1. Python The log sample

Logs can help us monitor the running state of programs . Through the log we can see if the program raises any warnings or errors . We record time stamps 、 news 、 Stack and so on .
Before using any logging function in the program , You need to explicitly import Python Of logging library .

import logging
# then some logging statement(s)

Here are Python in logging An example of library usage .

import logging
logging.warning("This is a simple logging example")

Execution and output ：

Next let's learn about the different levels of logging 、 How to set the log level and so on .

2. Log messages to log files

Use Python Of logging modular , You can try it out 、 Information 、 Warning 、 error 、 Serious errors are recorded in the log file , Instead of just echoing to the console .
To record data into a file , Use logging The basic configuration of , Send the address of the log file to filename Parameters . If the file does not exist ,Python The system will create a new file , So you should assign the new and write permissions of the file in the directory where the file is located to start the Python Users of the program . Use logging Modular basicConfig() Function to set filename The basic syntax for parameters is as follows .

logging.basicConfig(filename="mylog.log")

You can rewrite the file name according to your own needs .

2.1. Record logging Message to log file

In the next example , We will use basicConfig() Function settings logging To configure , Log the message to a message named mylog.log External files for . If the full path is not provided , This file will be created in the program's execution directory .

# Python Example for Logging Messages to Log File
import logging
logging.basicConfig(filename="mylog.log")
logging.warning("This is a WARNING message")
logging.error("This is an ERROR message")
logging.critical("This is a CRITICAL message")

Execution and output ：

Check the current program directory , New documents found mylog.log Generate ：

View its contents ：

so logging The message has been appended to the file .

2.3. Use handler Record logging Example of message to log file

This example is improved on the previous section , Used handler To handle the writing of log files .

# Python Example for Logging Messages to Log File using Handler
import logging
# create a logger
logger = logging.getLogger("mylogger")
handler = logging.FileHandler("mylog.log")
logger.addHandler(handler)
logger.warning("This is a WARNING message by handler")
logger.error("This is an ERROR message by handler")
logger.critical("This is a CRITICAL message by handler")

Execution and output ：

Check the log file again ：

logging The message has been appended to the log file , But the format is slightly different from that in the previous section .

3. Print DEBUG Level of logging

To use logging Print a row DEBUG Level of logging ,

• Check if the log level is at least DEBUG.
• Use logging.debug() Method , Pass the message to it as a parameter , In order to DEBUG Line logs are printed to the console or log file .

When logging Level set to DEBUG in the future ,logging It will put DEBUG Line to print to console or log file .
When you set logging The level of INFO、WARNING、ERROR or CRITICAL when ,DEBUG The line will not be written to the log file .
logging The order of rank is ：

DEBUG < INFO < WARNING < ERROR < CRITICAL

3.1. Print DEBUG Example of line

In this example , We will import logging modular , Set the log level to DEBUG, And then use debug() Method to print a DEBUG That's ok .

# Example to log DEBUG lines
import logging
# create a logger
logger = logging.getLogger("mylogger")
# set logging level
logger.setLevel(logging.DEBUG)
handler = logging.FileHandler("mylog.log")
# create a logging format
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
# write a debug message
logger.debug("This is a debug message")

Check... After executing the program mylog.log：

3.2. Record only DEBUG Example of row

You can also be right handler Filter to print only DEBUG Line to log file .

# Example to log only DEBUG Lines using Python Logger
import logging
class MyFilter(object):
def __init__(self, level):
self.__level = level
def filter(self, logRecord):
return logRecord.levelno <= self.__level
# create a logger
logger = logging.getLogger("mylogger")
logger.setLevel(logging.DEBUG)
handler = logging.FileHandler("mylog.log")
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
# set filter to log only DEBUG lines
handler.addFilter(MyFilter(logging.DEBUG))
logger.addHandler(handler)
# write a debug line to log file
logger.debug("This is a DEBUG message")
logger.info("This is a INFO message")
logger.warning("This is a WARNING message")
logger.error("This is an ERROR message")
logger.critical("This is a CRITICAL message")

Check... After executing the program mylog.log：

4. logging INFO That's ok

To use Python logging Print INFO That's ok ,

• Check logger Is the level at least INFO.
• Use logging.info() Method , Pass the message to it as a parameter , In order to INFO Line logs are printed to the console or log file .

When logging Level set to INFO or DEBUG in the future ,logging It will put INFO Line to print to console or log file .
When you set logging The level of WARNING、ERROR or CRITICAL when ,INFO The line will not be written to the log file .
logging The order of rank is ：

DEBUG < INFO < WARNING < ERROR < CRITICAL

4.1. Print INFO Example of line

In this example , We will import logging modular , Set the log level to INFO, And then use info() Method to print a INFO That's ok .

# Example to log INFO lines
import logging
# create a logger
logger = logging.getLogger("mylogger")
# set logging level
logger.setLevel(logging.INFO)
handler = logging.FileHandler("mylog.log")
# create a logging format
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
# write a info message
logger.info("This is a INFO message")

Check... After executing the program mylog.log：

4.2. Record only INFO Example of row

You can also be right handler Filter to print only INFO Line to log file .

