diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 31062030c..1df293f3a 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -16,10 +16,10 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
-      - name: Set up Python ${{ matrix.python }}
+      - name: Set up Python ${{ matrix.python-version }}
         uses: actions/setup-python@v5
         with:
-          python-version: ${{ matrix.python }}
+          python-version: ${{ matrix.python-version }}
 
       - name: Install dependencies
         run: |
diff --git a/src/monty/bisect.py b/src/monty/bisect.py
index 6261d8c2b..5047a8870 100644
--- a/src/monty/bisect.py
+++ b/src/monty/bisect.py
@@ -10,10 +10,6 @@
 from __future__ import annotations
 
 import bisect as bs
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from typing import Optional
 
 __author__ = "Matteo Giantomassi"
 __copyright__ = "Copyright 2013, The Materials Virtual Lab"
@@ -23,7 +19,7 @@
 __date__ = "11/09/14"
 
 
-def index(a: list[float], x: float, atol: Optional[float] = None) -> int:
+def index(a: list[float], x: float, atol: float | None = None) -> int:
     """Locate the leftmost value exactly equal to x."""
     i = bs.bisect_left(a, x)
     if i != len(a):
diff --git a/src/monty/dev.py b/src/monty/dev.py
index cf468bce4..c7525864b 100644
--- a/src/monty/dev.py
+++ b/src/monty/dev.py
@@ -17,15 +17,15 @@
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from typing import Callable, Optional, Type
+    from typing import Callable, Type
 
 logger = logging.getLogger(__name__)
 
 
 def deprecated(
-    replacement: Optional[Callable | str] = None,
+    replacement: Callable | str | None = None,
     message: str = "",
-    deadline: Optional[tuple[int, int, int]] = None,
+    deadline: tuple[int, int, int] | None = None,
     category: Type[Warning] = FutureWarning,
 ) -> Callable:
     """
@@ -34,7 +34,7 @@ def deprecated(
     Args:
         replacement (Callable | str): A replacement class or function.
         message (str): A warning message to be displayed.
-        deadline (Optional[tuple[int, int, int]]): Optional deadline for removal
+        deadline (tuple[int, int, int] | None): Optional deadline for removal
             of the old function/class, in format (yyyy, MM, dd). A CI warning would
             be raised after this date if is running in code owner' repo.
         category (Warning): Choose the category of the warning to issue. Defaults
diff --git a/src/monty/functools.py b/src/monty/functools.py
index 969f0d506..13c188dac 100644
--- a/src/monty/functools.py
+++ b/src/monty/functools.py
@@ -13,7 +13,7 @@
 from typing import TYPE_CHECKING
 
 if TYPE_CHECKING:
-    from typing import Any, Callable, Union
+    from typing import Any, Callable
 
 
 class _HashedSeq(list):  # pylint: disable=C0205
@@ -130,7 +130,7 @@ def invalidate(cls, inst: object, name: str) -> None:
 
 
 def return_if_raise(
-    exception_tuple: Union[list, tuple], retval_if_exc: Any, disabled: bool = False
+    exception_tuple: list | tuple, retval_if_exc: Any, disabled: bool = False
 ) -> Any:
     """
     Decorator for functions, methods or properties. Execute the callable in a
diff --git a/src/monty/io.py b/src/monty/io.py
index ccd24189e..b22ea6231 100644
--- a/src/monty/io.py
+++ b/src/monty/io.py
@@ -19,14 +19,11 @@
 from typing import TYPE_CHECKING, Literal, cast
 
 if TYPE_CHECKING:
-    from typing import IO, Any, Iterator, Union
-
-
-class EncodingWarning(Warning): ...  # Added in Python 3.10
+    from typing import IO, Any, Iterator
 
 
 def zopen(
-    filename: Union[str, Path],
+    filename: str | Path,
     /,
     mode: str | None = None,
     **kwargs: Any,
@@ -35,7 +32,7 @@ def zopen(
     This function wraps around `[bz2/gzip/lzma].open` and `open`
     to deal intelligently with compressed or uncompressed files.
     Supports context manager:
-        `with zopen(filename, mode="rt", ...)`.
+        `with zopen(filename, mode="rt", ...)`
 
