INTERNAL ERROR: 'NoneType' object is not iterable

[martincmartin]

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

[ilevkivskyi]

Could you please provide the code that causes this? But from the traceback it looks related to #3799 (maybe a duplicate).

[martincmartin]

I found a small repro:

from typing import NamedTuple


# MyNamedTuple = namedtuple('MyNamedTuple', 'parent')
class MyNamedTuple(NamedTuple):
    parent: 'MyNamedTuple'


def foo(mymap) -> MyNamedTuple:
    return next((mymap[key] for key in mymap), None)

[martincmartin]

Perhaps related, it looks like the resolution of the string type hint 'MyNamedTuple' is happening too late, or is incorrect? This code:

class MyNamedTuple(NamedTuple):
    parent: 'MyNamedTuple'

def bar(nt: MyNamedTuple) -> MyNamedTuple:
    return nt

produces

Deduction.py:9: error: Incompatible return value type (got "MyNamedTuple", expected "MyNamedTuple")

[ilevkivskyi]

Then it looks very similar to the crashes reported in #3419 (not sure if it is a duplicate).

In general, forward references (string type hints) to "synthetic" types is a known problem. I hope it will be fixed very soon.

[martincmartin]

Confirmed that a version with forward references, but not based on namedtuple, but rather "private" fields (leading underscore) and @Property getters (but not setters) works fine. Thanks! More verbose, but a fine work around until recursive NamedTuples are supported better.