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Python Gotcha's?

Evan Driscoll driscoll at cs.wisc.edu
Thu Apr 5 18:11:47 EDT 2012 
On 4/5/2012 13:44, Michael Hrivnak wrote:
> This is not a gotcha, and it's not surprising.  As John described,
> you're assigning a new value to an index of a tuple, which tuples
> don't support.

Um, at least for me personally, yes, it is surprising, and yes, it is a
gotcha.


This goes back to what languages you're used to.

I come from a strong C++ background. In C++, something like mytuple[0]
+= [3] would wind up calling 'operator+=' on the list, that += would
mutate the list, and then the expression would be done.

The C++ approach doesn't really work with Python because more stuff is
immutable, and Python's behavior is arguably the least-bad solution, but
it WAS definitely surprising for me (and others!) when I learned about
Python's behavior.

In particular, the translation of 'a+=b' to 'temp = a + b; a = temp' is
*not* a very natural one to me.

Evan




> 
> Michael
> 
> On Thu, Apr 5, 2012 at 10:15 AM, John Posner <jjposner at optimum.net> wrote:
>> On 4/4/2012 7:32 PM, Chris Angelico wrote:
>>> Don't know if it's what's meant on that page by the +=perator,
>>
>> Yes, it is.
>>
>>>> a=1],)
>>>> a[0].append(2) # This is fine
>>
>> [In the following, I use the term "name" rather loosely.]
>>
>> The append() method attempts to modify the object whose name is "a[0]".
>> That object is a LIST, so the attempt succeeds.
>>
>>>> a[0]+=] # This is not.
>>
>> The assignment attempts to modify the object whose name is "a". That
>> object is a TUPLE, so the attempt fails. This might be a surprise, but
>> I'm not sure it deserves to be called a wart.
>>
>>  Note the similarity to:
>>
>> temp =[0] + [3]   # succeeds
>> a[0] =emp         # fails
>>
>> -John
>>
>>
>>
>>
>> --
>> http://mail.python.org/mailman/listinfo/python-list
> 


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