Warning in Python

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A warning is the alert message which doesn't basically raise an exception and doesn't terminate program.

There are warning categories as shown below:

Class	Disposition
Warning	This is the base class of all warning category classes. It is a subclass of Exception.
UserWarning	The default category for warn().
DeprecationWarning	Base category for warnings about deprecated features when those warnings are intended for other Python developers (ignored by default, unless triggered by code in __main__).
SyntaxWarning	Base category for warnings about dubious syntactic features.
RuntimeWarning	Base category for warnings about dubious runtime features.
FutureWarning	Base category for warnings about deprecated features when those warnings are intended for end users of applications that are written in Python.
PendingDeprecationWarning	Base category for warnings about features that will be deprecated in the future (ignored by default).
ImportWarning	Base category for warnings triggered during the process of importing a module (ignored by default).
UnicodeWarning	Base category for warnings related to Unicode.
UnicodeWarning	Base category for warnings related to Unicode.
BytesWarning	Base category for warnings related to bytes and bytearray.
ResourceWarning	Base category for warnings related to resource usage (ignored by default).

warn() can manually occur a warning as shown below:

*Memos:

• The 1st argument is message(Required-Type:str).
• The 2nd argument is category(Optional-Default:None-Type:Warning). *If it's None, UserWarning is set to it.
• The 3rd argument is stacklevel(Optional-Default:1-Type:int). *It decides which code a warning refers to.
• The 4th argument is source(Optional-Default:None-Type:Any).
• There is skip_file_prefixes argument(Optional-Default:None-Type:tuple of str): *Memos:
  • skip_file_prefixes= must be used.
  • Manually setting None to it gets error.

import warnings

warnings.warn(message="This is a warning.")
# UserWarning: This is a warning.
#   warnings.warn(message="This is a warning.")

warnings.warn(message="This is a warning.",
              category=None,
              stacklevel=1,
              source=None,
              skip_file_prefixes=())
# UserWarning: This is a warning.
#   warnings.warn(message="This is a warning.",

warnings.warn(message="This is a warning.",
              category=Warning)
# Warning: This is a warning.
#   warnings.warn(message="This is a warning.",

warnings.warn(message="This is a warning.",
              category=DeprecationWarning)
# DeprecationWarning: This is a warning.
#   warnings.warn(message="This is a warning.",

def test1():
    warnings.warn(message="Warning 1",
                  stacklevel=-100)
    warnings.warn(message="Warning 2",
                  stacklevel=0)
    warnings.warn(message="Warning 3",
                  stacklevel=1)
    warnings.warn(message="Warning 4",
                  stacklevel=2)
    warnings.warn(message="Warning 5",
                  stacklevel=3)
    warnings.warn(message="Warning 6",
                  stacklevel=4)
    warnings.warn(message="Warning 7",
                  stacklevel=5)
    warnings.warn(message="Warning 8",
                  stacklevel=100)
def test2():
    test1()

def test3():
    test2()
test3()
# UserWarning: Warning 1
#   warnings.warn(message="Warning 1",
# UserWarning: Warning 2
#   warnings.warn(message="Warning 2",
# UserWarning: Warning 3
#   warnings.warn(message="Warning 3",
# UserWarning: Warning 4
#   test1()
# UserWarning: Warning 5
#   test2()
# UserWarning: Warning 6
#   test3()
# UserWarning: Warning 7
#   exec(code_obj, self.user_global_ns, self.user_ns)
# UserWarning: Warning 8