# Buggy code
def bind_function_type_variables(
    self, fun_type: CallableType, defn: Context
) -> List[TypeVarDef]:
    """Find the type variables of the function type and bind them in our tvar_scope"""
    if fun_type.variables:
        for var in fun_type.variables:
            var_expr = self.lookup(var.name, var).node
            assert isinstance(var_expr, TypeVarExpr)
            self.tvar_scope.bind(var.name, var_expr)
        return fun_type.variables
    typevars = self.infer_type_variables(fun_type)
    # Do not define a new type variable if already defined in scope.
    typevars = [
        (name, tvar)
        for name, tvar in typevars
        if not self.is_defined_type_var(name, defn)
    ]
    defs = []  # type: List[TypeVarDef]
    for name, tvar in typevars:
        if not self.tvar_scope.allow_binding(tvar.fullname()):
            self.fail(
                "Type variable '{}' is bound by an outer class".format(name), defn
            )
        self.tvar_scope.bind(name, tvar)
        binding = self.tvar_scope.get_binding(tvar.fullname())
        assert binding is not None
        defs.append(binding)

    return defs


# 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 bind_function_type_variables(
    self, fun_type: CallableType, defn: Context
) -> List[TypeVarDef]:
    """Find the type variables of the function type and bind them in our tvar_scope"""
    if not self.tvar_scope:
        return []  # We are in third pass, nothing new here
    if fun_type.variables:
        for var in fun_type.variables:
            var_expr = self.lookup(var.name, var).node
            assert isinstance(var_expr, TypeVarExpr)
            self.tvar_scope.bind(var.name, var_expr)
        return fun_type.variables
    typevars = self.infer_type_variables(fun_type)
    # Do not define a new type variable if already defined in scope.
    typevars = [
        (name, tvar)
        for name, tvar in typevars
        if not self.is_defined_type_var(name, defn)
    ]
    defs = []  # type: List[TypeVarDef]
    for name, tvar in typevars:
        if not self.tvar_scope.allow_binding(tvar.fullname()):
            self.fail(
                "Type variable '{}' is bound by an outer class".format(name), defn
            )
        self.tvar_scope.bind(name, tvar)
        binding = self.tvar_scope.get_binding(tvar.fullname())
        assert binding is not None
        defs.append(binding)

    return defs