, but not all errors. Some errors require you to customize

class LiYeError(BaseException):
    def __init__(self,msg):
        self.msg=msg
    def __str__(self):
        return self.msg

try:
    raise LiYeError('类型错误')
except LiYeError as e:
    print(e)

# 类型错误

Above, I hope to help you~