什么是异常

>>> 10 * (1/0)             # 0 不能作为除数，触发异常
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: division by zero

>>> 4 + spam*3             # spam 未定义，触发异常
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined

>>> '2' + 2               # int 不能与 str 相加，触发异常
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

try:
    num = int(input("Please enter a number: "))
    print(num)
except:
    print("You have not entered a number, please try again!")

PS D:\learning\git\work> python test.py
Please enter a number: 60
60
PS D:\learning\git\work> python test.py
Please enter a number: d
You have not entered a number, please try again!
PS D:\learning\git\work>

try:
    test_file = open("testfile.txt", "w")
    test_file.write("This is a test file!!!")
except IOError:
    print("Error: File not found or read failed")
else:
    print("The content was written to the file successfully")
    test_file.close()

PS D:\learning\git\work> python test.py
The content was written to the file successfully
PS D:\learning\git\work>

try:
    test_file = open("testfile.txt", "w")
    test_file.write("This is a test file!!!")
except IOError:
    print("Error: File not found or read failed")
else:
    print("The content was written to the file successfully")
    test_file.close()
finally:
    print("test")

PS D:\learning\git\work> python test.py
The content was written to the file successfully
test

def numb( num ):
    if num < 1:
        raise Exception("Invalid level!")
        # 触发异常后，后面的代码就不会再执行
try:
    numb(0)            # 触发异常
except Exception as err:
    print(1,err)
else:
    print(2)

PS D:\learning\git\work> python test.py
1 Invalid level!
PS D:\learning\git\work>