Diagnostics

The compiler does not use C++ exceptions for user errors. DiagnosticEngine collects error / warn / note with a DiagnosticCode. Messages print as error[NameError]: ….

--analyze runs the frontend only and prints JSON. The LSP uses the same path for textDocument/publishDiagnostics. It never touches LLVM.

Codes

Exception	Typical cause
`SyntaxError`	Parse
`IndentationError`	Mixed or inconsistent indent
`NameError`	Unknown name, type, function, macro, module
`AttributeError`	Unknown field or method
`TypeError`	Wrong type, operand, or argument
`IndexError`	Bad index or slice
`ImportError`	Missing module or prelude
`ValueError`	Invalid or uninferable value
`AssertionError`	Bad `assert`
`PermissionError`	Private field
`RuntimeError`	Control-flow / compiler internal
`RecursionError`	Macro expansion limit
`NotImplementedError`	Unsupported or leftover construct

These names match the runtime exception classes. An unknown name in # type[Bogus]: ignore is itself ValueError and lists this catalog.

Suppression

IgnoreDirective parses comments:

sere

# type[NameError]: ignore          # whole file if at the top
def main() -> i32:
    n: i32 = "nope"                # TypeError still reported
    return missing                 # NameError ignored (file rule)

    return missing  # type: ignore              # this line
    # type: ignore
    return missing                              # next statement

# type[Exception]: ignore hides every diagnostic in scope
# type: ignore[NameError] is accepted
A comment-only # type: ignore on the previous line applies to the next statement

Use this for generated code and known holes. Do not use it to hide a TypeError you have not looked at.

Incomplete lowering

If a construct parses but lowering is incomplete, you get NotImplementedError rather than silent wrong code. That is a compiler bug surface, not a language feature. File it.

Reserved / out of scope (they diagnose):

*args / **kwargs, keyword-only parameters, global / nonlocal
Nested def, async / await, yield
Lambda capture of enclosing locals
del name
print x as a statement (print is a call)

Introspection and platform

Native interop