Warnings in Python

Last Updated : 23 Jan, 2020

Warnings are provided to warn the developer of situations that aren't necessarily exceptions. Usually, a warning occurs when there is some obsolete of certain programming elements, such as keyword, function or class, etc. A warning in a program is distinct from an error. Python program terminates immediately if an error occurs. Conversely, a warning is not critical. It shows some message, but the program runs. The warn() function defined in the 'warning' module is used to show warning messages. The warning module is actually a subclass of Exception which is a built-in class in Python.

Python3

# program to display warning a message 


import warnings

print('Geeks')

# displaying the warning message 
warnings.warn('Warning Message: 4')

print('Geeks !')

Output:

Geeks
main.py:8: UserWarning: Warning Message: 4  
  warnings.warn('Warning Message: 4')
Geeks!

In the above program, the warn() function of the warning module is used to display the message Warning: Message: 4, the UserWarning is the default category of warn() function.

Warning Categories

In Python there are a variety of built-in exceptions which reflect categories of warning, some of them are:

Warning Class: It is the super class of all warning category classes and a subclass of the Exception class.
UserWarning Class: warn() function default category.
DeprecationWarning Class: Base category for alerts regarding obsolete features when those warnings are for other developers (triggered by code in __main__ unless ignored).
SyntaxWarning Class: Base class for warnings of suspicious syntactic attributes.
RuntimeWarning Class: Base class for warnings of suspicious run time attributes.
FutureWarning Class: Base class for warnings on obsolete features when certain warnings are meant for end-users of Python-written programs.
PendingDeprecationWarning Class: Base class for warnings of an outdated attribute.
ImportWarning Class: Base class for warnings caused during a module importation process.
UnicodeWarning Class: Base class for Unicode based warnings.
BytesWarning Class: Base class for bytes and bytearray based warnings.
ResourceWarning Class: Base class for resource-related warnings.

Warning Filters

The warning filter in Python handles warnings (presented, disregarded or raised to exceptions). The warnings filter establishes an organized list of filter parameters, any particular warnings are matched on each filter requirement throughout the list till the match is made, the filter determines the match arrangement. Every entry is indeed a tuple (action, message, category, module, lineno) of the form in which:

The action can be any of the following strings:

String	Explanation
"default"	Displays the first matching warnings for each position
"error"	Converts warnings to raise exceptions
"ignore"	Never display warnings which match
"always"	Always display the warnings which match
"module"	Displays the first matching warnings per module
"once"	Display just the first matching warnings, regardless of where they are located

The message is a string that has a regular expression that must match the beginning of the warning. (The expression compiled is always case-insensitive)
The category is a class (warning subclass) of which the warning class must be a subclass for matching.
The module is a string with a regular expression which must match the module name (The expression compiled is always case-insensitive).
The lineno is an integer to match the number of the line in which the warning appeared, or 0 to match any number of the line.

Warning Functions

Some of the commonly used functions of the warning module are:

warn(message, category=None, stacklevel=1, source=None): This function displays a warning, or disregard it or converts is to an exception.
Python3
# program to illustrate warn() # function in warning module # importing modules import warnings # displaying warning warnings.warn('Geeks 4 Geeks')
Output:
```
main.py:2: UserWarning: Geeks 4 Geeks
  warnings.warn('Geeks 4 Geeks')
```
In the above program warnings are displayed using the warn() function of warning module.
warn_explicit(message, category, filename, lineno, module=None, registry=None, module_globals=None, source=None): This function is a low-level method with warn() features

filterwarnings(action, message='', category=Warning, module='', lineno=0, append=False): This function adds an entry into the specifications of the warnings filter.

Python3

# program to illustrate filterwarnings()
# function in warning module

# importing module
import warnings

# adding entry into the specifications
# of the warnings filter.
warnings.filterwarnings('ignore', '.*do not.*', )

# displaying warinings
warnings.warn('Geeks 4 Geeks !')

# this warning will not be displayed
warnings.warn('Do not show this message')

Output:

main.py:8: UserWarning: Geeks 4 Geeks!
  warnings.warn('Geeks 4 Geeks!')

Here the second warning message is not displayed due to warnings.filterwarnings('ignore', '.*do not.*', ) in which action is "ignore" .

showwarning(message, category, filename, lineno, file=None, line=None): This function Writes a warning to a file.

simplefilter(action, category=Warning, lineno=0, append=False): This function adds a single entry into the warnings filter requirements list.

Python3 1==

# program to illustrate simplefilter() 
# function in warning module

# importing module
import warnings

# adding a single entry into warnings filter
warnings.simplefilter('error', UserWarning)

# displaying the warning
warnings.warn('This is a warning message')

Output:

Traceback (most recent call last):
  File "main.py", line 8, in     
     warnings.warn('This is a warning message')
UserWarning: This is a warning message

In the above program, a single entry is added to the warnings filter using warnings.simplefilter('error', UserWarning) in which the action is "error" and category is UserWrning and then the warning is displayed using the warn() method.