Fix RPN validation

CHANGELOG.md

@@ -23,7 +23,7 @@ This is a major release with significant upgrades under the hood of Cheetah. Des
 - Port Bmad-X tracking methods to Cheetah for `Quadrupole`, `Drift`, and `Dipole` (see #153, #240) (@jp-ga, @jank324)
 - Add `TransverseDeflectingCavity` element (following the Bmad-X implementation) (see #240, #278) (@jp-ga, @cr-xu, @jank324)
 - `Dipole` and `RBend` now take a focusing moment `k1` (see #235, #247) (@hespe)
-- Implement a converter for lattice files imported from Elegant (see #222, #251, #273) (@hespe)
+- Implement a converter for lattice files imported from Elegant (see #222, #251, #273, #281) (@hespe, @jank324)
 
 ### 🐛 Bug fixes
 

cheetah/converters/utils/rpn.py

@@ -3,7 +3,7 @@
 
 def is_valid_expression(expression: str) -> bool:
     """Checks if expression is a reverse Polish notation."""
-    stripped = expression[1:-1].strip()
+    stripped = expression.strip()
     return stripped[-1] in "+-/*" and len(stripped.split(" ")) == 3
 
 
@@ -13,5 +13,5 @@ def eval_expression(expression: str, context: dict) -> Any:
 
     NOTE: Does not support nested expressions.
     """
-    splits = expression[1:-1].strip().split(" ")
+    splits = expression.strip().split(" ")
     return eval(" ".join([splits[0], splits[2], splits[1]]), context)

tests/resources/cavity.lte

@@ -1,4 +1,4 @@
 C1E: EMATRIX, L = 0, ORDER = 1, C2=-0.0027, C4=-0.150, R11=1, R21=0.04, R22=1, R23=0.003, R33=1, R41=0.003, R43=-0.04, R44=1, R55=1, R66=1
-C1: RFCA, L = 0.7, PHASE = 90.000000, VOLT = 16175000.000000, FREQ = 1200000000.000000
+C1: RFCA, L = 0.7, PHASE = 90.000000, VOLT = 16175000.000000, FREQ = 1200000000.000000, CHANGE_P0 = 1, END1_FOCUS = 1, END2_FOCUS = 1, BODY_FOCUS_MODEL = "SRS"
 
 cavity: line=(C1E,C1)

tests/test_rpn.py

@@ -0,0 +1,33 @@
+from cheetah.converters.utils import rpn
+
+
+def test_valid_rpn_expression():
+    """
+    Test that a valid RPN expression without nesting is correctly recognised as a valid
+    RPN expression.
+    """
+    expression = "2 3 +"
+
+    assert rpn.is_valid_expression(expression)
+
+
+def test_valid_rpn_expression_with_single_quotes():
+    """
+    Test that a valid RPN expression that has single quotes around it and should
+    therefore not be recognised as valid because these should have been stripped off
+    before calling the function.
+    """
+    expression = "'2 3 +'"
+
+    assert not rpn.is_valid_expression(expression)
+
+
+def test_falsely_validated_normal_expression():
+    """
+    Tests that the expression `"ldsp2h +dldsp17h +lblxsph/2-lbxsph/2"`, which was
+    falsely recognised as a valid RPN expression in a previous version of Cheetah, is
+    correctly recognised as invalid.
+    """
+    expression = "ldsp2h +dldsp17h +lblxsph/2-lbxsph/2"
+
+    assert not rpn.is_valid_expression(expression)