Crash when using try/except in platform-specific code in a function that takes an TypeVar argument

[cptpcrd]

Crash Report

I added some platform-specific code to my program in a very specific context, and mypy started crashing with an INTERNAL ERROR. I managed to get it down to a minimal example below.

Traceback

$ mypy bad.py --show-traceback
bad.py:8: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.790
Traceback (most recent call last):
  File "/usr/bin/mypy", line 33, in <module>
    sys.exit(load_entry_point('mypy==0.790', 'console_scripts', 'mypy')())
  File "/usr/lib/python3.8/site-packages/mypy/__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/usr/lib/python3.8/site-packages/mypy/main.py", line 90, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 180, in build
    result = _build(
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 254, in _build
    graph = dispatch(sources, manager, stdout)
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 2630, in dispatch
    process_graph(graph, manager)
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 2953, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 3051, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/usr/lib/python3.8/site-packages/mypy/build.py", line 2111, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 294, in check_first_pass
    self.accept(d)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 677, in accept
    return visitor.visit_func_def(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 726, in visit_func_def
    self._visit_func_def(defn)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 730, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 792, in check_func_item
    self.check_func_def(defn, typ, name)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 975, in check_func_def
    self.accept(item.body)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1005, in accept
    return visitor.visit_block(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 1973, in visit_block
    self.accept(s)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1196, in accept
    return visitor.visit_if_stmt(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 3256, in visit_if_stmt
    self.accept(b)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1005, in accept
    return visitor.visit_block(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 1973, in visit_block
    self.accept(s)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1196, in accept
    return visitor.visit_if_stmt(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 3256, in visit_if_stmt
    self.accept(b)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1005, in accept
    return visitor.visit_block(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 1973, in visit_block
    self.accept(s)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 401, in accept
    stmt.accept(self)
  File "/usr/lib/python3.8/site-packages/mypy/nodes.py", line 1235, in accept
    return visitor.visit_try_stmt(self)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 3350, in visit_try_stmt
    self.visit_try_without_finally(s, try_frame=bool(s.finally_body))
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 3403, in visit_try_without_finally
    self.check_assignment(var, self.temp_node(t, var))
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 2053, in check_assignment
    lvalue_type, index_lvalue, inferred = self.check_lvalue(lvalue)
  File "/usr/lib/python3.8/site-packages/mypy/checker.py", line 2800, in check_lvalue
    assert isinstance(inferred, Var)
AssertionError: 
bad.py:8: : note: use --pdb to drop into pdb

To Reproduce

I managed to simplify my code down to this:

import sys
from typing import TypeVar

T = TypeVar("T", int, float)

def f(t: T) -> None:
    if sys.platform.startswith("anything"):
        try:
            pass
        except Exception as ex:
            pass

For some reason, it must be in a function that takes TypeVar/AnyStr argument, and it must be in an if statement that performs a sys.platform check that isn't true on the current platform (I've reproduced this on both Linux and Windows).

Your Environment

• Mypy version used: 0.790 (reproduced against the latest master)
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.8.6
• Operating system and version: Arch Linux

As mentioned above, I was also able to reproduce this on Windows (Python 3.8.4, mypy 0.790).

[hauntsaninja]

Thanks for the report! Stack trace looks similar to #9615 (seems like the cast to Var is just faulty). Probably something like the fact that we're in "unreachable" code means there isn't a SymbolTableNode where it expects one to be.

[cptpcrd]

Thanks for the report! Stack trace looks similar to #9615 (seems like the cast to Var is just faulty). Probably something like the fact that we're in "unreachable" code means there isn't a SymbolTableNode where it expects one to be.

That correlates with other things I've seen. I did some digging around in mypy's internals, and it seems that var.node isn't populated (so at the end, inferred=None).

I tried other some statements that would render it "unreachable." if False and if sys is not sys don't trigger it, but if sys.version_info < (3, 3) does.