Python Cookbook (3rd edition) Chinese version: 14.11 Output warning message

14.11 Output warning message? problem?

You want your program to generate warning messages (such as obsolete features or usage issues).

Solution?

To output a warning message, you can use a warning.warn() function. For example:

 import warningsdef func ( xylogfile=< Span class= "BP" >nonedebug=false ): if logfile is not none: warnings. Warn ( ' logfile argument deprecated ' ,  Deprecationwarning)           

warn()parameter is a warning message and a warning class with the following types of warnings: userwarning, deprecationwarning,
Syntaxwarning, runtimewarning, resourcewarning, or futurewarning.

The handling of warnings depends on how you run the interpreter and some other configurations.
For example, if you use the -W all option to run Python, you will get the following output:

 bash % python3 -w all example. Pyexample. Py:5: deprecationwarning: span class= "n" >logfile argument is deprecated  Warnings. Warn ( "logfile argument is deprecated '   Deprecationwarning)             

In general, warnings are output to standard errors. If you want to tell the warning to be converted to an exception, you can use the -W error option:

Bash%Python3-WErrorExample.PyTraceback(MostRecentPagerLast):File"Example.py",Line10,Inch<Module>func(2, 3, logfile=' Log.txt ') File "example.py", Line 5, in func warnings.  Warn(' logfile argument is deprecated ', deprecationwarning)deprecationwarning: logfile argument is deprecatedbash %          

Discuss?

When you maintain the software and prompt the user for certain information, but do not need to raise it to an exception level, it is useful to output a warning message.
For example, suppose you are ready to modify the functionality of a function library or framework, you can output a warning message for the part you want to change, and for a period of backwards compatibility.
You can also warn users about how to use the code in a problematic way.

As an example of a warning used by another built-in library, the following shows a warning message that is generated when you destroy a file without closing it:

 >>> import warnings>> > warnings. Simplefilter ( ' always ' ) >>> f = open ( '/etc/passwd ' ) >>> del f__main__:1: resourcewarning:unclosed file <_io. Textiowrapper name= '/etc/passwd '  mode= ' R ' encoding= ' UTF-8 ' >>>>  

By default, not all warning messages will appear. -w   option to control the output of the warning message.
-w all   All warning messages will be output, -w ignore   Ignore all warnings, -w error   Convert warnings to exceptions.
Another option, you can also use  warnings.simplefilter ()   function control output.
always   parameters will cause all warning messages to appear " Ignore   Ignore all warnings, error   Converts the warning to an exception.

This is enough for a simple case of generating a warning message.
warningsThe module provides a number of more advanced configuration options for filtering and warning message processing.
For more information, refer to the Python documentation

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Python Cookbook (3rd edition) Chinese version: 14.11 Output warning message