     Important Notes:
         - Default `mode` should not be used, and would not be allow
@@ -43,11 +40,12 @@ def zopen(
         - Always explicitly specify binary/text in `mode`, i.e.
             always pass `t` or `b` in `mode`, implicit binary/text
             mode would not be allow in future versions.
-        - Always provide an explicit `encoding` in text mode.
+        - Always provide an explicit `encoding` in text mode, it would
+            be set to UTF-8 by default otherwise.
 
     Args:
         filename (str | Path): The file to open.
-        mode (str): The mode in which the file is opened, you MUST
+        mode (str): The mode in which the file is opened, you should
             explicitly specify "b" for binary or "t" for text.
         **kwargs: Additional keyword arguments to pass to `open`.
 
@@ -79,14 +77,16 @@ def zopen(
             stacklevel=2,
         )
 
-    # Warn against default `encoding` in text mode
+    # Warn against default `encoding` in text mode if
+    # `PYTHONWARNDEFAULTENCODING` environment variable is set (PEP 597)
     if "t" in mode and kwargs.get("encoding", None) is None:
-        warnings.warn(
-            "We strongly encourage explicit `encoding`, "
-            "and we would use UTF-8 by default as per PEP 686",
-            category=EncodingWarning,
-            stacklevel=2,
-        )
+        if os.getenv("PYTHONWARNDEFAULTENCODING", False):
+            warnings.warn(
+                "We strongly encourage explicit `encoding`, "
+                "and we would use UTF-8 by default as per PEP 686",
+                category=EncodingWarning,
+                stacklevel=2,
+            )
         kwargs["encoding"] = "utf-8"
 
     _name, ext = os.path.splitext(filename)
@@ -141,7 +141,7 @@ def _get_line_ending(
         If file is empty, "\n" would be used as default.
     """
     if isinstance(file, (str, Path)):
-        with zopen(file, "rb") as f:
+        with zopen(file, mode="rb") as f:
             first_line = f.readline()
     elif isinstance(file, io.TextIOWrapper):
         first_line = file.buffer.readline()  # type: ignore[attr-defined]
@@ -169,7 +169,7 @@ def _get_line_ending(
 
 
 def reverse_readfile(
-    filename: Union[str, Path],
+    filename: str | Path,
 ) -> Iterator[str]:
     """
     A much faster reverse read of file by using Python's mmap to generate a
@@ -187,7 +187,7 @@ def reverse_readfile(
     l_end = _get_line_ending(filename)
     len_l_end = len(l_end)
 
-    with zopen(filename, "rb") as file:
+    with zopen(filename, mode="rb") as file:
         if isinstance(file, (gzip.GzipFile, bz2.BZ2File)):
             for line in reversed(file.readlines()):
                 # "readlines" would keep the line end character
diff --git a/src/monty/os/__init__.py b/src/monty/os/__init__.py
index 0158cec7a..24539971d 100644
--- a/src/monty/os/__init__.py
+++ b/src/monty/os/__init__.py
@@ -11,7 +11,7 @@
 
 if TYPE_CHECKING:
     from pathlib import Path
-    from typing import Generator, Union
+    from typing import Generator
 
 __author__ = "Shyue Ping Ong"
 __copyright__ = "Copyright 2013, The Materials Project"
@@ -22,7 +22,7 @@
 
 
 @contextmanager
-def cd(path: Union[str, Path]) -> Generator:
+def cd(path: str | Path) -> Generator:
     """
     A Fabric-inspired cd context that temporarily changes directory for
     performing some tasks, and returns to the original working directory
@@ -42,7 +42,7 @@ def cd(path: Union[str, Path]) -> Generator:
         os.chdir(cwd)
 
