# Buggy code
def visit_class_def(self, tdef: ClassDef) -> None:
    # NamedTuple base classes are validated in check_namedtuple_classdef; we don't have to
    # check them again here.
    if not tdef.info.is_named_tuple:
        for type in tdef.info.bases:
            self.analyze(type)
            if tdef.info.is_protocol:
                if not isinstance(type, Instance) or not type.type.is_protocol:
                    if type.type.fullname() != "builtins.object":
                        self.fail("All bases of a protocol must be protocols", tdef)
    # Recompute MRO now that we have analyzed all modules, to pick
    # up superclasses of bases imported from other modules in an
    # import loop. (Only do so if we succeeded the first time.)
    if tdef.info.mro:
        tdef.info.mro = []  # Force recomputation
        calculate_class_mro(tdef, self.fail_blocker)
        if tdef.info.is_protocol:
            add_protocol_members(tdef.info)
    if tdef.analyzed is not None:
        if isinstance(tdef.analyzed, TypedDictExpr):
            self.analyze(tdef.analyzed.info.typeddict_type)
        elif isinstance(tdef.analyzed, NamedTupleExpr):
            self.analyze(tdef.analyzed.info.tuple_type)
    super().visit_class_def(tdef)


# Traceback (TypeError)
mypy --show-traceback Deduction.py
Deduction.py:102: error: The return type of "__init__" must be None
Deduction.py:103: 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 754, in accept
return visitor.visit_class_def(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1109, in visit_class_def
self.accept(defn.defs)
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 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 974, in accept
return visitor.visit_if_stmt(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 1977, in visit_if_stmt
if_map, else_map = self.find_isinstance_check(e)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2559, in find_isinstance_check
return find_isinstance_check(n, self.type_map)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2810, in find_isinstance_check
return conditional_type_map(expr, vartype, type)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/checker.py", line 2613, in conditional_type_map
and is_proper_subtype(current_type, proposed_type)):
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 1114, in accept
return visitor.visit_union_type(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 693, in visit_union_type
return all([is_proper_subtype(item, self.right) for item in left.items])
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 693, in <listcomp>
return all([is_proper_subtype(item, self.right) for item in left.items])
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 900, in accept
return visitor.visit_tuple_type(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 671, in visit_tuple_type
if not is_proper_subtype(l, r):
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 569, in is_proper_subtype
return left.accept(ProperSubtypeVisitor(right))
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/types.py", line 435, in accept
return visitor.visit_instance(self)
File "/usr/local/Cellar/mypy/0.521/libexec/lib/python3.6/site-packages/mypy/subtypes.py", line 605, in visit_instance
for base in left.type.mro:
TypeError: 'NoneType' object is not iterable
Deduction.py:103: note: use --pdb to drop into pdb

# Fixed code
def visit_class_def(self, tdef: ClassDef) -> None:
    # NamedTuple base classes are validated in check_namedtuple_classdef; we don't have to
    # check them again here.
    if not tdef.info.is_named_tuple:
        types = list(tdef.info.bases)  # type: List[Type]
        for tvar in tdef.type_vars:
            if tvar.upper_bound:
                types.append(tvar.upper_bound)
            if tvar.values:
                types.extend(tvar.values)
        self.analyze_types(types, tdef.info)
        for type in tdef.info.bases:
            if tdef.info.is_protocol:
                if not isinstance(type, Instance) or not type.type.is_protocol:
                    if type.type.fullname() != "builtins.object":
                        self.fail("All bases of a protocol must be protocols", tdef)
    # Recompute MRO now that we have analyzed all modules, to pick
    # up superclasses of bases imported from other modules in an
    # import loop. (Only do so if we succeeded the first time.)
    if tdef.info.mro:
        tdef.info.mro = []  # Force recomputation
        calculate_class_mro(tdef, self.fail_blocker)
        if tdef.info.is_protocol:
            add_protocol_members(tdef.info)
    if tdef.analyzed is not None:
        # Also check synthetic types associated with this ClassDef.
        # Currently these are TypedDict, and NamedTuple.
        if isinstance(tdef.analyzed, TypedDictExpr):
            self.analyze(tdef.analyzed.info.typeddict_type, tdef.analyzed, warn=True)
        elif isinstance(tdef.analyzed, NamedTupleExpr):
            self.analyze(tdef.analyzed.info.tuple_type, tdef.analyzed, warn=True)
            for name in tdef.analyzed.info.names:
                sym = tdef.analyzed.info.names[name]
                if isinstance(sym.node, (FuncDef, Decorator)):
                    self.accept(sym.node)
                if isinstance(sym.node, Var):
                    self.analyze(sym.node.type, sym.node)
    super().visit_class_def(tdef)