assert versus print [was Re: The curious behavior of integer objects]

[Calvin Spealman]

On 1/15/07, Steven D'Aprano <steve at removeme.cybersource.com.au> wrote:
> On Mon, 15 Jan 2007 17:50:56 -0500, Calvin Spealman wrote:
>
> > assert foo(0x10) == 0  # Assertions are much better tests than prints :-)
>
> I dispute that assertion (pun intended).

Hah!

> Firstly, print statements work even if you pass the -O (optimize) flag
> to Python. Your asserts don't.

This is true, but the concept can be adapted to a things like an
assert_() function.

> Secondly, a bare assertion like that gives you very little information: it
> just tells you that it failed:
>
> >>> x = 1
> >>> assert x == 0
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> AssertionError
>
> To provide the same information that print provides, you need something
> like this:
>
> assert x == 0, "x == %s not 0" % x

What information would I need to know if my test passed? Nothing, I
say. I only want to know when the tests fail, especially as I add more
of them. Having no output is a great way to know nothing puked.

> Thirdly, and most importantly, assert and print aren't alternatives,
> but complementary tools. Assertions are good for automated testing and
> simple data validation, where you already know what values you should
> have. Printing is good for interactively exploring your data when you're
> uncertain about the values you might get: sometimes it is hard to know
> what you should be asserting until you've seen what results your function
> returns.

True, but I intended my statement as a nudge towards more proper
testing. Even when testing things out, I often use an assert rather
than a print, to verify some condition.

> --
> Steven D'Aprano
>
> --
> http://mail.python.org/mailman/listinfo/python-list
>


-- 
Read my blog! I depend on your acceptance of my opinion! I am interesting!
http://ironfroggy-code.blogspot.com/