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*My post explains Warning with warn() in Python.

A warnings filter can set which warnings to show using these filters(actions) below. *"once" may not work properly:

By default, these filters below are set to warnings.filters and they can be reset with resetwarnings() as shown below:

import warnings

print(warnings.filters)
# [('default', None, , '__main__', 0),
#  ('ignore', None, , None, 0),
#  ('ignore', None, , None, 0),
#  ('ignore', None, , None, 0),
#  ('ignore', None, , None, 0)]

warnings.resetwarnings()

print(warnings.filters)
# []

Now, filterwarnings() or simplefilter() can filter warnings as shown below:

*Memos for filterwarnings():

• The 1st argument is action(Required-Type:str). *"default", "error", "ignore", "always", "module" or "once" can be set.
• The 2nd argument is message(Optional-Default:""-Type:str).
• The 3rd argument is category(Optional-Default:Warning-Type:Warning).
• The 4th argument is module(Optional-Default:""-Type:str). *It may not work properly.
• The 5th argument is lineno(Optional-Default:0-Type:int): *Memos:
  • It decides a line number.
  • It must be x >= 0.
• The 6th argument is append(Optional-Default:False-Type:bool). *If it's False, a filter is add before other filters while if it's True, a filter is add after other filters.

*Memos for simplefilter():

• The 1st argument is action(Required-Type:str). *"default", "error", "ignore", "always", "module" or "once" can be set.
• The 2nd argument is category(Optional-Default:Warning-Type:Warning).
• The 3rd argument is lineno(Optional-Default:0-Type:int): *Memos:
  • It decides a line number.
  • It must be x >= 0.
• The 4th argument is append(Optional-Default:False-Type:bool). *If it's False, a filter is add before other filters while if it's True, a filter is add after other filters.

my_project
 |-main.py
 |-file1.py(module)
 └-file2.py(module)

file1.py:

import warnings                                                 # Line 1
                                                                # Line 2
warnings.warn(message="Warning 1", category=UserWarning)        # Line 3
warnings.warn(message="Warning 2", category=DeprecationWarning) # Line 4
warnings.warn(message="Warning 2", category=DeprecationWarning) # Line 5
warnings.warn(message="Warning 3", category=UserWarning)        # Line 6

file2.py:

import warnings                                                 # Line 1
                                                                # Line 2
warnings.warn(message="Warning 1", category=UserWarning)        # Line 3
warnings.warn(message="Warning 2", category=DeprecationWarning) # Line 4
warnings.warn(message="Warning 2", category=DeprecationWarning) # Line 5
warnings.warn(message="Warning 3", category=UserWarning)        # Line 6

main.py with "default" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="default")
warnings.filterwarnings(action="default", message="", category=Warning,
                        module="", lineno=0, append=False)
warnings.simplefilter(action="default")
warnings.simplefilter(action="default", category=Warning,
                      lineno=0, append=False)
print(warnings.filters)
# [('default', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)
# ...\my_project\file2.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)

main.py with "error" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="error")
warnings.simplefilter(action="error")

print(warnings.filters)
# [('error', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py", line 3, in 
#     warnings.warn(message="Warning 1", category=UserWarning)
# UserWarning: Warning 1

main.py with "ignore" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="ignore")
warnings.simplefilter(action="ignore")

print(warnings.filters)
# [('ignore', None, , None, 0)]

import file1, file2 # Warnings are not shown

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="ignore", category=UserWarning)
warnings.simplefilter(action="ignore", category=UserWarning)

print(warnings.filters)
# [('ignore', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="ignore", message="Warning 1", 
                        category=UserWarning)
print(warnings.filters)
# [('ignore', re.compile('Warning 1', re.IGNORECASE),
#  , None, 0)]

import file1, file2
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="ignore", lineno=6)
warnings.simplefilter(action="ignore", lineno=6)

print(warnings.filters)
# [('ignore', None, , None, 6)]

import file1, file2
# ...\my_project\file1.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)

main.py with "always" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="always")
warnings.simplefilter(action="always")

print(warnings.filters)
# [('always', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)
# ...\my_project\file2.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:5: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)

main.py with "module" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="module")
warnings.simplefilter(action="module")

print(warnings.filters)
# [('module', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)
# ...\my_project\file2.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)

main.py with "once" filter(action):

import warnings

warnings.resetwarnings()

warnings.filterwarnings(action="once")
warnings.simplefilter(action="once")

print(warnings.filters)
# [('once', None, , None, 0)]

import file1, file2
# ...\my_project\file1.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file1.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file1.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)
# ...\my_project\file2.py:3: UserWarning: Warning 1
#   warnings.warn(message="Warning 1", category=UserWarning)
# ...\my_project\file2.py:4: DeprecationWarning: Warning 2
#   warnings.warn(message="Warning 2", category=DeprecationWarning)
# ...\my_project\file2.py:6: UserWarning: Warning 3
#   warnings.warn(message="Warning 3", category=UserWarning)

main.py with append argument. *If append=False, a filter is add before other filters while if append=True, a filter is add after other filters:

import warnings

warnings.resetwarnings()

print(warnings.filters)
# []

warnings.filterwarnings(action="default", append=False)
warnings.simplefilter(action="default", append=False)

print(warnings.filters)
# [('default', None, , None, 0)]

warnings.filterwarnings(action="always", append=False)
warnings.simplefilter(action="always", append=False)

print(warnings.filters)
# [('always', None, , None, 0),
#  ('default', None, , None, 0)]

warnings.filterwarnings(action="once", append=True)
warnings.simplefilter(action="once", append=True)

print(warnings.filters)
# [('always', None, , None, 0),
#  ('default', None, , None, 0),
#  ('once', None, , None, 0)]