Implement cdef dataclasses (GH-3400)

New decorator/function "@cython.dataclasses.dataclass" and "cython.dataclasses.field()" to mark dataclasses and their fields.

Tries to match the interface provided by a regular dataclass as much as possible.
This means taking the types from the dataclasses module if available (so they match exactly) or a fallback Python version that just implements the core parts (executed with "PyRun_String()" in the C source).

Use of placeholders in generated "__init__" code means the code in the C file isn't hugely readable. Probably not a huge issue, but don't really see a way round that.

As part of this I've also also implemented a Cython version of "typing.ClassVar". Although really designed for use with dataclasses it behaves sensibly when used in types in a normal cdef class. This is worth documenting more thoroughly.

Closes https://github.com/cython/cython/issues/2903

4 files changed, 59 insertions, 0 deletions

diff --git a/tests/errors/dataclass_e1.pyx b/tests/errors/dataclass_e1.pyx
new file mode 100644
index 000000000..39337ba6d
--- /dev/null
+++ b/tests/errors/dataclass_e1.pyx
@@ -0,0 +1,22 @@
+# mode: error
+
+cimport cython
+
+@cython.dataclasses.dataclass(1, shouldnt_be_here=True, init=5, unsafe_hash=True)
+cdef class C:
+    a: list = []  # mutable
+    b: int = cython.dataclasses.field(default=5, default_factory=int)
+    c: int
+
+    def __hash__(self):
+        pass
+
+_ERRORS = """
+6:5: Arguments passed to cython.dataclasses.dataclass must be True or False
+6:5: Cannot overwrite attribute __hash__ in class C
+6:5: cython.dataclasses.dataclass() got an unexpected keyword argument 'shouldnt_be_here'
+6:5: cython.dataclasses.dataclass takes no positional arguments
+7:14: mutable default <class 'list'> for field a is not allowed: use default_factory
+8:37: cannot specify both default and default_factory
+9:4: non-default argument 'c' follows default argument in dataclass __init__
+"""
diff --git a/tests/errors/dataclass_e2.pyx b/tests/errors/dataclass_e2.pyx
new file mode 100644
index 000000000..e25965938
--- /dev/null
+++ b/tests/errors/dataclass_e2.pyx
@@ -0,0 +1,13 @@
+# mode: error
+# tag: dataclass
+
+import dataclasses
+
+@dataclasses.dataclass
+cdef class C:
+    pass
+
+_ERRORS = """
+6:0: Cdef functions/classes cannot take arbitrary decorators.
+6:0: Use '@cython.dataclasses.dataclass' on cdef classes to create a dataclass
+"""
diff --git a/tests/errors/dataclass_e3.pyx b/tests/errors/dataclass_e3.pyx
new file mode 100644
index 000000000..85a900172
--- /dev/null
+++ b/tests/errors/dataclass_e3.pyx
@@ -0,0 +1,13 @@
+# mode: compile
+# tag: dataclass, warnings
+
+cimport cython
+from dataclass import field
+
+@cython.dataclasses.dataclass
+cdef class E:
+    a: int = field()
+
+_WARNINGS="""
+9:18: Do you mean cython.dataclasses.field instead?
+"""
diff --git a/tests/errors/dataclass_e4.pyx b/tests/errors/dataclass_e4.pyx
new file mode 100644
index 000000000..007487bb8
--- /dev/null
+++ b/tests/errors/dataclass_e4.pyx
@@ -0,0 +1,11 @@
+# mode: error
+
+cimport cython
+
+@cython.dataclasses.dataclass
+cdef class C:
+    a: int = cython.dataclasses.field(unexpected=True)
+
+_ERRORS = """
+7:49: cython.dataclasses.field() got an unexpected keyword argument 'unexpected'
+"""