Introduces static attribute Operation.unpacked_args_to_init (#73)

* Introduces static attribute Operation.unpacked_args_to_init

Passing unpacked args is generally helpful if the class derived from
Operation has its constructor implemented as a dataclass.

* adds an example for initializing Operation sub-classes via dataclasses

* make 'dataclasses' a development dependency

matchpy/expressions/expressions.py

@@ -280,7 +280,10 @@ def __call__(cls, *operands: Expression, variable_name=None):
             return operands[0]
 
         operation = Expression.__new__(cls)
-        operation.__init__(operands, variable_name=variable_name)
+        if not cls.unpacked_args_to_init:
+            operation.__init__(operands, variable_name=variable_name)
+        else:
+            operation.__init__(*operands, variable_name=variable_name)
 
         return operation
 
@@ -358,6 +361,10 @@ class Operation(Expression, metaclass=_OperationMeta):
     infix = False
     """bool: True if the name of the operation should be used as an infix operator by str()."""
 
+
+    unpacked_args_to_init = False
+    """bool: True if the class must be instantiated with ``*operands`` instead of ``operands``."""
+
     def __init__(self, operands: List[Expression], variable_name=None) -> None:
         """Create an operation expression.
 

setup.cfg

@@ -52,6 +52,7 @@ develop = %(docs)s
           pytest-cov>=2.4,<3.0
           coveralls
           flake8
+          dataclasses; python_version<'3.7'
 
 [flake8]
 max-line-length = 120

tests/test_expressions.py

@@ -5,7 +5,11 @@
 import pytest
 from multiset import Multiset
 
-from matchpy.expressions.expressions import (Arity, Operation, Symbol, SymbolWildcard, Wildcard, Expression)
+from matchpy.expressions.expressions import (Arity, Operation, Symbol, SymbolWildcard, Wildcard, Expression, Pattern)
+from matchpy import match
+from dataclasses import dataclass, field, fields
+from typing import ClassVar, Optional
+
 from .common import *
 
 SIMPLE_EXPRESSIONS = [
@@ -349,3 +353,32 @@ def test_one_identity_error(self):
     def test_infix_error(self):
         with pytest.raises(TypeError):
             Operation.new('Invalid', Arity.unary, infix=True)
+
+
+class AbstractDataclassOp(Operation):
+    @property
+    def operands(self):
+        return tuple(getattr(self, field.name)
+                     for field in fields(self)
+                     if not field.metadata.get("not_an_operand", False))
+
+
+@dataclass
+class MySum(AbstractDataclassOp):
+    x1: Expression
+    x2: Expression
+    arity: ClassVar[Arity] = Arity.binary
+    variable_name: Optional[str] = field(default=None, metadata={"not_an_operand": True})
+    unpacked_args_to_init: ClassVar[bool] = True
+
+
+
+def test_dataclass_operation_subclass():
+    x1_ = Wildcard.dot("x1")
+    x2_ = Wildcard.dot("x2")
+
+    matches = match(MySum(Symbol("foo"), Symbol("bar")), Pattern(MySum(x1_, x2_)))
+    subst, = list(matches)
+
+    assert subst["x1"].name == "foo"
+    assert subst["x2"].name == "bar"