doctest with variable return value

[Peter Otten]

3KWA wrote:

> I am wondering what is the standard doctest (test) practice for
> functions who's returned value change all the time e.g. forex rate:
> 
> import urllib
> 
> def get_rate(symbol):
>     """get_rate(symbol) connects to yahoo finance to return the rate of
> symbol.
> 
>     >>>get_rate('AUDEUR')
> 
>     """
> 
>     url=
> "http://finance.yahoo.com/d/quotes.csv?s=%s=X&f=sl1d1t1c1ohgv&e=.csv" %
> \
>          symbol
>     f=urllib.urlopen(url)
>     return float(f.readline().split(',')[1])

You cannot test for an unknown value, but you can do some sanity checks:

    >>> rate = get_rate('AUDEUR')
    >>> rate > 0
    True
    >>> isinstance(rate, float)
    True

This will at least make sure that get_rate() does not throw an exception.
You can also spoonfeed it with handcrafted data...

    >>> def mock_urlopen(url):
    ...      from cStringIO import StringIO
    ...      return StringIO("yadda,0.1234")
    ...
    >>> urllib.urlopen = mock_urlopen
    >>> get_rate("AUDEUR")
    0.1234

but this has the disadvantage that the test has to know about the actual
implementation of the function about to be tested.

Peter