diff --git a/src/local_ops.py b/src/local_ops.py
index 327f608..a1f5874 100644
--- a/src/local_ops.py
+++ b/src/local_ops.py
@@ -513,7 +513,13 @@ def _read__binary(self, filename):
             assert type(content) is bytes
             return content
 
-    def readlines(self, filename, num_lines=0, binary=False, encoding=None):
+    def readlines(
+        self,
+        filename: str,
+        num_lines: int = 0,
+        binary: bool = False,
+        encoding: typing.Optional[str] = None,
+    ) -> typing.Union[typing.List[str], typing.List[bytes]]:
         """
         Read lines from a local file.
         If num_lines is greater than 0, only the last num_lines lines will be read.
diff --git a/src/os_ops.py b/src/os_ops.py
index 0b8c990..2f23648 100644
--- a/src/os_ops.py
+++ b/src/os_ops.py
@@ -120,7 +120,22 @@ def touch(self, filename):
     def read(self, filename, encoding, binary):
         raise NotImplementedError()
 
-    def readlines(self, filename):
+    def readlines(
+        self,
+        filename: str,
+        num_lines: int = 0,
+        binary: bool = False,
+        encoding: typing.Optional[str] = None,
+    ) -> typing.Union[typing.List[str], typing.List[bytes]]:
+        """
+        Read lines from a local file.
+        If num_lines is greater than 0, only the last num_lines lines will be read.
+        """
+        assert type(num_lines) is int
+        assert type(filename) is str
+        assert type(binary) is bool
+        assert encoding is None or type(encoding) is str
+        assert num_lines >= 0
         raise NotImplementedError()
 
     def read_binary(self, filename, offset):
diff --git a/src/remote_ops.py b/src/remote_ops.py
index 156b009..92f77a4 100644
--- a/src/remote_ops.py
+++ b/src/remote_ops.py
@@ -551,7 +551,13 @@ def _read__binary(self, filename):
         assert type(content) is bytes
         return content
 
-    def readlines(self, filename, num_lines=0, binary=False, encoding=None):
+    def readlines(
+        self,
+        filename: str,
+        num_lines: int = 0,
+        binary: bool = False,
+        encoding: typing.Optional[str] = None,
+    ) -> typing.Union[typing.List[str], typing.List[bytes]]:
         assert type(num_lines) is int
         assert type(filename) is str
         assert type(binary) is bool