 
-def makedirs_p(path: Union[str, Path], **kwargs) -> None:
+def makedirs_p(path: str | Path, **kwargs) -> None:
     """
     Wrapper for os.makedirs that does not raise an exception if the directory
     already exists, in the fashion of "mkdir -p" command. The check is
diff --git a/src/monty/os/path.py b/src/monty/os/path.py
index fa293a90f..5b3c0cb4e 100644
--- a/src/monty/os/path.py
+++ b/src/monty/os/path.py
@@ -12,7 +12,7 @@
 from monty.string import list_strings
 
 if TYPE_CHECKING:
-    from typing import Callable, Literal, Optional, Union
+    from typing import Callable, Literal
 
 
 def zpath(filename: str | Path) -> str:
@@ -41,9 +41,9 @@ def zpath(filename: str | Path) -> str:
 
 def find_exts(
     top: str,
-    exts: Union[str, list[str]],
-    exclude_dirs: Optional[str] = None,
-    include_dirs: Optional[str] = None,
+    exts: str | list[str],
+    exclude_dirs: str | None = None,
+    include_dirs: str | None = None,
     match_mode: Literal["basename", "abspath"] = "basename",
 ) -> list[str]:
     """
diff --git a/src/monty/re.py b/src/monty/re.py
index 76152ff13..feeaed248 100644
--- a/src/monty/re.py
+++ b/src/monty/re.py
@@ -49,7 +49,7 @@ def regrep(
     gen = (
         reverse_readfile(filename)
         if reverse
-        else zopen(filename, "rt", encoding="utf-8")
+        else zopen(filename, mode="rt", encoding="utf-8")
     )
     for i, line in enumerate(gen):
         for k, p in compiled.items():
diff --git a/src/monty/serialization.py b/src/monty/serialization.py
index 13f3fb004..ff2a53de2 100644
--- a/src/monty/serialization.py
+++ b/src/monty/serialization.py
@@ -22,10 +22,15 @@
 
 if TYPE_CHECKING:
     from pathlib import Path
-    from typing import Any, Optional, TextIO, Union
+    from typing import Any, Literal, TextIO
 
 
-def loadfn(fn: Union[str, Path], *args, fmt: Optional[str] = None, **kwargs) -> Any:
+def loadfn(
+    fn: str | Path,
+    *args,
+    fmt: Literal["json", "yaml", "mpk"] | None = None,
+    **kwargs,
+) -> Any:
     """
     Loads json/yaml/msgpack directly from a filename instead of a
     File-like object. File may also be a BZ2 (".BZ2") or GZIP (".GZ", ".Z")
@@ -39,9 +44,8 @@ def loadfn(fn: Union[str, Path], *args, fmt: Optional[str] = None, **kwargs) ->
     Args:
         fn (str/Path): filename or pathlib.Path.
         *args: Any of the args supported by json/yaml.load.
-        fmt (string): If specified, the fmt specified would be used instead
-            of autodetection from filename. Supported formats right now are
-            "json", "yaml" or "mpk".
+        fmt ("json" | "yaml" | "mpk"): If specified, the fmt specified would
+            be used instead of autodetection from filename.
         **kwargs: Any of the kwargs supported by json/yaml.load.
 
     Returns:
@@ -64,10 +68,10 @@ def loadfn(fn: Union[str, Path], *args, fmt: Optional[str] = None, **kwargs) ->
             )
         if "object_hook" not in kwargs:
             kwargs["object_hook"] = object_hook
-        with zopen(fn, "rb") as fp:
+        with zopen(fn, mode="rb") as fp:
             return msgpack.load(fp, *args, **kwargs)  # pylint: disable=E1101
     else:
-        with zopen(fn, "rt", encoding="utf-8") as fp:
+        with zopen(fn, mode="rt", encoding="utf-8") as fp:
             if fmt == "yaml":
                 if YAML is None:
                     raise RuntimeError("Loading of YAML files requires ruamel.yaml.")
@@ -81,7 +85,13 @@ def loadfn(fn: Union[str, Path], *args, fmt: Optional[str] = None, **kwargs) ->
             raise TypeError(f"Invalid format: {fmt}")
 
