write() argument must be str, not generator

This error occurs when you try to pass a generator object as an argument to the built-in `write()` function in Python. The `write()` function is used to write data to a file object or a stream, and it expects a string as its argument.

Here's an example of how this error can occur:

# Example 1: Passing a generator to write()

def my_generator():
    yield 'Hello'
    yield 'world'

with open('output.txt', 'w') as f:
    f.write(my_generator())  # ERROR: write() argument must be str, not generator

In this example, we define a generator function `my_generator()` that yields two strings: "Hello" and "world". We then try to write the output of this generator to a file using the `write()` function, which results in the error message "write() argument must be str, not generator".

To fix this error, we need to convert the generator output to a string before passing it to `write()`. One way to do this is to use the `join()` method of the string object to concatenate the generator output into a single string:

# Example 2: Converting generator output to string before passing to write()

def my_generator():
    yield 'Hello'
    yield 'world'

with open('output.txt', 'w') as f:
    f.write(''.join(my_generator()))  # OK: generator output is concatenated into a single string

In this example, we use the `join()` method to concatenate the output of the `my_generator()` function into a single string, which we then pass to the `write()` function. This should write the string "Helloworld" to the file "output.txt".