Behaviour of Python when attempting to iterate over characters in a string

Question

I am attempting to iterate over the following string, using a for loop:

>>> for a,b,c in "cat"
        print(a,b,c)

Now what I intended for this to do was print out each character in the string individually on one physical line, instead I receive an error. I am aware that this is very easily resolved by enclosing the string in the list operator []:

>>> for a,b,c in ["cat"]
        print(a,b,c)
c a t

But could someone explain why this is the case?

Answer 1

You are telling for to expand each iteration value to assign to three separate variables:

for a,b,c in "cat":
#   ^^^^^ the target for the loop variable, 3 different names

However, iteration over a string produces a string with a single character, you can't assign a single character to three variables:

>>> loopiterable = 'cat'
>>> loopiterable[0]  # first element
'c'
>>> a, b, c = loopiterable[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: not enough values to unpack (expected 3, got 1)

The error message tells you why this didn't work; you can't take three values out of a string of length 1.

When you put the string into a list, you changed what you loop over. You now have a list with one element, so the loop iterates just once, and the value for the single iteration is the string 'cat'. That string just happens to have 3 characters, so can be assigned to three variables:

>>> loopiterable = ['cat']
>>> loopiterable[0]  # first element
'cat'
>>> a, b, c = loopiterable[0]
>>> a
'c'
>>> b
'a'
>>> c
't'

This still would fail if the contained string has a different number of characters:

>>> for a, b, c in ['cat', 'hamster']:
...     print(a, b, c)
...
c a t
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: too many values to unpack (expected 3)

'hamster' is 7 characters, not 3; that's 4 too many.

The correct solution is to just use one variable for the loop target, not 3:

for character in 'cat':
    print(character)

Now you are printing each character separately:

>>> for character in 'cat':
...     print(character)
...
c
a
t

Now, if you wanted to pass all characters of a string to print() as separate arguments, just use * to expand the string to separate arguments to the call:

>>> my_pet = 'cat'
>>> print(*my_pet)
c a t

Answer 2

The for loop goes through each element in the iterable you supply and tries to perform an assignment.

In the first case, since your iterable is 'cat' you're essentially unpacking:

a, b, c = 'c'

during the first iteration and getting the appropriate error message:

ValueError: not enough values to unpack (expected 3, got 1)

because you'll iterate 3 times; once for each character in the string 'cat'.

In the second case, you're unpacking "cat" as expected, because the list has a single element (i.e 'cat') which is retrieved and unpacked into a, b and c.

Answer 3

You are iterating different objects in two blocks of code. In the former one, you iterate the string, which may be considered a special list.

While in the latter one, you iterate the list which contains only one object 'cat'.

You may change the first one to below:

for ch in 'cat':
    print(ch)

Feel free to ask more about this.