 
-def dumpfn(obj: object, fn: Union[str, Path], *args, fmt=None, **kwargs) -> None:
+def dumpfn(
+    obj: object,
+    fn: str | Path,
+    *args,
+    fmt: Literal["json", "yaml", "mpk"] | None = None,
+    **kwargs,
+) -> None:
     """
     Dump to a json/yaml directly by filename instead of a
     File-like object. File may also be a BZ2 (".BZ2") or GZIP (".GZ", ".Z")
@@ -95,6 +105,8 @@ def dumpfn(obj: object, fn: Union[str, Path], *args, fmt=None, **kwargs) -> None
     Args:
         obj (object): Object to dump.
         fn (str/Path): filename or pathlib.Path.
+        fmt ("json" | "yaml" | "mpk"): If specified, the fmt specified would
+            be used instead of autodetection from filename.
         *args: Any of the args supported by json/yaml.dump.
         **kwargs: Any of the kwargs supported by json/yaml.dump.
 
@@ -117,10 +129,10 @@ def dumpfn(obj: object, fn: Union[str, Path], *args, fmt=None, **kwargs) -> None
             )
         if "default" not in kwargs:
             kwargs["default"] = default
-        with zopen(fn, "wb") as fp:
+        with zopen(fn, mode="wb") as fp:
             msgpack.dump(obj, fp, *args, **kwargs)  # pylint: disable=E1101
     else:
-        with zopen(fn, "wt", encoding="utf-8") as fp:
+        with zopen(fn, mode="wt", encoding="utf-8") as fp:
             fp = cast(TextIO, fp)
 
             if fmt == "yaml":
diff --git a/src/monty/shutil.py b/src/monty/shutil.py
index 98047f576..5dc1c8c07 100644
--- a/src/monty/shutil.py
+++ b/src/monty/shutil.py
@@ -12,7 +12,7 @@
 from monty.io import zopen
 
 if TYPE_CHECKING:
-    from typing import Literal, Optional
+    from typing import Literal
 
 
 def copy_r(src: str | Path, dst: str | Path) -> None:
@@ -76,7 +76,7 @@ def gzip_dir(path: str | Path, compresslevel: int = 6) -> None:
 def compress_file(
     filepath: str | Path,
     compression: Literal["gz", "bz2"] = "gz",
-    target_dir: Optional[str | Path] = None,
+    target_dir: str | Path | None = None,
 ) -> None:
     """
     Compresses a file with the correct extension. Functions like standard
@@ -104,7 +104,7 @@ def compress_file(
         else:
             compressed_file = f"{str(filepath)}.{compression}"
 
-        with open(filepath, "rb") as f_in, zopen(compressed_file, "wb") as f_out:
+        with open(filepath, "rb") as f_in, zopen(compressed_file, mode="wb") as f_out:
             f_out.writelines(f_in)
 
         os.remove(filepath)
@@ -130,7 +130,7 @@ def compress_dir(path: str | Path, compression: Literal["gz", "bz2"] = "gz") ->
 
 
 def decompress_file(
-    filepath: str | Path, target_dir: Optional[str | Path] = None
+    filepath: str | Path, target_dir: str | Path | None = None
 ) -> str | None:
     """
     Decompresses a file with the correct extension. Automatically detects
@@ -157,7 +157,7 @@ def decompress_file(
         else:
             decompressed_file = str(filepath).removesuffix(file_ext)
 
-        with zopen(filepath, "rb") as f_in, open(decompressed_file, "wb") as f_out:
+        with zopen(filepath, mode="rb") as f_in, open(decompressed_file, "wb") as f_out:
             f_out.writelines(f_in)
 
         os.remove(filepath)
diff --git a/src/monty/string.py b/src/monty/string.py
index 1a56debca..72a6b42e0 100644
--- a/src/monty/string.py
+++ b/src/monty/string.py
@@ -7,7 +7,7 @@
 from typing import TYPE_CHECKING, Iterable, cast
 
 if TYPE_CHECKING:
-    from typing import Any, Union
+    from typing import Any
 
 
 def remove_non_ascii(s: str) -> str:
@@ -34,7 +34,7 @@ def is_string(s: Any) -> bool:
         return False
 
