Python for Indexing Loops
I'm taking an example from Mark Lutz's Learning Python book.
keys = ['spam','eggs','toast']
vals=[1,4,7]
D2={}
for (v,k) in zip(keys, vals): D2[k] = v
D2
{1: 'spam', 4: 'eggs', 7: 'toast'}
My example:
D1={}
for (k,v) in zip(keys, vals): D1[k] = v
D1
{'toast': 7, 'eggs': 4, 'spam': 1}
So, I still don't understand indexing, why for (v, k)?
It is unpacking the key and value from each set of zipped list of keys and values, then assigning the key / value pairs. No paranas for v, k in zip(keys, vals)
will also work. The difference between your code and the book code is the order v,k
, you use the list keys
as keys and the book does it in reverse order.
You can also create a dict in one step by calling the dict on zipped items, if you change the order of the lists passed in zip then you get the same behavior:
D2 = dict(zip(keys, vals))
print D2
D2 = dict(zip(vals, keys))
print(D2)
{'toast': 7, 'eggs': 4, 'spam': 1}
{1: 'spam', 4: 'eggs', 7: 'toast'}
The only difference is order. The fact that lists are called keys and values is probably a little confusing because values are ultimately keys and vice versa, but the main thing to understand is that you are assigning k
each item from the list in your loop keys
and the book code does the opposite.
zip will return list of tuples
:
Demo:
>>> keys = ['spam','eggs','toast']
>>> vals=[1,4,7]
>>> zip(keys, vals)
[('spam', 1), ('eggs', 4), ('toast', 7)]
Unpacking
Demo:
>>> t = (1,2,3)
>>> t
(1, 2, 3)
>>> a,b,c = t
>>> a
1
>>> b
2
>>> c
3
>>> a,b = t
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: too many values to unpack
>>>
- We iterate through the list, so its unboxing the first element from the tuple to
v
and the second valuek
. - Create a new key and value pair in dictionary D2.
code:
>>> D2={}
>>> for (v,k) in zip(keys, vals):
... print "v:", v
... print "k", k
... D2[k] = v
... # ^ ^
# Key Value
v: spam
k 1
v: eggs
k 4
v: toast
k 7
>>> D2
{1: 'spam', 4: 'eggs', 7: 'toast'}
>>>