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Python error and exception handling

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Release： 2018-09-30 13:55:44

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The content of this article is about error and exception handling in Python. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

1: Syntax errors syntax errors

Familiar with grammar!

2: Exception

①When printing error information, the exception type is displayed as the built-in name of the exception, and the specific information is displayed in the form of a call stack

②Common exceptions ：
NameError,
ZeropisionError,
TypeError
SyntaxError
IndexError Index exceeds sequence range
KeyError Requesting a dictionary key that does not exist
IOError use using using using using using using out out out out out out out out out out Out out out of ’ Try to access unknown object properties
Three: Exception handling

while True:
try:
x = int(input("Please enter a number: "))
break
except ValueError:
print("Oops!  That was no valid number.  Try again   ")

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The try statement works as follows;

①First, execute the try clause (between the keyword try and the keyword except Statements between)
② If no exception occurs, ignore the except clause and end after the try clause is executed.
③If an exception occurs during the execution of the try clause, the rest of the try clause will be ignored.
④If the exception type matches the name after except, the corresponding except clause will be executed. Finally execute the code after the try statement.
⑤If an exception does not match any except, then the exception will be passed to the upper try.
⑥A try statement may contain multiple except clauses to handle different specific exceptions. At most one branch will be executed.
⑦An except clause can handle multiple exceptions at the same time. These exceptions will be placed in parentheses to become a tuple.

except (RuntimeError, TypeError, NameError):
pass

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⑧The last except clause can ignore the name of the exception, it will be used as a wildcard. You can use this method to print an error message and then throw the exception again.

⑨The try except statement also has an optional else clause. If this clause is used, it must be placed after all except clauses. This clause will be executed when no exception occurs in the try clause.

Three: Throw an exception

Python uses the raise statement to throw a specified exception

raise the only one The parameter specifies the exception to be thrown.
It must be an exception instance or an exception class (that is, a subclass of Exception)

Four: User-defined exception

You can create a new exception class to Own your own exceptions.

The exception class inherits from the Exception class and can be inherited directly or indirectly.
When creating a module that may throw a variety of different exceptions,
A common approach is to create a basic exception class for this package,

5: Define cleanup behavior

try:
        raise KeyboardInterrupt
    finanlly:
        print("dooo")

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Regardless of whether an exception occurs in the try clause, the finally clause will be executed

If an exception is thrown in the try clause (or in the except and else clauses),
but not If any except intercepts it, then the exception will be thrown again after the finally clause is executed.

6: Extension------------->Assertion mechanism

assert is equivalent to a Boolean true judgment, and throws an exception AssertionError if it is not satisfied

Purpose: Defensive program
Requires logic checking during runtime
Reference: "The best time to use assertions in Python 》

# ---------------------------------------------------------------------#
# 异常处理
# ---------------------------------------------------------------------#
while True:
    try:
        x = int(input("Please enter a number: "))
        break
    except ValueError:
            print("Oops!  That was no valid number.  Try again   ")

# ---------------------------------------------------------------------#
# 抛出异常
# ---------------------------------------------------------------------#
"""
try:
    raise NameError(&#39;HiThere&#39;)
except NameError:
    print(&#39;An exception flew by!&#39;)
    raise
"""

# ---------------------------------------------------------------------#
# 用户自定义异常
# ---------------------------------------------------------------------#
class MyError(Exception):   # 继承自Exception
    def __init__(self, value):  # 构造函数重写
        self.value = value

    def __str__(self):
        return repr(self.value)
try:
    raise MyError(2*2)  # 抛出异常
except MyError as e:
        print(&#39;My exception occurred, value:&#39;, e.value)
# 当创建一个模块有可能抛出多种不同的异常时，
# 一种通常的做法是为这个包建立一个基础异常类，
# 然后基于这个基础类为不同的错误情况创建不同的子类
class Error(Exception):
    """Base class for exceptions in this module."""
    pass
class InputError(Error):
    """Exception raised for errors in the input.
    Attributes:
        expression -- input expression in which the error occurred
        message -- explanation of the error
    """
    def __init__(self, expression, message):
        self.expression = expression
        self.message = message
class TransitionError(Error):
    """Raised when an operation attempts a state transition that&#39;s not
    allowed.

    Attributes:
        previous -- state at beginning of transition
        next -- attempted new state
        message -- explanation of why the specific transition is not allowed
    """

    def __init__(self, previous, next1, message):
        self.previous = previous
        self.next = next1
        self.message = message


try:
    raise InputError(2*2, 45)   # 抛出异常
except InputError:         # 处理异常
        print(&#39;My exception occurred&#39;)

# ---------------------------------------------------------------------#
# 定义清理行为
# ---------------------------------------------------------------------#

def divide(x, y):
    try:
        result = x / y
    except ZeroDivisionError:
        print("division by zero!")
    else:                           # 没有异常的时执行
        print("result is", result)
    finally:                        # 无论在任何情况下都会执行的清理行为
        print("executing finally clause")


# 一个异常在 try 子句里（或者在 except 和 else 子句里）被抛出，
# 而又没有任何的 except 把它截住，那么这个异常会在 finally 子句执行后再次被抛出
# divide("2", "1")   #从测试代码

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