Tuple unpacking in for loops

Question

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?

Answer 1

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.

Answer 2

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