How are tuples unpacked 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 for information on for didn't yield any useful results. When I try and experiment 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

Answer 3

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.

Answer 4

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

Result:

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

Answer 5

You can combine the for index,value approach with direct unpacking of tuples using ( ). This is useful where you want to set up several related values in your loop that can be expressed without an intermediate tuple variable or dictionary, e.g.

users = [
    ('alice', 'alice@example.com', 'dog'),
    ('bob', 'bob@example.com', 'cat'),
    ('fred', 'fred@example.com', 'parrot'),
]

for index, (name, addr, pet) in enumerate(users):
    print(index, name, addr, pet)

prints

0 alice alice@example.com dog
1 bob bob@example.com cat
2 fred fred@example.com parrot

Answer 6

The enumerate function returns a generator object which, at each iteration, yields a tuple containing the index of the element (i), numbered starting from 0 by default, coupled with the element itself (a), and the for loop conveniently allows you to access both fields of those generated tuples and assign variable names to them.

Answer 7

Short answer, unpacking tuples from a list in a for loop works. enumerate() creates a tuple using the current index and the entire current item, such as (0, ('bob', 3))

I created some test code to demonstrate this:

    list = [('bob', 3), ('alice', 0), ('john', 5), ('chris', 4), ('alex', 2)]

    print("Displaying Enumerated List")
    for name, num in enumerate(list):
        print("{0}: {1}".format(name, num))

    print("Display Normal Iteration though List")
    for name, num in list:
        print("{0}: {1}".format(name, num))

The simplicity of Tuple unpacking is probably one of my favourite things about Python :D

Answer 8

let's get it through with an example:
list = [chips, drinks, and, some, coding]
  i = 0
  while i < len(list):
               
           if i % 2 != 0:
                print(i)
                  i+=1
       output:[drinks,some]

    now using EMUNERATE fuction:(precise)
   list = [chips, drinks, and, coding]
for index,items in enumerate(list):
            
             print(index,":",items)
OUTPUT:   0:drinks
          1:chips
          2:drinks
          3:and
          4:coding