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Argument Clinic generates invalid code for optional parameter if there are no mandatory parameters except self. Example:
/*[clinic input]
zlib.Decompress.flush
 self: self(type="compobject *")
 [
 length: uint
 the initial size of the output buffer.
 ]
 /
Return a bytes object containing any remaining decompressed data.
[clinic start generated code]*/
Generates:
...
 switch (PyTuple_Size(args)) {
 case 0:
 if (!PyArg_ParseTuple(args, ":flush", ))
 return NULL;
 break;
...
Note a comma in PyArg_ParseTuple.