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# What's causing mypy to give an `[assignment]` error in this nested for loop?

+4
−0

I started adding types to my (working) solution to Exercism's "Kindergarten Garden" exercise, to learn how typing with python and Mypy (strict) works. While doing so, I ran into a Mypy error that I can't figure out how to solve. Here's the full code:

``````class Garden:
SEEDCHAR_TO_SEED = {
"C": "Clover",
"G": "Grass",
"V": "Violets",
}
# Student list given when "students" isn't provided when initting instance of Garden
DEFAULT_STUDENTS = [
"Alice",
"Bob",
"Charlie",
"David",
"Eve",
"Fred",
"Ginny",
"Harriet",
"Ileana",
"Joseph",
"Kincaid",
"Larry",
]
# Init the instance of garden (params filled in when called)
def __init__(self, diagram: str, students: list[str] = DEFAULT_STUDENTS):
self.diagram: list[str] = diagram.splitlines()
# create the full seed garden (rather than using seedchars) in init
self.seed_diagram: list[list[str]] = []
for row in self.diagram:
self.seed_diagram.append([self.SEEDCHAR_TO_SEED[seed] for seed in row])
# Ensure that the list of students is in the correct (i.e. alphabetical) order
self.students: list[str] = sorted(students)
self.results: dict[str, list[str]] = {}
# Populate self.results with keys as each student, and values being empty lists
for student in self.students:
self.results[student] = []
for student in self.students:
for row in self.seed_diagram:
for seed in row[
self.students.index(student) * 2 : self.students.index(student) * 2
+ 2
]:
self.results[student].append(seed)

def plants(self, student: str) -> list[str]:
return self.results[student]
``````

running `mypy --strict --pretty` on the file gives this error:

``````kindergarten_garden.py:37: error: Incompatible types in assignment (expression has type "List[str]", variable has type "str")  [assignment]
for row in self.seed_diagram:
^
Found 1 error in 1 file (checked 1 source file)
``````

I've been running the code in a debugger to try to find the issue, but I don't see any assignments or expressions that could cause the error. All the types look correct. Am I missing something?

Why does this post require moderator attention?
Why should this post be closed?

+6
−0

The problem is that you are using `row` twice with different types.

``````for row in self.diagram: # row is str here
``````
``````for row in self.seed_diagram: # row is list[str] here
``````

Renaming one or the other makes the error go away.

Why does this happen? Python has some somewhat counterintuitive scoping rules. Take the following example:

``````def foo():
l = [["hello"], ["world"]]
for x in l:
for y in x:
pass
print(y)

foo()
``````

The `foo` call actually prints `world`! In Python, loops (and other conditionals) do not create a new block scope, and so in fact both `row` variables in your code are one and the same, scoped to the function, and so mypy gets upset when you try to use it with different types.

Why does this post require moderator attention?

+3
−1

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.

Why does this post require moderator attention?