[Python-ideas] Raise exception if (not) true

[Markus Unterwaditzer]

Painting the bikeshed, i'd rather have the syntax

assert x > 0, ValueError("x should be positive")

-- Markus

On 20 February 2014 21:44:57 CET, spir <denis.spir at gmail.com> wrote:
>On 02/20/2014 06:29 PM, MRAB wrote:
>> On 2014-02-20 17:10, Ryan Gonzalez wrote:
>>> In Python, you'll constantly see code like this:
>>>
>>> ```python
>>> if x != y:
>>>      raise ValueError('x != y!!')
>>> ```
>>>
>>> or:
>>>
>>> ```python
>>> if not isinstance(x,SomeType):
>>>      raise TypeError('x is not SomeType!')
>>> ```
>>>
>>> Assertion help a bit:
>>>
>>> ```python
>>> assert isinstance(x,SomeType), 'x is not SomeType!'
>>> ```
>>>
>>> Notice I said "a bit". If optimizations are on, they're disabled. In
>>> addition, the only type of error thrown is an AssertionError.
>>>
>>> I propose a `raise_if` function. If the given condition is True, an
>>> exception is raised. So, the above examples would be shortened to:
>>>
>>> ```python
>>>
>>> raise_if(x!=y, ValueError, 'x != y!!')
>>> raise_if(not isinstance(x,SomeType),TypeError, 'x is not SomeType!')
>>>
>>> ```
>>>
>>> There could also be a raise_if_not function that does the opposite:
>>>
>>> ```python
>>> raise_if_not(isinstance(x,SomeType), TypeError, 'x is not
>SomeType!')
>>> ```
>>>
>>> Thoughts?
>>>
>> So:
>>
>> raise_if_not(isinstance(x, SomeType), TypeError, 'x is not
>SomeType!')
>>
>> is equivalent to:
>>
>> if not isinstance(x, SomeType): raise TypeError('x is not SomeType!')
>>
>> ?
>
>And:
>     raise_if(x!=y, ValueError, 'x != y!!')
>is equivalent to:
>     if x != y: raise ValueError('x != y!!')
>
>> It doesn't improve the language much, IMHO! :-)
>
>Ditto.
>
>But I would like to be able to add an error type to assertions (in
>addition to 
>the optional message). This is particularly useful for people (like me)
>who 
>systematically check func inputs (for client debugging comfort), using
>assert's. 
>It would be mainly ValueError and TypeError.
>
>Example:
>     assert x > 0, "x should be positive", ValueError
>gives:
>     ValueError: x should be positive
>instead of:
>     AssertionError: x should be positive
>
>This is very similar to the proposal above, semantically and
>practically, except 
>we are here just reusing the builtin 'assert' instruction with an
>additional 
>parameter. (Seems backward-compatible to me, at first sight, provided
>the new 
>param comes last. Maybe an issue is the hypothetical mention of an
>error type, 
>without message.)
>
>d
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