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Warning control in Python Programs
Warning is different from error in a program. If error is encountered, Python program terminates instantly. Warning on the other hand is not fatal. It displays certain message but program continues. Warnings are issued to alert the user of certain conditions which aren't exactly exceptions. Typically warning appears if some deprecated usage of certain programming element like keyword/function/class etc. is found.
Warning messages are displayed by warn() function defined in 'warning' module of Python's standard library. Warning is actually a subclass of Exception in built-in class hierarchy. There are a number of built-in Warning subclasses. User defined subclass can also be defined.
| Warning | This is the base class of all warning category classes. |
| UserWarning | The default category for warn(). |
| DeprecationWarning | warnings about deprecated features when those warnings are intended for developers |
| SyntaxWarning | warnings about dubious syntactic features. |
| RuntimeWarning | warnings about dubious runtime features. |
| FutureWarning | warnings about deprecated features when those warnings are intended for end users. |
| PendingDeprecationWarning | warnings about features that will be deprecated in the future |
| ImportWarning | warnings triggered during the process of importing a module |
| UnicodeWarning | warnings related to Unicode. |
| BytesWarning | warnings related to bytes and byte array. |
| ResourceWarning | warnings related to resource usage. |
Warning example
Following code defines a class with a deprecated method and a method scheduled to be deprecated in a future version of the said class.
# warningexample.py
import warnings
class WarnExample:
def __init__(self):
self.text = "Warning"
def method1(self):
warnings.warn(
"method1 is deprecated, use new_method instead",
DeprecationWarning
)
print ('method1', len(self.text))
def method2(self):
warnings.warn(
"method2 will be deprecated in version 2, use new_method instead",
PendingDeprecationWarning
)
print ('method2', len(self.text))
def new_method(self):
print ('new method', len(self.text))
if __name__=='__main__':
e = WarnExample()
e.method1()
e.method2()
e.new_method()If above script is executed from command prompt as
E:\python37>python warningexample.py
No warning messages are displayed on terminal. For that you have to use _Wd switch as below
E:\python37>python -Wd warningexample.py warningexample.py:10: DeprecationWarning: method1 is deprecated, use new_method instead DeprecationWarning method1 7 warningexample.py:19: PendingDeprecationWarning: method2 will be deprecated in version 2, use new_method instead PendingDeprecationWarning method2 7 new method 7
Similarly, following interactive session too doesn't show any warning messages.
E:\python37>python >>> from warningexample import WarnExample >>> e = WarnExample() >>> e.method1() method1 7 >>> e.method2() method2 7 >>> e.new_method() new method 7
You have to start Python session with –Wd
E:\python37>python -Wd >>> from warningexample import WarnExample >>> e=WarnExample() >>> e.method1() E:\python37\warningexample.py:10: DeprecationWarning: method1 is deprecated, use new_method instead DeprecationWarning method1 7 >>> e.method2() E:\python37\warningexample.py:17: PendingDeprecationWarning: method2 will be deprecated in version 2, use new_method instead PendingDeprecationWarning method2 7 >>> e.new_method() new method 7
Warnings Filters
The warnings filter controls whether warnings are ignored, displayed, or turned into errors (raising an exception).
| Action | Meaning |
|---|---|
| error | Turn the warning into an exception. |
| ignore | Discard the warning. |
| always | Always emit a warning. |
| default | Print the warning the first time it is generated from each location. |
| module | Print the warning the first time it is generated from each module. |
| once | Print the warning the first time it is generated. |
Following interactive session sets filter to default by simplefilter() function.
E:\python37>python
>>> import warnings
>>> warnings.simplefilter('default')
>>> from warningexample import WarnExample
>>> e=WarnExample()
>>> e.method1()
E:\python37\warningexample.py:10: DeprecationWarning: method1 is deprecated, use new_method instead
DeprecationWarning
method1 7
>>> e.method2()
E:\python37\warningexample.py:17: PendingDeprecationWarning: method2 will be deprecated in version 2, use new_method instead
PendingDeprecationWarning
method2 7
>>> e.new_method()
new method 7In order to temporarily suppress warnings, set simplefilter to 'ignore'.
import warnings
def function():
warnings.warn("deprecated", DeprecationWarning)
with warnings.catch_warnings():
warnings.simplefilter("ignore")
function()
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