Python exception handling and built-in modules

exception handling

Sometimes when we write a program, an error or exception occurs, causing the program to terminate, as in the following example:

#!/usr/bin/env python
a = 2/0
print(a)

The result shows the following error:

Traceback (most recent call last):
  File "002.py", line 2, in <module>
    a = 2/0
ZeroDivisionError: integer division or modulo by zero

The above prompts that the dividend cannot be 0, which will cause the program to run interrupted. In order to allow the program to execute normally, we can add a tey...except... statement:

try:
    a = 2/ 0
     print (a)
 except Exception as e:
     print ( "The divisor cannot be 0 " )
     # raise e # Raise the exception 
finally :
     print ( " No matter what happens, this step is executed. " )

result:
Divisor cannot be 0
Do this step no matter what happens.

If raise e is added above, the exception information will be printed: ZeroDivisionError: integer division or modulo by zero. The except part is to process the error information, and finally this step will be executed regardless of whether there is an exception before.

Python standard exception class:

exception name	describe
BaseException	Base class for all exceptions
SystemExit	interpreter request to exit
KeyboardInterrupt	User interrupts execution (usually by typing ^C)
Exception	base class for general errors
StopIteration	iterator has no more values
GeneratorExit	Generator (generator) exception to notify exit
SystemExit	Python interpreter request to quit
StandardError	Base class for all built-in standard exceptions
ArithmeticError	Base class for all numerical computation errors
FloatingPointError	floating point calculation error
OverflowError	Numerical operation exceeds 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, reaching the EOF marker
EnvironmentError	Base class for operating system errors
IOError	I/O operation failed
OSError	operating system error
WindowsError	system call failed
ImportError	Failed to import module/object
KeyboardInterrupt	User interrupts execution (usually by typing ^C)
LookupError	Base class for invalid data queries
IndexError	There is no such index in the sequence (index)
KeyError	There is no such key in the map
MemoryError	Out of memory error (not fatal to the Python interpreter)
NameError	undeclared/initialized object (no properties)
UnboundLocalError	access uninitialized local variable
ReferenceError	Weak reference attempts to access objects that have been garbage collected
RuntimeError	General runtime errors
NotImplementedError	Method not yet implemented
SyntaxError	Python syntax error
IndentationError	Indentation error
TabError	Mixing Tabs and Spaces
SystemError	General interpreter system errors
TypeError	Invalid operation on type
ValueError	Invalid parameter passed in
UnicodeError	Unicode related errors
UnicodeDecodeError	Error in Unicode decoding
UnicodeEncodeError	Error in Unicode encoding
UnicodeTranslateError	Error during Unicode conversion
Warning	base class for warnings
DeprecationWarning	Warning about deprecated features
FutureWarning	Warning about future semantic changes of constructs
OverflowWarning	Old warning about auto-promotion to long
PendingDeprecationWarning	Warning that features will be deprecated
RuntimeWarning	可疑的运行时行为(runtime behavior)的警告
SyntaxWarning	可疑的语法的警告
UserWarning	用户代码生成的警告

　　

模块

模块和目录的区别，看整个文件夹里面是否有__init__.py文件，有就是模块，没有就是普通目录。__init__.py一般是一个空文件。

通常一个.py文件我们就可以称之为一个模块。

a.py
#!/usr/bin/env python
def hello():
    print("hello")
hello()

def world():
    print("world")
world()

b.py
#!/usr/bin/env python
import a

运行python b.py结果：
hello
world

如上 ，当我们在b.py中将a.py作为模块导入之后，在运行b.py的时候，直接会运行a.py里面的所有函数，但是如果我们只想要在b.py中调用a.py中指定函数的时候运行，就需要在a.py中加入if __name__ == "__main__":语句：

a.py
#!/usr/bin/env python
def hello():
    print("hello")
    
def world():
    print("world")

if __name__ == "__main__":
    hello()
    world()

b.py
#!/usr/bin/env python
import a
a.hello()
a.world()

