why I cannot suppress warning with regex using warnings.filterwarnings

Question

I want to suppress a specific type of warning using regular expression. The warning message is:

C:\Anaconda3\lib\site-packages\pandas\core\indexing.py:420: SettingWithCopyWarning:

A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy 
self.obj[item] = s

My way of suppressing filter:

import warnings
warnings.filterwarnings("ignore", message= ".*A value is trying to.*")

However, it failed. I did try pasting different part of the warning message into the regex but still failed. I want to know why.

Answer 1

Your regular expression does not match the correct message string .

r".*A Value is trying to.*" does not match "\\nA value is trying to be.*" because r"." matches everything except the newline character .

Sometimes it can be tricky to figure out what the actual message string is without looking at the source code of the module that generated the warning.

Answer 2

Try giving only the below code(without message).. May be the message you've mentioned is not matching with the warnings.

import warnings warnings.filterwarnings("ignore")

Answer 3

Was this the error you got?

AssertionError: message must be a string

warnings.filterwarnings only takes a string for the message argument and regex wouldn't compile there. If you really want to suppress this error, do:

import pandas as pd
warnings.simplefilter("error", pd.core.common.SettingWithCopyWarning)

Otherwise, you might want to check out ways to avoid SettingWithCopyWarning as many other people have encounted the same issue: How to deal with SettingWithCopyWarning in Pandas?

Answer 4

This is not how the filterwarnings works. In the documentation you can see Omitted arguments default to a value that matches everything. and also you can see: message (default '') : is a string containing a regular expression that start of the warning message must match .

This can be understood as using action "once" will affect each unique text message to display once. If you have a field in message that may change (for example filename), the warning will be displayed once for each filename .

If you set the message argument, than each matching unique text message will be diplayed once.

Here is a small example:

import warnings
import random

warnings.filterwarnings("ignore", category=UserWarning)
warnings.warn("You won't see this warning")

message_formater = "this message with number {} will be displayed once"

# deactivating previously created filter
warnings.resetwarnings()
# applying new filter
warnings.filterwarnings("once", message_formater.format("(.*)"), category=UserWarning)

for i in range(100):
    warnings.warn(message_formater.format(random.randint(0, 3)))

/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py:8: UserWarning: this message with number 0 will be displayed once

/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py:8: UserWarning: this message with number 3 will be displayed once

/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py:8: UserWarning: this message with number 1 will be displayed once

/usr/local/lib/python3.7/dist-packages/ipykernel_launcher.py:8: UserWarning: this message with number 2 will be displayed once