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As the error message indicates, the assignment is to the variable `row`. The `for` loop will repeatedly assign to `row`.

But why is there a problem? Because Python's scoping rules are bad/broken. There is no block scoping. The scope of any variable within a function is that entire function except for the bodies of nested functions. The scope of the iterating variable in a `for` loop is not just the `for` loop. The scope of variables within a block, e.g. within the body of a `for` loop, is not just that`for` loop. There is no shadowing of variables except by nested functions.

The upshot for your example is that you use `row` in two different `for` loops looping over two differently typed expressions, so `row` ends up being assigned two different types because there is only one `row` variable.

Specifically, `for row in self.diagram:` leads to `row` having type `str`, but `for row in self.seed_diagram:` would assign `row` with a value of type `list[str]`. Hence the error.

The most direct resolution would be to use a different variable name for each of these loops.