fix: allow creating UnevaluatedExpr with doit arg (#113)

src/ampform/dynamics/__init__.py

@@ -11,7 +11,11 @@
 import sympy as sp
 from sympy.printing.latex import LatexPrinter
 
-from .decorator import UnevaluatedExpression, implement_doit_method
+from .decorator import (
+ UnevaluatedExpression,
+ create_expression,
+ implement_doit_method,
+)
 from .math import ComplexSqrt
 
 try:
@@ -56,10 +60,7 @@ def __new__( # pylint: disable=arguments-differ
  **hints: Any,
  ) -> "BlattWeisskopfSquared":
  args = sp.sympify((angular_momentum, z))
- if evaluate:
- # pylint: disable=no-member
- return sp.Expr.__new__(cls, *args, **hints).evaluate()
- return sp.Expr.__new__(cls, *args, **hints)
+ return create_expression(cls, evaluate, *args, **hints)
 
  def evaluate(self) -> sp.Expr:
  angular_momentum, z = self.args

src/ampform/dynamics/decorator.py

@@ -1,7 +1,7 @@
 """Tools for defining lineshapes with `sympy`."""
 
 from abc import abstractmethod
-from typing import Any, Callable, Type
+from typing import Any, Callable, Optional, Type
 
 import sympy as sp
 from sympy.printing.latex import LatexPrinter
@@ -99,3 +99,20 @@ def doit_method(self: Any, **hints: Any) -> sp.Expr:
  return decorated_class
 
  return decorator
+
+
+def create_expression(
+ cls: Type[UnevaluatedExpression], evaluate: bool, *args: Any, **kwargs: Any
+) -> sp.Expr:
+ """Helper function for implementing :code:`Expr.__new__`.
+
+ See e.g. source code of `.BlattWeisskopfSquared`.
+ """
+ # pylint: disable=no-member
+ deep: Optional[bool] = kwargs.pop("deep", None)
+ expr = sp.Expr.__new__(cls, *args, **kwargs)
+ if evaluate:
+ expr = expr.evaluate()
+ if deep:
+ expr = expr.doit(deep=deep)
+ return expr