How do I get warnings.warn to issue a warning and not ignore the line?

Question

I'm trying to raise a `DeprecationWarning`, with a code snippet based on the example shown in the docs. http://docs.python.org/2/library/warnings.html#warnings.warn Official ``` def deprecation(message): warnings.warn(message, DeprecationWarning, stacklevel=2) ``` Mine ``` import warnings warnings.warn("This is a warnings.", DeprecationWarning, stacklevel=2) is None # returns True ``` I've tried removing the stacklevel argument, setting it to negative, 0, 2 and 20000. The warning is always silently swallowed. It doesn't issue a warning or raise an exception. It just ignores the line and returns `None`. The docs doesn't mention the criteria for ignoring. Giving a message, makes warnings.warn correctly issue a `Userwarning.` What can be causing this and how do I get warn to actually warn?

Accepted Answer

From the docs:

By default, Python installs several warning filters, which can be overridden by the command-line options passed to -W and calls to filterwarnings().

- DeprecationWarning and PendingDeprecationWarning, and ImportWarning are ignored.

- BytesWarning is ignored unless the -b option is given once or twice; in this case this warning is either printed (-b) or turned into an exception (-bb).

By default, `DeprecationWarning` is ignored. You can change the filters using the following:

```
warnings.simplefilter('always', DeprecationWarning)
```

Now your warnings should be printed:

```
>>> import warnings
>>> warnings.simplefilter('always', DeprecationWarning)
>>> warnings.warn('test', DeprecationWarning)
/home/guest/.env/bin/ipython:1: DeprecationWarning: test
  #!/home/guest/.env/bin/python
```

How do I get warnings.warn to issue a warning and not ignore the line?

Solution

Problem