[BUG] Compiler error when casting to `ctuple`

[jakirkham]

Describe the bug

When attempting to cast to a ctuple, Cython runs into a compiler error.

To Reproduce

Code to reproduce the behavior:

# cython: language_level=3

cdef double d
cdef int i

o = (3.0, 2)
d, i = <(double, int)>o  # <--- compiler errors here

Exception:

Traceback (most recent call last):
  File "/Users/jkirkham/miniconda/envs/cython/bin/cython", line 11, in <module>
    sys.exit(setuptools_main())
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 840, in setuptools_main
    return main(command_line = 1)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 858, in main
    result = compile(sources, options)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 780, in compile
    return compile_multiple(source, options)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 757, in compile_multiple
    result = run_pipeline(source, options, context=context)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 515, in run_pipeline
    err, enddata = Pipeline.run_pipeline(pipeline, source)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Pipeline.py", line 355, in run_pipeline
    data = run(phase, data)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Pipeline.py", line 335, in run
    return phase(data)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Pipeline.py", line 34, in parse
    tree = context.parse(source_desc, scope, pxd = 0, full_module_name = full_module_name)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Main.py", line 369, in parse
    tree = Parsing.p_module(s, pxd, full_module_name)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 3707, in p_module
    body = p_statement_list(s, ctx(level=level), first_statement = 1)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 2341, in p_statement_list
    stat = p_statement(s, ctx, first_statement = first_statement)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 2333, in p_statement
    return p_simple_statement_list(s, ctx, first_statement=first_statement)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 2154, in p_simple_statement_list
    stat = p_simple_statement(s, first_statement = first_statement)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 2148, in p_simple_statement
    node = p_expression_or_assignment(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 1523, in p_expression_or_assignment
    expr = p_testlist_star_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 1453, in p_testlist_star_expr
    expr = p_test_or_starred_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 210, in p_test_or_starred_expr
    return p_test(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 142, in p_test
    expr = p_or_test(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 163, in p_or_test
    return p_rassoc_binop_expr(s, ('or',), p_and_test)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 166, in p_rassoc_binop_expr
    n1 = p_subexpr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 179, in p_and_test
    return p_rassoc_binop_expr(s, ('and',), p_not_test)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 166, in p_rassoc_binop_expr
    n1 = p_subexpr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 189, in p_not_test
    return p_comparison(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 195, in p_comparison
    n1 = p_starred_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 219, in p_starred_expr
    expr = p_bit_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 261, in p_bit_expr
    return p_binop_expr(s, ('|',), p_xor_expr)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 266, in p_xor_expr
    return p_binop_expr(s, ('^',), p_and_expr)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 271, in p_and_expr
    return p_binop_expr(s, ('&',), p_shift_expr)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 276, in p_shift_expr
    return p_binop_expr(s, ('<<', '>>'), p_arith_expr)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 281, in p_arith_expr
    return p_binop_expr(s, ('+', '-'), p_term)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 286, in p_term
    return p_binop_expr(s, ('*', '@', '/', '%', '//'), p_factor)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 97, in p_binop_expr
    n1 = p_sub_expr(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 292, in p_factor
    return _p_factor(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 308, in _p_factor
    return p_typecast(s)
  File "/Users/jkirkham/miniconda/envs/cython/lib/python3.8/site-packages/Cython/Compiler/Parsing.py", line 322, in p_typecast
    and base_type.name is None):
AttributeError: 'CTupleBaseTypeNode' object has no attribute 'name'

Workaround (ctypedef of ctuple):

ctypedef (double, int) double_int

cdef double d
cdef int i

o = (3.0, 2)
d, i = <double_int>o

Expected behavior

The compiler error does not occur.

Environment (please complete the following information):

• OS: macOS
• Python version 3.8.5
• Cython version 0.29.21

Additional context

NA

[da-woods]

I realize this is probably a minimized example to demonstrate the issue, but I can't see why the cast to a ctuple is useful here? I'd think d, i = o should work just as efficiently?

[jakirkham]

Yeah it’s an oversimplified example just to demonstrate the issue.

[scoder]

d, i = <(double, int)>o # <--- compiler errors here

You didn't say what error occurred, but it probably fails to parse it, right? Such a cast is not really beautiful syntax, and most likely not something that was considered when ctuples were implemented.

Is there really a use case for such a cast? In an assignment, you could always type the left hand side rather than casting the right hand side. In what kind of code have you noticed this?

[jakirkham]

Yeah it seems to be a parsing error. Have added the exception to the detail in the OP.

Fair enough. FWIW it seems readable and seems comparable in complexity to things like <float[:n:1]>.

Only recently learned about ctuples in Cython (which is a neat feature btw :), so was doing some exploration with some existing code that could be made a little clearer by bundling a few related values together. Stumbled across this issue in the process.

No strong feelings on the resolution here (including closing if that is what we deem appropriate).

[da-woods]

It looks like it's being parsed basically correctly and only a small change is needed to make it work so I've submitted a fix.

I'm still not convinced that there's a real need to do it, but it probably should work.

[jakirkham]

Thanks for doing that :)