Infinite loop for unmatched parenthesis

isort/core.py

@@ -7,7 +7,7 @@
 from isort.settings import DEFAULT_CONFIG, Config
 
 from . import output, parse
-from .exceptions import FileSkipComment
+from .exceptions import ExistingSyntaxErrors, FileSkipComment
 from .format import format_natural, remove_whitespace
 from .settings import FILE_SKIP_COMMENTS
 
@@ -303,6 +303,10 @@ def process(
                         else:
                             while ")" not in stripped_line:
                                 line = input_stream.readline()
+
+                                if not line:  # end of file without closing parenthesis
+                                    raise ExistingSyntaxErrors("Parenthesis is not closed")
+
                                 stripped_line = line.strip().split("#")[0]
                                 import_statement += line
 

tests/unit/test_isort.py

@@ -5562,3 +5562,21 @@ def seekable(self):
     test_input = NonSeekableTestStream("import m2\n" "import m1\n" "not_import = 7")
     identified_imports = list(map(str, api.find_imports_in_stream(test_input)))
     assert identified_imports == [":1 import m2", ":2 import m1"]
+
+
+def test_infinite_loop_in_unmatched_parenthesis() -> None:
+    test_input = "from os import ("
+
+    # ensure a syntax error is raised for unmatched parenthesis
+    with pytest.raises(ExistingSyntaxErrors):
+        isort.code(test_input)
+
+    test_input = """from os import (
+    path,
+
+    walk
+)
+"""
+
+    # ensure other cases are handled correctly
+    assert isort.code(test_input) == "from os import path, walk\n"