Python Warning – Tutorial with Examples

Python is a popular programming language among developers all over the world. It provides various built-in functions and features that make the development process much faster and more convenient. One such feature is warnings.

Python warnings are used to indicate that something unexpected or unusual has happened in the code, but the code can still continue to run. It is a way to alert developers to potential problems and suggest ways to fix them. Warnings are very helpful in debugging, as they help to identify the source of problems.

In this article, we’ll cover everything you need to know about warnings in Python, including how to ignore them, how to change their behavior, and how to create your own custom warning messages.

What Are Warnings?

Python warnings are messages that appear during the execution of a program, informing the developer of potential issues with the code. They do not stop the code from running, but they do suggest that there may be problems that need to be addressed. Warnings can be used to identify bugs, performance issues, and other problems that would otherwise go unnoticed.

Warnings may be related to deprecation, resource usage, coding standards, or other issues.

Types of Warnings

Python has many built-in warnings, which are classified into different categories like DeprecationWarning, SyntaxWarning, OverflowWarning, ImportWarning, and so on.

The following are some of the most commonly used warnings in Python:

• DeprecationWarning − This warning is issued when a method or property has been deprecated and will be removed in future versions of Python.
• RuntimeWarning − This warning is issued when a runtime problem occurs that is not serious enough to halt the execution of the code.
• SyntaxWarning − This warning is issued when a statement or expression has syntax errors that are not fatal to the interpretation of the code.
• PendingDeprecationWarning − This warning is issued when a method or property is set to be deprecated in the future, but not immediately.

Using Warnings in Python

Python provides a standard warnings module that can be used to issue and control warnings.

Issuing Warnings

You can issue warnings in Python using the warnings.warn() function. This function takes the following arguments:

• message − The warning message to be displayed. This argument is mandatory.
• warning type − The type of warning to be displayed. This argument is optional and defaults to Warning.
• stacklevel − The stack level at which the warning is issued. This argument is optional and defaults to 1.

Here’s an example:

import warnings

def myfunc(x):
    if x > 100:
        warnings.warn("x is greater than 100.", UserWarning)

myfunc(101)

The code above will issue a warning because the value of x is greater than 100. The warning message will be displayed as follows:

UserWarning: x is greater than 100.
  warnings.warn("x is greater than 100.", UserWarning)

Ignoring Warnings

If you want to temporarily ignore warnings, you can use the warnings.filterwarnings() function. This function takes the following arguments:

• action − This argument specifies the action to be performed on the warning. It can be set to 'error', 'ignore', 'always', 'default', or 'module'. The default value is 'default'.
• category − This argument specifies the category of warnings to be filtered. It can be set to any valid warning category name.
• module − This argument specifies the name of the module to which the warning applies. It can be set to None, which means all modules.
• lineno − This argument specifies the line number at which the warning occurred. It can be set to None, which means all lines.
• append − This argument specifies whether to append the new warning filter to the existing filters or to replace the existing filters. It can be set to True (append) or False (replace). The default value is True.

Here’s an example that shows how to temporarily ignore warnings:

import warnings

warnings.warn("This is a warning message.")
warnings.filterwarnings("ignore")
warnings.warn("This is another warning message.")

In the code above, the first warning message will be displayed, but the second message will be ignored because we have used the ignore action to filter warnings.

Changing Warning Behaviors

You can control the behavior of warnings using the warnings.simplefilter(action, category) function. This function takes two arguments:

• action − This argument specifies the action to be performed on the warning. It can be set to 'error', 'ignore', 'always', 'default', or 'module'. The default value is 'default'.
• category − This argument specifies the category of warnings to be filtered. It can be set to any valid warning category name.

You can use this function to do the following:

• Set the default warning action using warnings.simplefilter('default').
• Change the default behavior of a specific warning using warnings.simplefilter('action', category=SomeWarning).

Here’s an example:

import warnings

warnings.simplefilter("error", DeprecationWarning)

def myfunc():
    warnings.warn("myfunc() is deprecated.", DeprecationWarning)

myfunc()

In the code above, the DeprecationWarning is set to error, which means that the code will raise an exception if this warning is issued. Thus, when we call the myfunc() function, an exception will be raised because we have issued a DeprecationWarning warning.

Traceback (most recent call last):
  File "test.py", line 8, in <module>
    myfunc()
DeprecationWarning: myfunc() is deprecated.

Creating Custom Warnings

You can create your own custom warnings in Python using the warnings.warn() function. To do this, you should define a new warning class that inherits from the standard Warning class.

Here’s an example:

import warnings

class MyWarning(Warning):
    pass

def myfunc(x):
    if x > 100:
        warnings.warn("x is greater than 100.", MyWarning)

myfunc(101)

The code above defines a new warning class called MyWarning. It then issues a warning if the value of x is greater than 100, using this new warning class.

MyWarning: x is greater than 100.
  warnings.warn("x is greater than 100.", MyWarning)

Conclusion

Python warnings are very helpful in identifying potential problems with your code. They let you know when something unexpected or unusual has happened, allowing you to take corrective action before it becomes a serious problem. In this tutorial, we have covered everything you need to know about using warnings in Python, including how to issue warnings, ignore them, change their behavior, and create your own custom warning messages.