Question

why this code is wrong?

for i,j in range(100),range(200,300):
    print i,j

when I test this for statement I see this error

ValueError: too many values to unpack

but when I test

for i, j in range(2),range(2):
     print i,j

every thing is correct!

Answer 1

range(2) gives a list [0, 1]. So, your i, j will be fetched from first list and and then from the second list.

So, your loop is similar to: –

for i, j in [0, 1], [0, 1]:
    print i, j

Prints: –

0 1
0 1

Now, if you have range(3) there, then it will fail, because, range(3) gives a 3-element list, which cannot be unpacked in two loop variables.

So, you cannot do: –

for (i, j) in [[0, 1, 2]]:
    print i, j

It will fail, giving you the error that you are getting.

Try using zip, to zip your both list into one.: –

>>> for (i, j) in (zip(range(2), range(3))):
    print i, j


0 0
1 1
>>> 

zip converts your lists into list of tuples with 2 elements in the above case, as you are zipping 2 lists.

>>> zip(range(2), range(3))
[(0, 0), (1, 1)]

Similarly, if you zip three lists, you will get list of 3-elements tuple.