[BUG] Syntax error in C array declaration and initialization

[GalaxySnail]

Describe the bug

I found it in #5177.

def main():
    cdef int arr[4] = [1, 2, 3, 4]

This code doesn't compile:

Error compiling Cython file:
------------------------------------------------------------
...
def main():
    cdef int arr[4] = [1, 2, 3, 4]
                    ^
------------------------------------------------------------

test.pyx:2:20: Syntax error in C variable declaration

But these two do:

def main():
    cdef int arr1[4]
    arr1 = [1, 2, 3, 4]

    cdef int[4] arr2 = [1, 2, 3, 4]

I think it is already known, because it has been mentioned in the documentation:

• cython/docs/src/userguide/language_basics.rst

  Lines 124 to 129 in b2cad76

  	def func():
  	cdef int i = 10, j, k
  	cdef float f = 2.5
  	# cdef float g[4] = [1,2,3,4] # currently not supported
  	cdef float *g = [1, 2, 3, 4]
  	cdef float *h = &f

but I couldn't find the related issue.

Anyway, if it can't be fixed easily, it will be good to document it more clearly.

Code to reproduce the behaviour:

No response

Expected behaviour

No response

Environment

Cython version: both 0.29.32 and 3.0.0a11

Additional context

No response

[matusvalo]

Just a thought. Should Cython support both syntaxes to declare array?

cdef int arr1[4]   # C style array declaration
cdef int[4] arr1   # Java style array declaration

it seems that both of them generate same C code but the Java style (cdef int[4] arr1) is more consistent because:

1. Memoryviews are defined in the same way: cdef int [:, :, :] narr_view = narr [1]
2. As mentioned in the bug report Cython currently supports cdef int[4] arr2 = [1, 2, 3, 4] but not cdef int arr[4] = [1, 2, 3, 4]
3. Defining fused types uses syntax of TYPE[n]:

   ctypedef fused my_fused_type:
       int[3]
       float[2]

Hence my proposal is following:

Soft-deprecate C style definition - use only Java style array definition (cdef int[4] arr1) in Cython Documentation in all places but still support the C style (to keep backward compatibility).

Any thoughts?

[1] https://cython.readthedocs.io/en/latest/src/userguide/memoryviews.html

[GalaxySnail]

it seems that both of them generate same C code but the Java style (cdef int[4] arr1) is more consistent

Soft-deprecate C style definition - use only Java style array definition (cdef int[4] arr1) in Cython Documentation in all places but still support the C style (to keep backward compatibility).

Fair enough.

BTW, Java style cdef int[4] a, b is different from C style cdef int a[4], b (whether b is an array or an int), but the latter seems to have been deprecated for almost 10 years: 9e3a2d7 8ce5dbc.

However, the documentation said:

• cython/docs/src/userguide/language_basics.rst

  Lines 89 to 91 in d888842

  	def func():
  	cdef int i, j, k
  	cdef float f, g[42], *h

which will generate warnings:

warning: test.pyx:3:19: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.
warning: test.pyx:3:25: Non-trivial type declarators in shared declaration (e.g. mix of pointers and values). Each pointer declaration should be on its own line.

IMO it's better to mention it in the documentation.

[da-woods]

I'll leave this open for now. I think I'd accept a simple parser change to make the other syntax work. But I suspect it's low priority and may not happen

[williamhCode]

I was just wondering how would I declare an array of pointers with java style declaration. I stumbled across this while trying to fit some linked lists in an array.

cdef Node *data[4]
data = [NULL, NULL, NULL, NULL]

This is the only way that I can think of initialising data for now.
I was expecting it to be something like this to work:

cdef Node*[4] data = [NULL, NULL, NULL, NULL]

[scoder]

This should work:

…

cdef Node* data = [NULL, NULL, NULL, NULL]

[williamhCode]

Doing cdef Node** data = [NULL, NULL, NULL, NULL] works as a replacement but it's not quite the same. One problem is if I have to define Node *data[4] in a struct or as an instance variable inside a class, then I wouldn't be able to have stack allocated array of pointers in java style syntax. Sorry for being nit picky but I feel like if Cython wants to deprecate C style syntax and move to Java style, it should support everything. Personally I'm having trouble deciding which style to use for my project due to limitations of the Java style syntax, but also C style syntax not supporting one-liners, so I end up mixing the two.

[GalaxySnail]

If I understand correctly, "soft-deprecating" something doesn't mean that we will actually deprecate it. "soft-deprecated" only means it's not recommended, but will be still supported without any warnings. So if you really need the C style syntax, you can use it as expected, it's OK.

However, I agree that:

cdef Node*[4] data = [NULL, NULL, NULL, NULL]

can be a useful feature that will improve code consistency.

[matusvalo]

"soft-deprecated" only means it's not recommended, but will be still supported without any warnings.

Yes. It should be the preferred way and should be used in all examples in documentation.