Tuple unpacking in for loops

I stumbled across the following code:

for i, a in enumerate(attributes):
   labels.append(Label(root, text = a, justify = LEFT).grid(sticky = W))
   e = Entry(root)
   e.grid(column=1, row=i)
   entries.append(e)
   entries[i].insert(INSERT,"text to insert")

I don't understand the i, a bit, and searching google for information on for is a pain in the bum. When I try and experement with the code I get the error:

ValueError: need more than 1 value to unpack

Does anyone know what it does, or a more specific term associated with it that I can google to learn more?

You could google "tuple unpacking". This can be used in various places in Python. The simplest is in assignment:

>>> x = (1,2)
>>> a, b = x
>>> a
1
>>> b
2

In a for-loop it works similarly. If each element of the iterable is a tuple, then you can specify two variables, and each element in the loop will be unpacked to the two.

>>> x = [(1,2), (3,4), (5,6)]
>>> for item in x:
...     print "A tuple", item
A tuple (1, 2)
A tuple (3, 4)
A tuple (5, 6)
>>> for a, b in x:
...     print "First", a, "then", b
First 1 then 2
First 3 then 4
First 5 then 6

The enumerate function creates an iterable of tuples, so it can be used this way.

Enumerate basically gives you an index to work with in the for loop. So:

for i,a in enumerate([4, 5, 6, 7]):
    print i, ": ", a

Would print:

0: 4
1: 5
2: 6
3: 7

Take this code as an example:

elements = ['a', 'b', 'c', 'd', 'e']
index = 0

for element in elements:
  print element, index
  index += 1

You loop over the list and store an index variable as well. enumerate() does the same thing, but more concisely:

elements = ['a', 'b', 'c', 'd', 'e']

for index, element in enumerate(elements):
  print element, index

The index, element notation is required because enumerate returns a tuple ((1, 'a'), (2, 'b'), ...) that is unpacked into two different variables.

[i for i in enumerate(['a','b','c'])]

Result:

[(0, 'a'), (1, 'b'), (2, 'c')]