 
-def list_strings(arg: Union[str, Iterable[str]]) -> list[str]:
+def list_strings(arg: str | Iterable[str]) -> list[str]:
     """
     Always return a list of strings, given a string or list of strings as
     input.
diff --git a/src/monty/subprocess.py b/src/monty/subprocess.py
index 104261c2e..e17f95c55 100644
--- a/src/monty/subprocess.py
+++ b/src/monty/subprocess.py
@@ -13,8 +13,6 @@
 from monty.string import is_string
 
 if TYPE_CHECKING:
-    from typing import Optional
-
     from typing_extensions import Self
 
 __author__ = "Matteo Giantomass"
@@ -63,7 +61,7 @@ def __init__(self, command: str):
     def __str__(self):
         return f"command: {self.command}, retcode: {self.retcode}"
 
-    def run(self, timeout: Optional[float] = None, **kwargs) -> Self:
+    def run(self, timeout: float | None = None, **kwargs) -> Self:
         """
         Run a command in a separated thread and wait timeout seconds.
         kwargs are keyword arguments passed to Popen.
diff --git a/src/monty/tempfile.py b/src/monty/tempfile.py
index 979dc5126..f5ac9556e 100644
--- a/src/monty/tempfile.py
+++ b/src/monty/tempfile.py
@@ -11,9 +11,6 @@
 
 from monty.shutil import copy_r, gzip_dir, remove
 
-if TYPE_CHECKING:
-    from typing import Union
-
 
 class ScratchDir:
     """
@@ -42,7 +39,7 @@ class ScratchDir:
 
     def __init__(
         self,
-        rootpath: Union[str, Path, None],
+        rootpath: str | Path | None,
         create_symbolic_link: bool = False,
         copy_from_current_on_enter: bool = False,
         copy_to_current_on_exit: bool = False,
diff --git a/tests/test_io.py b/tests/test_io.py
index f4422bf2a..5617e847a 100644
--- a/tests/test_io.py
+++ b/tests/test_io.py
@@ -9,7 +9,6 @@
 import pytest
 
 from monty.io import (
-    EncodingWarning,
     FileLock,
     FileLockException,
     _get_line_ending,
@@ -427,16 +426,19 @@ def test_lzw_files(self):
         # Cannot decompress a real LZW file
         with (
             pytest.raises(gzip.BadGzipFile, match="Not a gzipped file"),
+            pytest.warns(FutureWarning, match="compress LZW-compressed files"),
             zopen(f"{TEST_DIR}/real_lzw_file.txt.Z", "rt", encoding="utf-8") as f,
         ):
             f.read()
 
     @pytest.mark.parametrize("extension", [".txt", ".bz2", ".gz", ".xz", ".lzma"])
-    def test_warnings(self, extension):
+    def test_warnings(self, extension, monkeypatch):
         filename = f"test_warning{extension}"
         content = "Test warning"
 
         with ScratchDir("."):
+            monkeypatch.setenv("PYTHONWARNDEFAULTENCODING", "1")
+
             # Default `encoding` warning
             with (
                 pytest.warns(EncodingWarning, match="use UTF-8 by default"),
@@ -444,6 +446,19 @@ def test_warnings(self, extension):
             ):
                 f.write(content)
 
+            # No encoding warning if `PYTHONWARNDEFAULTENCODING` not set
+            monkeypatch.delenv("PYTHONWARNDEFAULTENCODING", raising=False)
+
+            with warnings.catch_warnings():
+                warnings.filterwarnings(
+                    "error",
+                    "We strongly encourage explicit `encoding`",
+                    EncodingWarning,
+                )
+
+                with zopen(filename, "wt") as f:
+                    f.write(content)
+
             # Implicit text/binary `mode` warning
             warnings.filterwarnings(
                 "ignore", category=EncodingWarning, message="argument not specified"