Python Gotcha's?

[John Posner]

On 4/4/2012 7:32 PM, Chris Angelico wrote:
> Don't know if it's what's meant on that page by the += operator,

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]+=[3] # 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 = a[0] + [3]   # succeeds
a[0] = temp         # fails

-John