Bug: INTERNAL ERROR from := assignment expression within list comprehension

[Jasha10]

I've encountered a bug when running mypy on a file with an assignment expression contained within a list comprehension.

# mwe.py
lst = [1, 2, 3, 4]
processed = [e for elt in lst if (e := elt - 1) > 1]
print(processed)

I am running the following:
mypy version: 0.790+dev.068594eb38fc550d834b7e61745177c423f2b224
CPython version: 3.8.3, installed with anaconda

Here is the traceback:

$ mypy --show-traceback mwe.py
mwe.py:3: 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+dev.068594eb38fc550d834b7e61745177c423f2b224
Traceback (most recent call last):
  File "/home/jbss/anaconda3/envs/tmp/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/main.py", line 89, in main
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/build.py", line 180, in build
    result = _build(
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/build.py", line 252, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/build.py", line 2626, in dispatch
    process_graph(graph, manager)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/build.py", line 2949, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/build.py", line 3041, in process_stale_scc
    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal_main.py", line 78, in semantic_analysis_for_scc
    process_top_levels(graph, scc, patches)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal_main.py", line 199, in process_top_levels
    deferred, incomplete, progress = semantic_analyze_target(next_id, state,
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal_main.py", line 326, in semantic_analyze_target
    analyzer.refresh_partial(refresh_node,
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 357, in refresh_partial
    self.refresh_top_level(node)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 368, in refresh_top_level
    self.accept(d)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 4691, in accept
    node.accept(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/nodes.py", line 1063, in accept
    return visitor.visit_assignment_stmt(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 1906, in visit_assignment_stmt
    s.rvalue.accept(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/nodes.py", line 1946, in accept
    return visitor.visit_list_comprehension(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 3755, in visit_list_comprehension
    expr.generator.accept(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/nodes.py", line 1933, in accept
    return visitor.visit_generator_expr(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 3770, in visit_generator_expr
    self.analyze_comp_for(expr)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 3790, in analyze_comp_for
    cond.accept(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/nodes.py", line 1762, in accept
    return visitor.visit_comparison_expr(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 3648, in visit_comparison_expr
    operand.accept(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/nodes.py", line 1619, in accept
    return visitor.visit_assignment_expr(self)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 1896, in visit_assignment_expr
    self.analyze_lvalue(s.target, escape_comprehensions=True)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 2565, in analyze_lvalue
    self.analyze_name_lvalue(lval, explicit_type, is_final, escape_comprehensions)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 2616, in analyze_name_lvalue
    added = self.add_symbol(name, var, lvalue, escape_comprehensions=escape_comprehensions)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 4217, in add_symbol
    return self.add_symbol_table_node(name, symbol, context, can_defer, escape_comprehensions)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 4265, in add_symbol_table_node
    names = self.current_symbol_table(escape_comprehensions=escape_comprehensions)
  File "/home/jbss/anaconda3/envs/tmp/lib/python3.8/site-packages/mypy/semanal.py", line 4553, in current_symbol_table
    assert names is not None
AssertionError:
mwe.py:3: : note: use --pdb to drop into pdb

[ilevkivskyi]

Thanks for reporting! Crashes are bad so setting priority to high.

[dhood]

out of curiosity I've been looking into this and think I have an idea of what's causing it. Planning to open a PR with some regression tests and a fix!