diff --git a/flake8_annotations/checker.py b/flake8_annotations/checker.py
index 594495d..54c1777 100644
--- a/flake8_annotations/checker.py
+++ b/flake8_annotations/checker.py
@@ -1,9 +1,14 @@
 from functools import lru_cache
-from pathlib import Path
-from typing import Generator, List, Tuple
+from typing import Generator, List
 
 from flake8_annotations import (
-    Argument, Function, FunctionVisitor, PY_GTE_38, __version__, enums, error_codes
+    Argument,
+    Function,
+    FunctionVisitor,
+    PY_GTE_38,
+    __version__,
+    enums,
+    error_codes,
 )
 
 # Check if we can use the stdlib ast module instead of typed_ast
@@ -20,11 +25,10 @@ class TypeHintChecker:
     name = "flake8-annotations"
     version = __version__
 
-    def __init__(self, tree: ast.Module, filename: str):
-        # Unfortunately no way that I can find around requesting the ast-parsed tree from flake8
-        # Removing tree unregisters the plugin, and per the documentation the alternative is
-        # requesting per-line information
-        self.tree, self.lines = self.load_file(Path(filename))
+    def __init__(self, lines: List[str]):
+        # Request `lines` here and join to allow for correct handling of input from stdin
+        self.lines = lines
+        self.tree = self.get_typed_tree("".join(lines))  # flake8 doesn't strip newlines
 
     def run(self) -> Generator[error_codes.Error, None, None]:
         """
@@ -68,11 +72,8 @@ def run(self) -> Generator[error_codes.Error, None, None]:
                 yield classify_error(function, arg).to_flake8()
 
     @staticmethod
-    def load_file(src_filepath: Path) -> Tuple[ast.Module, List[str]]:
-        """Parse the provided Python file and return an (typed AST, source) tuple."""
-        with src_filepath.open("r", encoding="utf-8") as f:
-            src = f.read()
-
+    def get_typed_tree(src: str) -> ast.Module:
+        """Parse the provided source into a typed AST."""
         if PY_GTE_38:
             # Built-in ast requires a flag to parse type comments
             tree = ast.parse(src, type_comments=True)
@@ -80,9 +81,7 @@ def load_file(src_filepath: Path) -> Tuple[ast.Module, List[str]]:
             # typed-ast will implicitly parse type comments
             tree = ast.parse(src)
 
-        lines = src.splitlines()
-
-        return tree, lines
+        return tree
 
 
 def classify_error(function: Function, arg: Argument) -> error_codes.Error:
diff --git a/testing/helpers.py b/testing/helpers.py
index f3f7a71..56f5455 100644
--- a/testing/helpers.py
+++ b/testing/helpers.py
@@ -19,17 +19,15 @@ def parse_source(src: str) -> Tuple[ast.Module, List[str]]:
         # typed-ast will implicitly parse type comments
         tree = ast.parse(src)
 
-    lines = src.splitlines()
+    lines = src.splitlines(keepends=True)
 
     return tree, lines
 
 
 def check_source(src: str) -> Generator[Error, None, None]:
     """Helper for generating linting errors from the provided source code."""
-    # Because TypeHintChecker is expecting a filename to initialize, rather than change this logic
-    # we can use this file as a dummy, then update its tree & lines attributes as parsed from source
-    checker_instance = TypeHintChecker(None, __file__)
-    checker_instance.tree, checker_instance.lines = parse_source(src)
+    _, lines = parse_source(src)
+    checker_instance = TypeHintChecker(lines)
 
     return checker_instance.run()
 
diff --git a/testing/test_cases/variable_formatting_test_cases.py b/testing/test_cases/variable_formatting_test_cases.py
index cb7b693..eb10ad0 100644
--- a/testing/test_cases/variable_formatting_test_cases.py
+++ b/testing/test_cases/variable_formatting_test_cases.py
@@ -20,9 +20,9 @@ def foo(some_arg, *some_args, **some_kwargs) -> int:
     "protected_function": FormatTestCase(
         src=dedent(
             """\
-        def _foo(some_arg, *some_args, **some_kwargs) -> int:
-            pass
-        """
+            def _foo(some_arg, *some_args, **some_kwargs) -> int:
+                pass
+            """
         ),
     ),
     "private_function": FormatTestCase(