运行python b.py之后的结果：
hello
world

由上可以看出，加上if __name__ == "__main__":语句之后，就能满足我们的需求。

总结：

    1、文件夹里面需要有__init__.py文件的才能当做模块使用。

    2、if __name__ == "__main__":语句的使用。

 

内置模块

 

datetime

import datetime
# 下面我们使用的是datetime模块下面的datetime模块，所以使用的时候需要datetime.datetime，为了更方便的使用，也可以直接使用from datetime import datetime
print(datetime.datetime.now())          # 打印当前时间  2018-04-23 09:33:32.055974
print(datetime.datetime.now().year)     # 打印当前时间中的年份 2018
print(datetime.datetime.now().month)    # 打印当前时间中的月份 4
print(datetime.datetime.now().day)      # 打印当前时间中的天 23
print(datetime.datetime.now().hour)     # 打印当前时间中的小时 9
print(datetime.datetime.now().minute)   # 打印当前时间中的分钟 33
print(datetime.datetime.now().second)   # 打印当前时间中的秒   32
print(datetime.datetime.now().microsecond)  # 打印当前时间中的毫秒 56063
print(datetime.datetime.now().strftime("%Y-%m-%d"))  # 从时间格式转换成字符串，满足"%Y-%m-%d"格式的字符串格式    2018-04-23 09:33:32.055974  -->  2018-04-23
print(datetime.datetime.now().strftime("%c"))        # 标准时间，类似于这种格式  Mon Apr 23 09:50:45 2018
print(datetime.datetime.now().strftime("%a"))        # 本地简化星期名称  Mon
print(datetime.datetime.now().strftime("%b"))        # 本地简化月份名称  Apr
print(datetime.datetime.now().strftime("%d"))        # 当前这天是一个月中的第几天  23
# 直接导入datetime模块下面的datetime
#from datetime import datetime
#print(datetime.now())

%Y    带世纪部分的十进制年份

%m   十进制表示的月份

%d    十进制表示的每月的第几天

%H    24小时制的小时

%M    十进制表示的分钟数

%S    十进制的秒数

 

如果我们需要表示昨天、上周等情况：

#!/usr/bin/env python
from datetime import datetime
from datetime import timedelta
now_time = datetime.now()   # 当前时间
print(now_time)
b = now_time + timedelta(days = -1)  # 一天前
print(b)
c = now_time + timedelta(days = -1,weeks = -1)  # 一个周前的前一天
print(c)

结果：
2018-04-23 10:35:40.245370
2018-04-22 10:35:40.245370
2018-04-15 10:35:40.245370

time模块

这个time模块不是datetime下面的那个模块，它是一个单独的模块。

#!/usr/bin/env python
import time
time.sleep(2)  # 暂停2秒后，打印
print("Hello")
print(time.time()) # 打印时间戳，即从1970-01-01到现在的秒数


print(time.localtime())
# time.struct_time(tm_year=2018, tm_mon=4, tm_mday=23, tm_hour=10, tm_min=47, tm_sec=59, tm_wday=0, tm_yday=113, tm_isdst=0)
time.strptime(string,[,format])  # 把一个格式化时间字符串转化为struct_time，它和strftime是逆操作。

commands模块

有时候我们需要使用shell命令，就用到了commands模块。

#!/usr/bin/env python
import commands
output = commands.getoutput("ls -ll")  # 返回执行完命令后的结果
print(output)
status, output = commands.getstatusoutput("ls -l")  # 返回一个元组，如果shell执行成功，第一个值（状态码）是0表示成功，第二个值是shell执行结果
print(status,output)

subprocess模块

和commands模块用法类似，都是用来执行shell命令的。

#!/usr/bin/env python
from subprocess import PIPE,Popen
p = Popen(['ifconfig'],stdout=PIPE)
data = p.stdout.read()
print(data)