# Example to log only INFO Lines using Python Logger
import logging
class MyFilter(object):
def __init__(self, level):
self.__level = level
def filter(self, logRecord):
return logRecord.levelno == self.__level
# create a logger
logger = logging.getLogger("mylogger")
logger.setLevel(logging.INFO)
handler = logging.FileHandler("mylog.log")
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
# set filter to log only INFO lines
handler.addFilter(MyFilter(logging.INFO))
logger.addHandler(handler)
# write a INFO line to log file
logger.debug("This is a DEBUG message")
logger.info("This is a INFO message")
logger.warning("This is a WARNING message")
logger.error("This is an ERROR message")
logger.critical("This is a CRITICAL message")

Check... After executing the program mylog.log：

5. logging WARNING That's ok

To use Python logging Print WARNING That's ok ,

• Check logger Is the level at least WARNING.
• Use logging.warning() Method , Pass the message to it as a parameter , In order to WARNING Line logs are printed to the console or log file .

When logging Level set to WARNING、INFO or DEBUG in the future ,logging It will put WARNING Line to print to console or log file .
When you set logging The level of ERROR or CRITICAL when ,WARNING The line will not be written to the log file .
logging The order of rank is ：

DEBUG < INFO < WARNING < ERROR < CRITICAL

5.1. Print WARNING Example of line

In this example , We will import logging modular , Set the log level to WARNING, And then use warning() Method to print a WARNING That's ok .

# Example to log WARNING lines
import logging
# create a logger
logger = logging.getLogger("mylogger")
# set logging level
logger.setLevel(logging.WARNING)
handler = logging.FileHandler("mylog.log")
# create a logging format
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
# write a warning message
logger.warning("This is a WARNING message")

Check... After executing the program mylog.log：

5.2. Record only WARNING Example of row

You can also be right handler Filter to print only WARNING Line to log file .

# Example to log only WARNING Lines using Python Logger
import logging
class MyFilter(object):
def __init__(self, level):
self.__level = level
def filter(self, logRecord):
return logRecord.levelno == self.__level
# create a logger
logger = logging.getLogger("mylogger")
logger.setLevel(logging.WARNING)
handler = logging.FileHandler("mylog.log")
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
# set filter to log only WARNING lines
handler.addFilter(MyFilter(logging.WARNING))
logger.addHandler(handler)
# write a WARNING line to log file
logger.debug("This is a DEBUG message")
logger.info("This is a INFO message")
logger.warning("This is a WARNING message")
logger.error("This is an ERROR message")
logger.critical("This is a CRITICAL message")

Check... After executing the program mylog.log：

6. logging ERROR That's ok

To use Python logging Print ERROR That's ok ,

• Check logger Is the level at least ERROR.
• Use logging.error() Method , Pass the message to it as a parameter , In order to ERROR Line logs are printed to the console or log file .

When logging Level set to ERROR、WARNING、INFO or DEBUG in the future ,logging It will put ERROR Line to print to console or log file .
When you set logging The level of CRITICAL when ,WARNING Lines or lower will not be written to the log file .
logging The order of rank is ：

DEBUG < INFO < WARNING < ERROR < CRITICAL

6.1. Print ERROR Example of line

In this example , We will import logging modular , Set the log level to ERROR, And then use warning() Method to print a ERROR That's ok .

# Example to log ERROR lines
import logging
# create a logger
logger = logging.getLogger("mylogger")
# set logging level
logger.setLevel(logging.ERROR)
handler = logging.FileHandler("mylog.log")
# create a logging format
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
# write an error message
logger.error("This is an ERROR message")

Check... After executing the program mylog.log：

6.2. Record only ERROR Example of row

You can also be right handler Filter to print only ERROR Line to log file .

# Example to log only ERROR Lines using Python Logger
import logging
class MyFilter(object):
def __init__(self, level):
self.__level = level
def filter(self, logRecord):
return logRecord.levelno == self.__level
# create a logger
logger = logging.getLogger("mylogger")
logger.setLevel(logging.ERROR)
handler = logging.FileHandler("mylog.log")
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
# set filter to log only ERROR lines
handler.addFilter(MyFilter(logging.ERROR))
logger.addHandler(handler)
# write a ERROR line to log file
logger.debug("This is a DEBUG message")
logger.info("This is a INFO message")
logger.warning("This is a WARNING message")
logger.error("This is an ERROR message")
logger.critical("This is a CRITICAL message")

Check... After executing the program mylog.log：

7. logging CRITICAL That's ok

To use Python logging Print CRITICAL That's ok ,

• Check logger Is the level at least CRITICAL.
• Use logging.critical() Method , Pass the message to it as a parameter , In order to CRITICAL Line logs are printed to the console or log file .

When logging Level set to CRITICAL、ERROR、WARNING、INFO or DEBUG in the future ,logging It will put CRITICAL Line to print to console or log file .
When you set logging The level of CRITICAL when ,ERROR Lines or lower will not be written to the log file .
logging The order of rank is ：

DEBUG < INFO < WARNING < ERROR < CRITICAL

7.1. Print CRITICAL Example of line

In this example , We will import logging modular , Set the log level to CRITICAL, And then use critical() Method to print a CRITICAL That's ok .

# Example to log CRITICAL lines
import logging
# create a logger
logger = logging.getLogger("mylogger")
# set logging level
logger.setLevel(logging.CRITICAL)
handler = logging.FileHandler("mylog.log")
# create a logging format
formatter = logging.Formatter("%(asctime)s - %(name)s - %(levelname)s - %(message)s")
handler.setFormatter(formatter)
logger.addHandler(handler)
# write an CRITICAL message
logger.critical("This is a CRITICAL message")

Check... After executing the program mylog.log：

Reference material

• Python Logging
• Python Logging Messages to a Log File
• Python Logging – Set DEBUG Level
• Python Logging – Set INFO Level
• Python Logging – Set WARNING Level
• Python Logging – Set ERROR Level
• Python Logging – Set CRITICAL Level