On Sat, Apr 28, 2012 at 4:20 PM, [email protected]
<[email protected]> wrote:
> On 29 April 2012 00:10, Joachim Durchholz <[email protected]> wrote:
>> Hi all,
>>
>> I'm having a "will this cause too much surprise" question here.
>> I need to explain the situation, so I'm sorry for any tl;dr effects.
>>
>> as those who're watching https://github.com/sympy/sympy/pull/911 already
>> know, I have upgraded raises() by backporting some code from pytest.
>> Raises will then be able accept code in three forms:
>>
>> As a string like before:
>> raises(DivisionByZero, "1/0")
>>
>> As a lambda:
>> raises(DivisionByZero, lambda: 1/0)
>> The code inside the lambda does not execute before raises() is called, but
>> inside raises(), after all the mechanics has been set up to catch any
>> exceptions and report them if they aren't the expected one.
>>
>> As a with statement:
>> with raises(DivisionByZero):
>> 1/0
>> This works just like the lambda but can contain statements (assignments,
>> loops etc.)
>>
>> Now the question is what people would do it they have two such tests to run.
>> For a silly example, assume testing whether 1/0 and -1/0 both throw an
>> exception. Would you write
>>
>> with raises(DivisionByZero):
>> 1/0
>> with raises(DivisionByZero):
>> -1/0
>>
>> or
>>
>> with raises(DivisionByZero):
>> 1/0
>> -1/0
>>
>> The potential issue here is that if people instinctively use the second
>> form, they won't get the -1/0 test: the 1/0 will raise the exception and
>> shortcut the entire with block.
>>
>> Now the question is whether that's a problem in practice or not.
>> Points to consider:
>>
>> - That an exception shortcuts the entire with block is part of the intended
>> purpose of with blocks, so we might say we're expecting everybody using them
>> to know their semantics.
>> - This construction with the exact same semantics is also available in
>> pytest. So the pytest people either consider the issue to be a nonproblem,
>> or they have accepted it as a necessary evil. (Is anybody able to dig up any
>> discussion the pytest people might have had on the issue?)
>> - Those who get hit by this won't notice. There won't be an error message,
>> just a slightly worse test coverage.
>> - It's a rare problem. I have applied the new lambda: and with: stuff to 99%
>> of all tests in SymPy, and less than a dozen or so tests actually had
>> statements in them. With a lambda, nobody is going to make this error.
>>
>> Should we worry, or not?
>
> I vote for "not worry". If somebody is new and does not how to use the
> "with" block the reviewer will probably notice it.
>
> By the way, this is a really aesthetically pleasing way to write a
> test. (my main reason to vote "not worry")
>
>> Any feedback welcome.
>>
It is a convenient feature, but I can see even myself getting caught
off guard and writing
with raises(ValueError):
statement1
statement2
Aaron Meurer
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