1.语法错误
语法错误, 也称为解析错误:
>>> while True print('Hello world')
File "<stdin>", line 1, in ?
while True print('Hello world')
^
SyntaxError: invalid syntax
2.异常
语法正常, 运行时检测到错误, 则为异常.
>>> 10 * (1/0)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
ZeroDivisionError: int division or modulo by zero
>>> 4 + spam*3
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: Can't convert 'int' object to str implicitly
3.异常处理
通过编程来处理异常.
示例: 它会一直要求用户输入, 直到输入一个合法的整数为止, 但允许用户中断这个程序.
用户中断程序会引发一个KeyboardInterrupt异常.
>>> while True:
... try:
... x = int(input("Please enter a number: "))
... break
... except ValueError:
... print("Oops! That was no valid number. Try again...")
...
一个try可以包含多个except, 一个except可以列出多个异常的名字:
... except (RuntimeError, TypeError, NameError):
... pass
最后一个except通常省略异常名, 作为通配. 可以重新抛出异常raise.
import sys
try:
f = open('myfile.txt')
s = f.readline()
i = int(s.strip())
except OSError as err:
print("OS error: {0}".format(err))
except ValueError:
print("Could not convert data to an integer.")
except:
print("Unexpected error:", sys.exc_info()[0])
raise
try...except可以在所有except后面带上一个else, 当try语句没有抛出异常时, 将执行else中的代码:
for arg in sys.argv[1:]:
try:
f = open(arg, 'r')
except IOError:
print('cannot open', arg)
else:
print(arg, 'has', len(f.readlines()), 'lines')
f.close()
在异常名之后, 可以指定一个变量, 这个变量可以获取到一些异常信息:
>>> try:
... raise Exception('spam', 'eggs')
... except Exception as inst:
... print(type(inst)) # the exception instance
... print(inst.args) # arguments stored in .args
... print(inst) # __str__ allows args to be printed directly,
... # but may be overridden in exception subclasses
... x, y = inst.args # unpack args
... print('x =', x)
... print('y =', y)
...
<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam
y = eggs
异常处理器也捕获函数内部发生的异常:
>>> def this_fails():
... x = 1/0
...
>>> try:
... this_fails()
... except ZeroDivisionError as err:
... print('Handling run-time error:', err)
...
Handling run-time error: int division or modulo by zero
4.抛出异常
raise允许强制抛出一个异常:
>>> raise NameError('HiThere')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: HiThere
抛出的异常必须是一个异常实例或者异常类, 继承自Exception.
捕获异常后重新抛出:
>>> try:
... raise NameError('HiThere')
... except NameError:
... print('An exception flew by!')
... raise
...
An exception flew by!
Traceback (most recent call last):
File "<stdin>", line 2, in ?
NameError: HiThere
5.自定义异常
异常类通常直接或间接从Exception类派生:
>>> class MyError(Exception):
... def __init__(self, value):
... self.value = value
... def __str__(self):
... return repr(self.value)
...
>>> try:
... raise MyError(2*2)
... except MyError as e:
... print('My exception occurred, value:', e.value)
...
My exception occurred, value: 4
>>> raise MyError('oops!')
Traceback (most recent call last):
File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'
定义异常基类, 再派生多种不同的异常类型:
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'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, next, message):
self.previous = previous
self.next = next
self.message = message
大多数异常的命名都以Error结尾.
6.定期清理行为
无论异常是否捕获, finally都将在try执行完毕后执行.
示例:
>>> try:
... raise KeyboardInterrupt
... finally:
... print('Goodbye, world!')
...
Goodbye, world!
KeyboardInterrupt
Traceback (most recent call last):
File "<stdin>", line 2, in ?
即使在try中执行了break/continue/return退出了try, 同样也会执行finally子句.
示例:
>>> def divide(x, y):
... try:
... result = x / y
... except ZeroDivisionError:
... print("division by zero!")
... else:
... print("result is", result)
... finally:
... print("executing finally clause")
...
>>> divide(2, 1)
result is 2
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'
7.预定义清理行为
with语句使得文件之类的对象可以确保及时准确地进行清理:
with open("myfile.txt") as f:
for line in f:
print(line)
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