python - 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?

Answer 1

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