Added documentation for multi-line preconditions (#1305)

Author: PraneethS42

packages/python-ta/docs/checkers/index.md

@@ -2697,7 +2697,7 @@ def f(x: int) -> int:
 
 This error occurs when a function precondition contains invalid Python expression syntax. Valid Python statements
 (e.g. assignment statements like x = 5) are also flagged, as they aren't valid Python expressions.
-Does not currently support multi-line preconditions.
+Multi-line preconditions are supported using a backslash ('\') following a space at the end of each line except the last.
 
 Example:
 

packages/python-ta/docs/contracts/index.md

@@ -124,13 +124,16 @@ PythonTA uses the [typeguard] library to check types.
 
 ### Functions: custom preconditions
 
-You can write arbitrary preconditions as Python expressions in the function docstring, using the following syntax:
+You can write arbitrary preconditions as Python expressions in the function docstring, using the following syntax.
+Multi-line preconditions can be created by using a backslash ('\') following a space at the end of each line except the last:
 
 ```
 Preconditions:
     - <expr>
     - <expr>
-    - <expr>
+    - <expr> \
+      <expr> \
+      <expr>
 ```
 
 Each expression is evaluated in the same scope as the function body, in top-down order.

packages/python-ta/src/python_ta/checkers/contract_checker.py

@@ -3,6 +3,7 @@
 """
 
 import ast
+import re
 
 from astroid import nodes
 from pylint.checkers import BaseChecker
@@ -33,10 +34,13 @@ def visit_functiondef(self, node: nodes.FunctionDef) -> None:
 
         preconditions = parse_assertions(node, parse_token="Precondition")
         for condition in preconditions:
+            cleaned_condition = re.sub(r"\s+", " ", condition)
             try:
-                ast.parse(condition, mode="eval")
+                ast.parse(cleaned_condition, mode="eval")
             except SyntaxError:
-                self.add_message("invalid-precondition-syntax", node=node, args=(condition,))
+                self.add_message(
+                    "invalid-precondition-syntax", node=node, args=(cleaned_condition,)
+                )
 
 
 def register(linter: PyLinter) -> None:

packages/python-ta/tests/test_custom_checkers/test_contract_checker.py

@@ -127,3 +127,55 @@ def f(x: int) -> int:
         func_node, *_ = mod.nodes_of_class(nodes.FunctionDef)
         with self.assertNoMessages():
             self.checker.visit_functiondef(func_node)
+
+    def test_valid_multi_line_preconditions(self) -> None:
+        src = """
+        def f(data: list, threshold: int) -> int:
+            '''Return list length of a positive integer array
+
+            Preconditions:
+                - len(data) > 0 and \
+                  all(isinstance(x, int) for x in data) and \
+                  threshold >= 0
+                - data != []
+            '''
+            return len(data)
+        """
+
+        mod = parse(src)
+        func_node, *_ = mod.nodes_of_class(nodes.FunctionDef)
+        with self.assertNoMessages():
+            self.checker.visit_functiondef(func_node)
+
+    def test_invalid_multi_line_preconditions(self) -> None:
+        src = """
+        def f(data: list, threshold: int) -> int:
+            '''Return list length of a positive integer array
+
+            Preconditions:
+                - len(data) > 0 and \
+                  all(isinstance(x, int) for x in data) and \
+                  threshold > = 0
+                - data !== []
+            '''
+            return len(data)
+        """
+
+        mod = parse(src)
+        func_node, *_ = mod.nodes_of_class(nodes.FunctionDef)
+        with self.assertAddsMessages(
+            pylint.testutils.MessageTest(
+                msg_id="invalid-precondition-syntax",
+                node=func_node,
+                args=(
+                    "len(data) > 0 and all(isinstance(x, int) for x in data) and threshold > = 0",
+                ),
+            ),
+            pylint.testutils.MessageTest(
+                msg_id="invalid-precondition-syntax",
+                node=func_node,
+                args=("data !== []",),
+            ),
+            ignore_position=True,
+        ):
+            self.checker.visit_functiondef(func_node)