Add several methods to handle decay descriptors

src/decaylanguage/decay/decay.py

@@ -602,6 +602,33 @@ def ndecays(self) -> int:
         """
         return len(self.decays)
 
+    def to_string(self, arrow="->") -> str:
+        """
+        One-line string representation of the entire decay chain. Sub-decays are
+        enclosed in round parentheses.
+        """
+
+        def _str(
+            decay: dict[str, list[dict[str, float | str | list[Any]]]] | str,
+            top=False,
+        ) -> str:
+            if isinstance(decay, dict):
+                mother = list(decay.keys())[0]
+                fs = [_str(fsp) for i_decay in decay[mother] for fsp in i_decay["fs"]]
+                descriptor = " ".join([mother, arrow, *fs])
+                if not top:
+                    return f"({descriptor})"
+                return descriptor
+
+            elif isinstance(decay, str):
+                return decay
+
+            else:
+                raise TypeError(f"Don't know how to handle {type(decay)}")
+
+        dc_dict = self.to_dict()
+        return _str(dc_dict, True)
+
     def print_as_tree(self) -> None:  # pragma: no cover
         """
         Tree-structure like print of the entire decay chain.

tests/decay/test_descriptor.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+import pytest
+
+from decaylanguage import DecayChain, DecayMode
+
+dm1 = DecayMode(0.6770, "D0 pi+")  # D*+
+dm2 = DecayMode(0.0124, "K_S0 pi0")  # D0
+dm3 = DecayMode(0.692, "pi+ pi-")  # K_S0
+dm4 = DecayMode(0.98823, "gamma gamma")  # pi-
+dm5 = DecayMode(0.0105, "D- tau+ nu_tau")  # B0
+dm6 = DecayMode(0.0938, "K+ pi- pi-")  # D-
+dm7 = DecayMode(0.0931, "pi+ pi+ pi- anti-nu_tau")  # tau+
+dm8 = DecayMode(1.85e-6, "phi phi'")  #  B_s0
+dm9a = DecayMode(0.491, "K+ K-")  #  phi
+dm9b = DecayMode(0.154, "pi+ pi- pi0")  #  phi
+
+
+@pytest.mark.parametrize(
+    ("dc", "expected"),
+    [
+        (
+            DecayChain("D0", {"D0": dm2, "K_S0": dm3, "pi0": dm4}),
+            "D0 -> (K_S0 -> pi+ pi-) (pi0 -> gamma gamma)",
+        ),
+        (
+            DecayChain("D*+", {"D*+": dm1, "D0": dm2, "K_S0": dm3, "pi0": dm4}),
+            "D*+ -> (D0 -> (K_S0 -> pi+ pi-) (pi0 -> gamma gamma)) pi+",
+        ),
+        (
+            DecayChain("B0", {"B0": dm5, "D-": dm6, "tau+": dm7}),
+            "B0 -> (D- -> K+ pi- pi-) nu_tau (tau+ -> anti-nu_tau pi+ pi+ pi-)",
+        ),
+        (
+            DecayChain("B_s0", {"B_s0": dm8, "phi": dm9a, "phi'": dm9b}),
+            "B_s0 -> (phi -> K+ K-) (phi' -> pi+ pi- pi0)",
+        ),
+    ],
+)
+def test_descriptor(dc: DecayChain, expected: str):
+    descriptor = dc.to_string()
+    assert descriptor == expected