PERF: delay expensive imports (netcdf4, importlib.metadata, multiproc…

…essing, tarfile, tomlib, tomli_w)

yt/loaders.py

@@ -5,15 +5,11 @@
 import atexit
 import os
 import sys
-import tarfile
 import time
 import types
 import warnings
-from importlib.metadata import entry_points
-from multiprocessing import Pipe, Process
-from multiprocessing.connection import Connection
 from pathlib import Path
-from typing import Any, Dict, List, Optional, Tuple, Union, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union, cast
 from urllib.parse import urlsplit
 
 import numpy as np
@@ -44,6 +40,8 @@
 )
 from yt.utilities.on_demand_imports import _pooch as pooch, _ratarmount as ratarmount
 
+if TYPE_CHECKING:
+    from multiprocessing.connection import Connection
 # --- Loaders for known data formats ---
 
 
@@ -90,6 +88,8 @@ def load(
     yt.utilities.exceptions.YTAmbiguousDataType
         If the data format matches more than one class of similar specialization levels.
     """
+    from importlib.metadata import entry_points
+
     fn = os.path.expanduser(fn)
 
     if any(wildcard in fn for wildcard in "[]?!*"):
@@ -1503,6 +1503,7 @@ def load_sample(
     - Corresponding sample data live at https://yt-project.org/data
 
     """
+    import tarfile
 
     if fn is None:
         print(
@@ -1636,7 +1637,7 @@ def safe_extract(tar, path=".", members=None, *, numeric_owner=False):
 
 
 def _mount_helper(
-    archive: str, mountPoint: str, ratarmount_kwa: Dict, conn: Connection
+    archive: str, mountPoint: str, ratarmount_kwa: Dict, conn: "Connection"
 ):
     try:
         fuseOperationsObject = ratarmount.TarMount(
@@ -1697,6 +1698,8 @@ def load_archive(
     - This function requires ratarmount to be installed.
     - This function does not work on Windows system.
     """
+    import tarfile
+    from multiprocessing import Pipe, Process
 
     warnings.warn(
         "The 'load_archive' function is still experimental and may be unstable.",

yt/utilities/configure.py

@@ -4,16 +4,10 @@
 from pathlib import Path
 from typing import Callable, List
 
-import tomli_w
 from more_itertools import always_iterable
 
 from yt.utilities.configuration_tree import ConfigLeaf, ConfigNode
 
-if sys.version_info >= (3, 11):
-    import tomllib
-else:
-    import tomli as tomllib
-
 configuration_callbacks: List[Callable[["YTConfig"], None]] = []
 
 
@@ -89,6 +83,10 @@ def read(self, file_names):
             if not os.path.exists(fname):
                 continue
             metadata = {"source": f"file: {fname}"}
+            if sys.version_info >= (3, 11):
+                import tomllib
+            else:
+                import tomli as tomllib
             try:
                 with open(fname, "rb") as fh:
                     data = tomllib.load(fh)
@@ -104,6 +102,8 @@ def read(self, file_names):
         return file_names_read
 
     def write(self, file_handler):
+        import tomli_w
+
         value = self.config_root.as_dict()
         config_as_str = tomli_w.dumps(value)
 

yt/utilities/on_demand_imports.py

@@ -294,6 +294,8 @@ def __init__(self):
         # imported first, can get file lock errors on some systems (including travis-ci)
         # so we need to do this before initializing h5py_imports()!
         # similar to this issue https://github.com/pydata/xarray/issues/2560
+        if find_spec("h5py") is None or find_spec("netCDF4") is None:
+            return
         try:
             import netCDF4  # noqa F401
         except ImportError: