1. What is an exception

When an error is detected, the interpreter cannot continue to execute. Instead, some error prompts appear, which is the so-called "exception".

2. Exception presentation

# operator
# print(10/0)

# File exception
f = open('python.txt', 'r')
content = f.readlines()
print(content)

3. Exception capture

Basic grammar

try:
    Possible error codes
except(capture):
    Code to execute if an exception occurs

try... Exception is mainly used to catch code runtime except ions

case

try:
    f = open('python.txt', 'r')
    content = f.readline()
    print(content, end='')
except:
    f = open('python.txt', 'w', encoding='utf-8')
    f.write('Exception occurred, execute except Code in statement')

f.close()

4. Catch specified exception

In the above case code, exception is equivalent to capturing all exceptions. No matter what error is encountered, the code encapsulated in exception will be automatically executed. However, in some cases, we capture exceptions to the targeted and execute the corresponding code.

Basic grammar

try:
    Code that may encounter exceptions
except Exception type:
    Code executed after the corresponding error is captured

① If the exception type of the code you are trying to execute is inconsistent with the exception type you are trying to catch, you cannot catch an exception.

② Generally, there is only one line of code to try to execute below try.

case

Caught FileNotFoundError exception

try:
    f = open('python.txt', 'r')
except FileNotFoundError as e:
    print(e)

5. Catch multiple exceptions at the same time

try:
    print(name)
    # print(10/0)
except (NameError, ZeroDivisionError) as e:
    print(e)

6. Catch all unknown exceptions

No matter how many Exception types we define after Exception, unknown exceptions that cannot be captured may occur in practical applications. At this time, we consider using the Exception exception type to catch all unknown exceptions that may be encountered:

try:
    Possible error codes
except Exception as e:
    print(e)

Case: print an undefined variable and use the Exception class to catch it

try:
    print(name)
except Exception as e:
    print(e)

7. else statement in exception capture

else statement: it represents the code to be executed if there is no exception.

try:
    print(1)
except Exception as e:
    print(e)
else:
    print('Ha ha, it's really Kaisen. I haven't encountered any exceptions')

case

try:
    f = open('python.txt', 'r')
except Exception as e:
    print(e)
else:
    content = f.readlines()
    print(content, end='')
    f.close()

8. finally statement in exception capture

finally indicates the code to be executed regardless of whether there is an exception, such as closing the file and closing the database connection.

try:
    f = open('python.txt', 'r')
except:
    f = open('python.txt', 'w')
else:
    print('Ha ha, it's really Kaisen. I haven't encountered any exceptions')
finally:
    print('Close file')
    f.close()

9. Abnormal comprehensive cases

☆ abnormal transmission

demand

• Try to open the python.txt file in read-only mode. If the file exists, read the file content. If the file does not exist, prompt the user.
• Read content requirements: try to read the content circularly. During the reading process, if it is detected that the user accidentally terminates the program, except ion will be captured

import time

try:
   f = open('python.txt', 'r')
   try:
       while True:
           content = f.readline()
           if len(content) == 0:
               break
           time.sleep(3)
           print(content, end='')
   except:
       # Ctrl + C (in the terminal, it represents the continuation of the terminated program)
       print('python.txt Not all reads completed, interrupted...')
   finally:
       f.close()
except:
   print('python.txt file cannot be found...')

☆ raise throws a custom exception

In Python, the syntax for throwing custom exceptions is raise exception class object.

demand

If the password length is insufficient, an exception will be reported (if the user enters the password, if the length is less than 6 digits, an error will be reported, that is, a user-defined exception will be thrown and caught).

def input_password():
    password = input('Please enter your password, no less than 6 digits:')
    if len(password) < 6:
        # Throw exception
        raise Exception('Your password is less than 6 digits long')
        return
    # If the password length is normal, the password will be displayed directly
    print(password)
        
input_password()

Object oriented throw custom exception:

class ShortInputError(Exception):
    # Length represents the length of the input password, min_length represents the minimum length of ShortInputError
    def __init__(self, length, min_length):
        self.length = length
        self.min_length = min_length

    # Define a__ str__ Method to output string information
    def __str__(self):
        return f'The length of the password you entered is{self.length}，Not less than{self.min_length}Characters'

try:
    password = input('Please enter your password, no less than 6 digits:')
    if len(password) < 6:
        raise ShortInputError(len(password), 6)
except Exception as e:
    print(e)
else:
    print(f'The password is entered. Your password is:{password}')

10. Common exception types

Exception name	describe
BaseException	Base class for all exceptions
SystemExit	Interpreter requests exit
KeyboardInterrupt	User interrupts execution (usually enter ^ C)
Exception	Base class for general errors
StopIteration	The iterator has no more values
GeneratorExit	An exception occurred in the generator to notify it to exit
SystemExit	Python interpreter requests exit
StandardError	Base class for all built-in standard exceptions
ArithmeticError	Base class for all numeric errors
FloatingPointError	Floating point calculation error
OverflowError	The numeric operation exceeds the maximum limit
ZeroDivisionError	Divide (or modulo) zero (all data types)
AssertionError	Assertion statement failed
AttributeError	Object does not have this property
EOFError	No built-in input, EOF flag reached
EnvironmentError	Base class for operating system error
IOError	Input / output operation failed
OSError	Operating system error
WindowsError	system call filed
ImportError	Failed to import module / object
KeyboardInterrupt	User interrupts execution (usually enter ^ C)
LookupError	Base class for invalid data query
IndexError	This index does not exist in the sequence
KeyError	This key is not in the map
MemoryError	Memory overflow error (not fatal for Python interpreter)
NameError	Object not declared / initialized (no properties)
UnboundLocalError	Accessing uninitialized local variables
ReferenceError	A weak reference attempts to access an object that has been garbage collected
RuntimeError	General runtime error
NotImplementedError	Methods not yet implemented
SyntaxError	Python syntax error
IndentationError	Indent error
TabError	Mixing tabs and spaces
SystemError	General interpreter system error
ValueError	Invalid parameter passed in
UnicodeError	Unicode related errors
UnicodeDecodeError	Error in Unicode decoding
UnicodeEncodeError	Error encoding Unicode
Warning	Warning base class
DeprecationWarning	Warning about deprecated features
FutureWarning	Warning about future semantic changes in constructs
OverflowWarning	Old warning about automatically promoting to long
PendingDeprecationWarning	Warning that features will be discarded
RuntimeWarning	Warning of suspicious runtime behavior
SyntaxWarning	Warning of suspicious syntax
UserWarning	Warnings generated by user code
FileNotFoundError	File not found error