Unexpected python behaviour

[Duncan Booth]

Richard Philips <richard.philips at ua.ac.be> wrote in news:mailman.1174.1070029879.702.python-list at python.org:

> Consider the script:
> 
> def test(drivers={}):
>     if drivers:
>         print drivers
>     else:
>         drivers["A"] = "B"
> 
> if __name__  == "__main__":
>     test()
>     test()
> 
> prints in stdout:
> {'A': 'B'}
> 
> I expected that nothing would be displayed.
> I do not understand this behaviour (python2.3 on WinXP and Linux).

You are not alone, this is one of the commonest questions from beginners.

> 
> If anybody can explain this situation, you are welcome.

The best explanation I know is in the FAQ, entry 4.21.

See http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects

> 
> Thank you.
> 
You are welcome.

-- 
Duncan Booth                                             duncan at rcp.co.uk
int month(char *p){return(124864/((p[0]+p[1]-p[2]&0x1f)+1)%12)["\5\x8\3"
"\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?