Python join method on a tuple

I'd like some clarification on the way the str.join() method works when passed a tuple. Take for example this code snippet:

a = ("A", "B")
print(''.join(a))

This gives expected output: AB

So from this I ascertained that passing join an iterable (Such as a tuple) would allow it to unpack and "join" the contents together.

With this knowledge in mind I wrote the following generator expression:

import random
import string

generated_string = ''.join((random.choice(string.ascii_letters), random.choice(string.digits)) for _ in range(4))

This results in the following error:

Traceback (most recent call last):
  File "main.py", line 5, in <module>
    generated_string = ''.join((random.choice(string.ascii_letters), random.choice(string.digits)) for _ in range(4))
TypeError: sequence item 0: expected str instance, tuple found

I'm confused as to why this isn't working. I can manually iterate through the contents of the generator and "join" the tuples manually, for example:

generated_string = ((random.choice(string.ascii_letters), random.choice(string.digits)) for _ in range(4))

for val in generated_string:
    print(''.join(val))

Which gives the following output:

U7
W1
y7
G8

What am I missing here? It feels like it's obvious but I feel like I've covered all bases.

Thank you for your time.

Solution

As you said, you pass an iterable to join, and the items must be strings. The iterable can be a tuple, generator, list, or whatever. However:

''.join((random.choice(string.ascii_letters), random.choice(string.digits)) for _ in range(4))

That's not a tuple of strings, it's a sequence of tuples of strings.

print(list((random.choice(string.ascii_letters), random.choice(string.digits)) for _ in range(4)))
# [('w', '7'), ('X', '6'), ('W', '7'), ('P', '1')]

Edit: To fix it you can either nest two ''.join like so:

''.join(''.join((random.choice(string.ascii_letters), random.choice(string.digits))) for _ in range(4))

or, since you know you're always joining exactly two items, use an f-string:

''.join(f'{random.choice(string.ascii_letters)}{random.choice(string.digits)}' for _ in range(4))