On Jun 15, 2009, at 7:57 AM, Jason Bandlow wrote:
> Robert Bradshaw wrote:
>> On Jun 9, 2009, at 9:47 AM, Jason Bandlow wrote:
>>
>>>> On Jun 9, 2009, at 8:29 AM, Jason Bandlow wrote:
>>>>
>>>>> Hi,
>>>>>
>>>>> I ran across the following behavior in sage-3.4.1 and sage-4.0 (I
>>>>> don't
>>>>> have 4.0.1 yet), and I find it fairly disturbing.
>>>>>
>>>>> sage: d = {'a': 1} # Create some object
>>>>> sage: d
>>>>> {'a': 1}
>>>>> sage: type(d) = type({}) # Attempt to check the type but
>>>>> # foolishly use = instead of ==
>>>>>
>>>>> TypeError: cannot coerce type '<type 'type'>' into a
>>>>> SymbolicExpression. # An error is expected
>>>>>
>>>>> sage: d # But now my data is gone!
>>>>> d
>>>>> sage: type(d) # Replaced by a symbolic variable
>>>>> <class 'sage.calculus.calculus.SymbolicVariable'>
>>> Robert Bradshaw wrote:
>>>> This isn't a coercion issue, it's because of how functions are
>>>> defined in Sage using the "f(x) = expr" notation using the
>>>> preparser.
>>>> I think that's too useful to get rid of. We could special case a
>>>> warning for "type(x) = ..." but I'm not sure if that's the best
>>>> idea.
>>> Interesting! This means the problem is worse than I thought.
>>> Anytime we
>>> have data 'd' and 'e' and a function 'f', doing
>>>
>>> sage: f(d) = e # Instead of f(d) == e
>>>
>>> will raise a TypeError and replace 'd' with a symbolic variable.
>>> Fortunately, though,
>>>
>>> sage: if f(d) = e: # The usual reason to check equality
>>>
>>> raises a SyntaxError first, so maybe this problem isn't so serious.
>>
>> Yes, I think it's fairly rare to want to do this. Probably should be
>> added to an FAQ somewhere at least, as it's a surprising and subtle
>> side effect.
>
> First of all, thanks very much for the explanations, Robert. Is
> there a
> particular FAQ where I could add a brief description of this?
Here's a good a place as any:
http://wiki.sagemath.org/faq
- Robert
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