# Buggy code
def check_multi_assignment(
    self,
    lvalues: List[Lvalue],
    rvalue: Expression,
    context: Context,
    infer_lvalue_type: bool = True,
    msg: Optional[str] = None,
) -> None:
    """Check the assignment of one rvalue to a number of lvalues."""

    # Infer the type of an ordinary rvalue expression.
    rvalue_type = self.expr_checker.accept(rvalue)  # TODO maybe elsewhere; redundant
    undefined_rvalue = False

    if isinstance(rvalue_type, UnionType):
        # If this is an Optional type in non-strict Optional code, unwrap it.
        relevant_items = rvalue_type.relevant_items()
        if len(relevant_items) == 1:
            rvalue_type = relevant_items[0]

    if isinstance(rvalue_type, AnyType):
        for lv in lvalues:
            if isinstance(lv, StarExpr):
                lv = lv.expr
            temp_node = self.temp_node(
                AnyType(TypeOfAny.from_another_any, source_any=rvalue_type), context
            )
            self.check_assignment(lv, temp_node, infer_lvalue_type)
    elif isinstance(rvalue_type, TupleType):
        self.check_multi_assignment_from_tuple(
            lvalues, rvalue, rvalue_type, context, undefined_rvalue, infer_lvalue_type
        )
    else:
        self.check_multi_assignment_from_iterable(
            lvalues, rvalue_type, context, infer_lvalue_type
        )


# Traceback (AssertionError)
mypy --py2 --show-traceback test.py

test.py:6: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty
Traceback (most recent call last):
File "/usr/local/Cellar/mypy/0.521/libexec/bin/mypy", line 11, in <module>
load_entry_point('mypy===0.521-858f7512cf3b4e39c0f4e8de5a13eee0e1e138fb-dirty', 'console_scripts', 'mypy')()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
main(None)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 50, in main
res = type_check_only(sources, bin_dir, options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/main.py", line 97, in type_check_only
options=options)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 196, in build
graph = dispatch(sources, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1801, in dispatch
process_graph(graph, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2044, in process_graph
process_stale_scc(graph, scc, manager)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 2147, in process_stale_scc
graph[id].type_check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/build.py", line 1716, in type_check_first_pass
self.type_checker.check_first_pass()
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 185, in check_first_pass
self.accept(d)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 565, in accept
return visitor.visit_func_def(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 519, in visit_func_def
self.check_func_item(defn, name=defn.name())
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 578, in check_func_item
self.check_func_def(defn, typ, name)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 738, in check_func_def
self.accept(item.body)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 815, in accept
return visitor.visit_block(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1202, in visit_block
self.accept(s)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 273, in accept
stmt.accept(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/nodes.py", line 859, in accept
return visitor.visit_assignment_stmt(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1209, in visit_assignment_stmt
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1236, in check_assignment
infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1483, in check_assignment_to_multiple_lvalues
self.check_multi_assignment(lvalues, rvalue, context, infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1522, in check_multi_assignment
context, undefined_rvalue, infer_lvalue_type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1550, in check_multi_assignment_from_tuple
assert isinstance(reinferred_rvalue_type, TupleType)
AssertionError:
test.py:6: note: use --pdb to drop into pdb

# Fixed code
def check_multi_assignment(
    self,
    lvalues: List[Lvalue],
    rvalue: Expression,
    context: Context,
    infer_lvalue_type: bool = True,
    rv_type: Optional[Type] = None,
    undefined_rvalue: bool = False,
) -> None:
    """Check the assignment of one rvalue to a number of lvalues."""

    # Infer the type of an ordinary rvalue expression.
    # TODO: maybe elsewhere; redundant.
    rvalue_type = rv_type or self.expr_checker.accept(rvalue)

    if isinstance(rvalue_type, UnionType):
        # If this is an Optional type in non-strict Optional code, unwrap it.
        relevant_items = rvalue_type.relevant_items()
        if len(relevant_items) == 1:
            rvalue_type = relevant_items[0]

    if isinstance(rvalue_type, AnyType):
        for lv in lvalues:
            if isinstance(lv, StarExpr):
                lv = lv.expr
            temp_node = self.temp_node(
                AnyType(TypeOfAny.from_another_any, source_any=rvalue_type), context
            )
            self.check_assignment(lv, temp_node, infer_lvalue_type)
    elif isinstance(rvalue_type, TupleType):
        self.check_multi_assignment_from_tuple(
            lvalues, rvalue, rvalue_type, context, undefined_rvalue, infer_lvalue_type
        )
    elif isinstance(rvalue_type, UnionType):
        self.check_multi_assignment_from_union(
            lvalues, rvalue, rvalue_type, context, infer_lvalue_type
        )
    else:
        self.check_multi_assignment_from_iterable(
            lvalues, rvalue_type, context, infer_lvalue_type
        )