The member variable name up causes a collision with the generated c function MyClass_setup. This causes an error during C compile time. This is mainly a coincidence that set + up is the same as setup.

Version: mypy 0.950 (compiled: yes) with MSVC++ 19.21.27702.2 for x64
Edit: also confirmed with mypy 0.970+dev.74b35bef35ad5a7b1aa635f52ffcfd850eadc589.dirty (compiled: no)


Example
Running mypyc on this python class:

class MyClass:
    def __init__(self, arg1):
        self.up = arg1

produces this generated C function:
static int MyClass_setup(example1___MyClassObject *self, PyObject *value, void *closure);

which conflicts with the existing MyClass_setup function that is generated for classes:
static PyObject *MyClass_setup(PyTypeObject *type);


Expected behavior
A member variable "up" would not cause a collision. For example, other variable names like this:

class MyClass:
    def __init__(self, arg1):
        self.membervar1 = arg1

produce generated code like this:
static int MyClass_setmembervar1(example1___MyClassObject *self, PyObject *value, void *closure);
which does not conflict with other function names.