From 9322c27af93718df6938672d585590ae4e5d583c Mon Sep 17 00:00:00 2001
From: Nicolas Mounet <nicolas.mounet@cern.ch>
Date: Wed, 26 May 2021 18:57:56 +0200
Subject: [PATCH] Introducing _MaskBase class, of which Mask is a subclass

---
 cara/models.py | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/cara/models.py b/cara/models.py
index c2500d68..f5e80d1a 100644
--- a/cara/models.py
+++ b/cara/models.py
@@ -472,8 +472,21 @@ Virus.types = {
 
 
 @dataclass(frozen=True)
-class Mask:
-    #: Filtration efficiency. (In %/100)
+class _MaskBase:
+    """
+    Represents the filtration of aerosols by a mask, both inward and 
+    outward.
+    The nature of the various air exchange schemes means that it is expected
+    for subclasses of _MaskBase to exist.
+    """
+    def exhale_efficiency(self, diameter: float) -> _VectorisedFloat:
+        # Overall efficiency, including the effect of the leaks.
+        raise NotImplementedError("Subclass must implement")
+
+
+@dataclass(frozen=True)
+class Mask(_MaskBase):
+    #: Filtration efficiency.
     η_exhale: _VectorisedFloat
 
     #: Leakage through side of masks.