Why only one warning in a loop?

Question

I want a warning raise for each problem detected in a loop, but the warning is only raised once, the first time. For example :

import warnings  for i in range(10):    print i    warnings.warn('this is a warning message') 

I expect :

0 UserWarning: this is a warning message 1 UserWarning: this is a warning message 2 UserWarning: this is a warning message 3 UserWarning: this is a warning message 4 

but the result is :

0 __main__:4: UserWarning: this is a warning message 1 2 3 4 

Why do I have only one warning? How can I get a warning for each iteration?

Answer 1

It is by design. See the docs at http://docs.python.org/2/library/warnings.html:

Repetitions of a particular warning for the same source location are typically suppressed.

You can override this behavior by adding a filter with the keyword always, as in:

import warnings  warnings.simplefilter('always', UserWarning) for i in range(10):    print i    warnings.warn('this is a warning message') 

Answer 2

From the warnings module documentation:

Repetitions of a particular warning for the same source location are typically suppressed.

This is by design.

I'd not use the warnings module if you want your message to be printed always; you could reset the filters (using warnings.resetwarnings(), but that's very much not recommended as you'd discard any user-configured filters too. You could add an explicit filter that always allows the message with the warnings.simplefilter() function:

warnings.simplefilter('always', UserWarning) 

but I'd just write to sys.stderr directly instead.