Hi there, In this article, we will explore how to use warnings filter and warning categories in python. Also how we can use different warnings for different scenarios.
Warning messages are issued in situations where we warn the developer of situations that are not necessarily exceptions and it does not terminate the program. For example, We might issue a warning when a program uses an obsolete module or certain programming elements such as some keywords,
functions, classes, etc. A warning in the program is distinct from an error. In simple words, the program gets terminated if an error occurred. but in case of a warning, it displays a warning message but the program runs.
Warning categories are useful to filter out groups of warnings. Also, developers can define additional warnings by subclassing one of the warning categories
Note: A warning category must be always a subclass of the warning class.
1. exception Warning
This is a base class of all warning category classes. And it is a subclass of Exception.
# program to illustrate warn() # importing modules import warnings # displaying warning warnings.warn('Warning Message') Output: main.py:2: UserWarning: Geeks 4 Geeks warnings.warn('Geeks 4 Geeks')
2. exception UserWarning
Base class for warning generated by code.
# program to illustrate warn() # importing modules import warnings # displaying warning x = 3 if x<4 : warnings.warn('Please Enter valid Number') Output: main.py:7: UserWarning: Please Enter valid Number warnings.warn('Please Enter valid Number')
Similarly, We can use warn methods listed below.
3. exception DeprecationWarning
It is a base class for alerts regarding obsolete features when those warnings are intended for other python developers
By default, it’s ignored, but it can be triggered by code in the main.
4. exception PendingDeprecationWarning
This exception is also ignored by default. It is a base class for warning about features that are obsolete and can be deprecated in future but not at the moment.
5. exception SyntaxWarning
It is a base class of warnings about dubious syntactic features.
6. exception RuntimeWarning
Base class of warnings for dubious behavior at run time or suspicious run time attributes.
#runtime warning #dividing 1 by zero cause runtime error import warnings try: print(1/0) except : warnings.warn("caught an runtimeWarning", RuntimeWarning) Output: main.py:8: RuntimeWarning: caught an runtimeWarning warnings.warn("caught an runtimeWarning", RuntimeWarning)
7. exception FutureWarning
It is a base class of warnings on obsolete features when certain warnings are meant for end-user of python written programs.
8. exception ImportWarning
Base class of warnings when there is a probable mistake in importing modules. By default, it is ignored by the filter. We can enable it with the python developer mode.
9. exception UnicodeWarning
Used to Display warnings that are related to the Unicode
10. exception EncodingWarning
Base class of warning related to encoding. Introduced in 3.10
11. exception BytesWarning
It is used to display a warning that is related to the bytes or byteArray.
12. exception ResourceWarning.
Base class for warnings related to resource usage. Also, it is ignored by default and can be used with python developer mode.
The Warning Filter:
It is used to handle warnings in python. The warning filter maintains an ordered list of filter specifications. and warning is matched against each filter specification till the warning gets matched. And Each entry is a tuple of the form (action, message, category, module, line no)
action values (string):
1. default: display the first matching warning for each position.
2. error: Converts warning to raise an exception.
3. ignore: Never display warnings that match.
4. always: Always print matching warnings
5. module: First occurrence of warning for each module where warning issued. regardless of line no
6. once: Display the first matching warning regardless of location.
Message (string) :
The message is the string that contains a regular expression that must match the beginning of the warning.
The category is the class of which the warning category must be a subclass in order to match.
The module is a string with a regular expression that must match the module name.
An integer to match the number of line in which the warning appeared. or 0 to match any number of line.
Here is a list of some commonly used warning functions.
warn(message, category=none, stacklevel=1, source=none)
#function warn() import warnings # displaying warning warnings.warn('The Code Hubs') Output: main.py:4: UserWarning: The Code Hubs warnings.warn('The Code Hubs')
filterwarnings(action, message=”, category=Warning, module=”, lineno=0, append=False)
This function adds entry into the specifications of warning filter.
Program: #program for filterwarnings import warnings # adding entry into the specifications # of the warnings filter. warnings.filterwarnings('ignore', '.*do not.*', ) # displaying warinings warnings.warn('The Code Hubs !') # this warning will not be displayed warnings.warn('Do not show this message') Output: main.py:12: UserWarning: The Code Hubs ! warnings.warn('The Code Hubs !')
simplefilter(action, category=Warning, lineno=0, append=False):
Used to add a single entry to the filter requirements list.
# simplefilter() import warnings warnings.simplefilter('error', UserWarning) # display warning warnings.warn('Hope You Find It Useful') Output: Traceback (most recent call last): File "main.py", line 5, in <module> warnings.warn('This is a warning message') UserWarning: This is a warning message
With that, it covers almost every concept of warning for more give a visit to The Warning Section of official documentation.
Thanks for the read.