decompressing a compressed string - string

Question

My question is that I don't know where to go now with the code I have to create a decompress code. I get the error (TypeError: can't multiply sequence by non-int of type 'str') and assume its because I'm not multiplying the string correctly. Also, I can't use lists, just string manipulation for this assignment
Just as an example, the output's suppose to look like this-> cat2dog1qwerty3 -> catcatdogqwertyqwertyqwerty
Function:
def decompress(compressed_in):
new_word = True
char_holder = ""
decompressed_out = ""
for char in compressed_in:
if char.isalpha and new_word:
char_holder += char
new_word = False
elif char.isalnum:
decompressed_out += char * char_holder
new_word = True
return decompressed_out
Main:
# Import
from compress import decompress
# Inputs
compressed_in = str(input("Enter a compressed string: ")) # compressed
# Outputs
decompressed_out = decompress(compressed_in)
print(decompressed_out)

Answer 1

Since this is apparently a homework assignment, I won't give you the code, but here are several problems I see with what you're presented.
Indentation. This is probably an artifact of copying-and-pasting, but every line after the def should be indented.
Not calling functions. When you write char.isalpha, that probably isn't doing what you want it to. .isalpha() is a function, so you need to call it with parentheses, like char.isalpha().
isalnum() is probably not the function you want. That checks if something is a letter or a number, but you've already checked for letters, so you probably want the function that checks if something is a number. This isn't strictly necessary, since the other if condition will still trigger first, but it's something you could get marked down for.
You never clear char_holder. It looks like you meant to, since you have a boolean new_word that you keep track of, but you aren't using it properly. At some point, you should be doing char_holder = char (ie. not +=). I'll let you decide where to put that logic.
Finally, for the error you're getting. You are correct that you are not multiplying things together correctly. Think about what the types are in the multiplication statement, and what values the variables would have. For example, in the first pass, char_holder would be equal to 'cat', and char would be equal to '3'. Try typing '3' * 'cat' into a Python interpreter and see what happens. It should be evident from here what you need to do to fix this.