mjhjmtu

One block of code works but the other does not. Which would make sense except the second block is the same as the first only with an operation written in shorthand. They are practically the same operation.

l = ['table']
i = 

Version 1

for n in l:
 i += n
print(i)

Output: ['t', 'a', 'b', 'l', 'e']

Version 2

for n in l:
 i = i + n
print(i)

Output:

TypeError: can only concatenate list (not "str") to list

What is causing this strange error?

They don't have to be the same.

Using the + operator calls the method __add__ while using the += operator calls __iadd__. It is completey up to the object in question what happens when one of these methods is called.

If you use x += y but x does not provide an __iadd__ method, __add__ is used as a fallback, e.g. x = x + y happens.

In the case of lists, using l += iterable actually extends the list l with the elements of iterable. In your case, every character from the string (which is an iterable) is appended during the extend operation.

Demo 1: using __iadd__

>>> l = 
>>> l += 'table'
>>> l
['t', 'a', 'b', 'l', 'e']

Demo 2: using extend does the same

>>> l = 
>>> l.extend('table')
>>> l
['t', 'a', 'b', 'l', 'e']

Demo 3: adding a list and a string raises a TypeError.

>>> l = 
>>> l = l + 'table'
[...]
TypeError: can only concatenate list (not "str") to list

Not using += gives you the TypeError here because only __iadd__ implements the extending behavior.

Demo 4: common pitfall: += does not build a new list (note the identical ids)

>>> l = 
>>> id(l)
140359172117512
>>> l += [1, 2, 3]
>>> id(l)
140359172117512

... but the l = l + iterable syntax does.

>>> l = 
>>> id(l)
140359172142472
>>> l = l + [1, 2, 3]
>>> id(l)
140359172167560

In some cases, this can produce subtle bugs, because += mutates the original list, while
l = l + iterable builds a new list and reassigns the name l.

BONUS

Ned Batchelder's challenge to find this in the docs

If in the second case you cast the n to a list to avoid errors:

for n in l:
 i = i + [n]
print(i)

you get

['table']

so they do different operations.