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Python Error and Exception Concepts (General)
1. How to handle errors and exceptions
a: NameError
if True: SyntaxError
f = oepn('1.txt'): IOError
10/0: ZeropisionError
a = int('d'): ValueError
Program Running interruption: KeyboardInterrupt
try: try_suite except Exception [e]: exception_block
try is used to capture errors in try_suite, and Handle the error to except
except is used to handle exceptions. If handling exceptions is consistent with setting captured exceptions, use exception_block to handle exceptions
# case 1 try: undef except: print 'catch an except'
# case 2 try: if undef except: print 'catch an except'
case1: The exception can be caught because it is a runtime error
case2: The exception cannot be caught because it is a syntax error and a pre-run error
--
# case 3 try: undef except NameError,e: print 'catch an except',e
# case 4 try: undef except IOError,e: print 'catch an except',e
case3: The exception can be caught because the catch NameError exception is set
case4: The exception cannot be caught because Setting IOError will not handle NameError
import random num = random.randint(0, 100) while True: try: guess = int(raw_input("Enter 1~100")) except ValueError, e: print "Enter 1~100" continue if guess > num: print "guess Bigger:", guess elif guess < num: print "guess Smaller:", guess elif guess == num: print "Guess OK,Game Over" break print '\n'
try -except: handle multiple exceptions
try: try_suite except Exception1[e]: exception_block1 except Exception2[e]: exception_block2 except ExceptionN[e]: exception_blockN
try: try_suite finally: do_finally
If the try statement does not catch the error, the code executes the do_finally statement
If the try statement catches an error, the program first executes the do_finally statement, and then hands the captured error to the python interpreter for processing
try: try_suite except: do_except finally: do_finally
If the try statement does not catch the exception, after executing the try code segment, execute finally
If try catches the exception, first executes except to handle the error, and then executes finally
with context [as var]: with_suite
with statement is used to replace try_except_finall statement to make the code more concise
The context expression returns an object
var is used to save the context return object, a single return value or a tuple
with_suite uses var variables to operate on the context return object
Context management protocol: Contains methods __enter__()
and __exit()__
. Objects that support this protocol must implement these two methods
Context manager: defines the runtime context to be established when executing the with statement, and is responsible for executing the entry and exit operations in the context of the with statement block
Enter the context manager: call Manager __enter__
method, if the as var statement is set, the var variable accepts __enter__()
method return value
Exit the context manager: call Manager__exit__
Method
class Mycontex(object): def __init__(self, name): self.name = name def __enter__(self): print "__enter__" return self def do_self(self): print "do_self" def __exit__(self, exc_type, exc_val, exc_tb): print "__exit__" print "Error:", exc_type, " info:", exc_val if __name__ == "__main__": with Mycontex('test context') as f: print f.name f.do_self()
File operation
Mutual exclusion objects between process threads, such as mutex locks
Other objects that support context
rais statement
reise statement is used Actively throw exceptions
Syntax format: raise[exception[,args]]
exception: exception class
args: Tuple describing exception information
raise TypeError, 'Test Error'
raise IOError, 'File Not Exit'
assert statement
Assertion statement: The assert statement is used to check whether the expression is true. If it is false, an AssertionError is raised.
Syntax format: assert expression[,args]
experession: expression
args: description information of the judgment condition
assert 0, 'test assert'
assert 4==5, 'test assert'
Standard exceptions
Python built-in exceptions already exist before the program is executed
Custom exceptions:
Python allows custom exceptions, Used to describe exceptions not involved in python
Custom exceptions must inherit the Exception class
Custom exceptions can only be actively triggered
class CustomError(Exception): def __init__(self, info): Exception.__init__(self) self.message = info print id(self) def __str__(self): return 'CustionError:%s' % self.message try: raise CustomError('test CustomError') except CustomError, e: print 'ErrorInfo:%d,%s' % (id(e), e)
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