[Tutor] Why doesn't this filter FutureWarning?

[Kent Johnson]

Wow, this one had me stumped for a while!

It turns out that this warning is generated at *compile* time, not at 
runtime! Here is a way to demonstrate this:

In FW.py:
def foo():
     num=0x1234
     num=num & 0xffffffff
     print 'foo'

In the interpreter:
 >>> import FW
FW.py:3: FutureWarning: hex/oct constants > sys.maxint will return positive 
values in Python 2.4 and up
   num=num & 0xffffffff
 >>> FW.foo()
foo

Notice I get the warning on import, even though all the code in FW is in a 
function which is not executed! Then when I execute foo(), I *don't* get 
the warning.

When you run from the command line, each line of input is compiled 
independently. So setting the warning filter affects everything after.

When you run from a file, the whole file is compiled, generating the 
warning, then the file is executed, disabling the warning. The warning 
filter isn't set until after the file is compiled, so it is too late.

Two workarounds:
- Use 0xffffffffL and avoid the warning completely
- Put the call to filterwarnings() in a separate file. Call 
filterwarnings(), then import the file of interest. You can simulate this 
from the command line:
 >>> import warnings
 >>> warnings.filterwarnings(action='ignore', category=FutureWarning)
 >>> import FW
 >>> FW.foo()
foo

Kent

At 06:00 PM 10/17/2004 -0700, Terry Carroll wrote:
>Here's what I get in the interactive session:
>
> >>> num=0x1234
> >>> import warnings
> >>> num=num & 0xffffffff
><stdin>:1: FutureWarning: hex/oct constants > sys.maxint will return 
>positive va
>lues in Python 2.4 and up
> >>> warnings.filterwarnings('ignore', "hex/oct constants", FutureWarning)
> >>> num=num & 0xffffffff
>
>
>That's just what I'd expect.  I get the FutureWarning on the first try,
>but after invoking filterwarnings, I no longer get it.
>
>But when I do the same code from inside a file:
>
><begin quote>
>
>C:\test>cat fw.py
>num=0x1234
>import warnings
>num=num & 0xffffffff
>warnings.filterwarnings('ignore', "hex/oct constants", FutureWarning)
>num=num & 0xffffffff
>
>C:\test>python fw.py
>fw.py:3: FutureWarning: hex/oct constants > sys.maxint will return 
>positive valu
>es in Python 2.4 and up
>   num=num & 0xffffffff
>fw.py:5: FutureWarning: hex/oct constants > sys.maxint will return 
>positive valu
>es in Python 2.4 and up
>   num=num & 0xffffffff
>
><end-quote>
>
>What gives?  Why would this work in an interactive session, but not from
>a file?
>
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