Implementing the recursive definition of Grays Code from Knuth I need to add values to tuples, I have:
In [54]: tuple_one = (1,0,0)
In [55]: tuple_one
Out[55]: (1, 0, 0)
In [56]: tuple_one += (1,)
In [57]: tuple_one
Out[57]: (1, 0, 0, 1)
However if I try and assign the above I get a syntax error
In [63]: B = tuple_one += (1,)
File "<ipython-input-63-537be8a059fc>", line 1
B = tuple_one += (1,)
^
SyntaxError: invalid syntax
Why is this ? Regards Paul
An augmented assignment evaluates the target (which, unlike normal assignment statements) is evaluated only once. An augmented assignment expression like x += 1 can be rewritten as x = x + 1 to achieve a similar, but not exactly equal effect. In the augmented version, x is only evaluated once.
Also, when possible, the actual operation is performed in-place, meaning that rather than creating a new object and assigning that to the target, the old object is modified instead.
With the exception of assigning to tuples and multiple targets in a single statement, the assignment done by augmented assignment statements is handled the same way as normal assignments
So, while assigning to tuples or/and multiple targets in a single statement, do not use augmented assignment statements.
Instead, use
B = tuple_one = tuple_one + (1,)
x evaluated twice in a x = x + 1 statement -- in other words what's the purpose of the second evaluation?
That's not a list of tuples. Its just a tuple.
And the reason it doesn't work is because it is actually invalid syntax. It is literally equal to
B = tuple_one = tuple_one + (1,)
if you want to save the value of tuple_one then do it in two line
tuple_one += (1,)
B = tuple_one
if you don't then just use the + operator
B = tuple_one + (1,)
Your "list of tuples" is not a list of tuples, it is a single one. You can tell that it isn't a list, because there are no square brackets ([]).
+= is used to modify in place, so a+=1 would be used instead of a = a+1. The difference is that a+1 returns the value of a plus 1, but +=1 does not return anything:
>>> a = 1
>>> x = a+=1 #DOES NOT WORK
File "<stdin>", line 1
x = a+=1
^
SyntaxError: invalid syntax
>>> x = a+1
>>> x
2
>>>
Here is your edited line:
B = tuple_one + (1,)
B = tuple_one += (1,)really is invalid syntax. TryB = tuple_one + (1,)appendto).