typecheck:explain

Command /types:explain

Explain the type error and show the correct solution without type: ignore.

Principle

NEVER suggest type: ignore. Every type error has a correct solution.

Instructions

Step 1: Get context

If file:line is specified — read the code around the error.

# Get the error
mypy src/file.py:{{ line }} --show-error-codes

Step 2: Identify the error type

Common mypy errors:

• error: Incompatible types in assignment
• error: Argument X has incompatible type
• error: Item "None" of "..." has no attribute
• error: Missing return statement
• error: Function is missing a return type annotation
• error: Need type annotation for "..."

Step 3: Show the solution

Solutions for Common Errors

Incompatible types in assignment

# Error
x: str = 123  # Incompatible types in assignment (expression has type "int", variable has type "str")

# Solution 1: Change the variable type
x: int = 123

# Solution 2: Convert the value
x: str = str(123)

# Solution 3: Use Union
x: int | str = 123

Argument has incompatible type

# Error
def process(items: list[str]) -> None: ...
process(["a", "b", 1])  # Argument 1 has incompatible type "list[str | int]"

# Solution: Fix data or signature
process(["a", "b", str(1)])
# or
def process(items: list[str | int]) -> None: ...

Item "None" has no attribute

# Error
user = get_user(id)  # Returns User | None
print(user.name)  # Item "None" of "User | None" has no attribute "name"

# Solution 1: Check for None
user = get_user(id)
if user is not None:
    print(user.name)

# Solution 2: Early return
user = get_user(id)
if user is None:
    raise ValueError("User not found")
print(user.name)  # Now mypy knows user is not None

# Solution 3: assert (only for internal code)
user = get_user(id)
assert user is not None, "User must exist"
print(user.name)

Missing return statement

# Error
def process(x: int) -> str:
    if x > 0:
        return "positive"
    # Missing return statement

# Solution: Add return for all paths
def process(x: int) -> str:
    if x > 0:
        return "positive"
    return "non-positive"