Python exception handling and debugging

Python provides a powerful exception handling mechanism, can improve the robustness of the program by capturing exceptions. Exception handling has also released an object, such as suspension of the role of the cycle of operation. The program is running, if an error occurs, you can return an error code prior agreement.

"Try ... except" statement

To deal with the problem statement, catch exceptions may exist. try block placed in clause exception statements may occur, except clause code for handling exceptions. When an exception occurs, Python will automatically generate an exception object.

>>> try:
...     f = open('test.txt', 'r')
...     print("该文件是正常的")
    # 捕获IO异常
... except IOError:
...     print("该文件不存在")
    # 其他异常情况
... except:
...     print('程序异常')
... else:
...     print('文件打开成功')
        f.close()
该文件不存在
>>>

try...exceptYou can also add back a finallystatement, regardless of whether an exception occurs, the finallyclause will be executed. All finallyclauses are used to turn off the system resources can not be released due to an exception.

try:
    f = open('test.txt', 'r')
    try:
        print(f.read())
    except:
        print('该文件是正常的')
    finally:
        print('释放资源')
        f.close()
except IOError:
    print('文件不存在')

with...as

with...as(Context tube) may achieve the above functions very simple

with open('test.txt', 'r') as f:
    f.write('hrllo ')
    f.write('world ')

with...asCircumstances can handle unusual circumstances, and avoids open () after a file forget to close () method

raise

When a program error occurs, Python will automatically raise an exception, can also raiseexception that is thrown display statement, once executed raisestatement, raisethe code after the statement will not be executed

try:
    s = None
    if s is None:
        print('s是空对象')
        raise NameError
    print(len(s))
except TypeError:
    print('空对象是没有长度的')
    
    
s是空对象
Traceback (most recent call last):
  File "异常处理.py", line 21, in <module>
    raise NameError
NameError

Python debugging

General debugging method used

print method
Assertion (assert) method
logging module
pdb
Editor comes with debugging features

Assertion (assert) method for detecting whether an expression is true

>>> assert 1 == 0, '1 不等于 0'        # '1 不等于 0' 是为断言语句加的异常参数
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: 1 不等于 0
>>>

logging module

If the Python program code amount to a certain number, use the logging module is a good choice, logging can not only output to the console, you can also write to the file, TCP send the logs to the network can also be used.

import logging


logging.debug('debug message')
logging.info('info message')
logging.warning('warning message')
logging.error('error message')
logging.critical('critical message')

# 输出结果：
WARNING:root:warning message
ERROR:root:error message
CRITICAL:root:critical message

By default, logging module logs print to the screen (stdout), log level is WARNING (only WARNING log level is higher than the output of the log will)

pdb

Python debugger that allows single-step program execution, at any time to view the running status

carried outpython3 -m pdb test.py

"L" represents the code for the full experience, "n" represents a step by step through the code, "p + variable name" variable names can print out the program at any time, "q" to exit