Warn and discourage lazy.load of subpackages (#57)

* Add warning and change docs to discourage subpackages with load()

* Update test to use builtin lib stable over python versions

* Minor edits

* Also update README

---------

Co-authored-by: Stefan van der Walt <sjvdwalt@gmail.com>
Co-authored-by: Stefan van der Walt <stefanv@berkeley.edu>

Author: dschult

README.md

@@ -104,10 +104,16 @@ internal imports.
 Use `lazy.load` to lazily import external libraries:
 
 ```python
-linalg = lazy.load('scipy.linalg')  # `linalg` will only be loaded when accessed
+sp = lazy.load('scipy')  # `sp` will only be loaded when accessed
+sp.linalg.norm(...)
 ```
 
-You can also ask `lazy.load` to raise import errors as soon as it is called:
+_Note that lazily importing *sub*packages,
+i.e. `load('scipy.linalg')` will cause the package containing the
+subpackage to be imported immediately; thus, this usage is
+discouraged._
+
+You can ask `lazy.load` to raise import errors as soon as it is called:
 
 ```
 linalg = lazy.load('scipy.linalg', error_on_import=True)

lazy_loader/__init__.py

@@ -11,6 +11,7 @@
 import os
 import sys
 import types
+import warnings
 
 __all__ = ["attach", "load", "attach_stub"]
 
@@ -121,34 +122,44 @@ def load(fullname, error_on_import=False):
     We often see the following pattern::
 
       def myfunc():
-          from numpy import linalg as la
-          la.norm(...)
+          import numpy as np
+          np.norm(...)
           ....
 
-    This is to prevent a module, in this case `numpy`, from being
-    imported at function definition time, since that can be slow.
+    Putting the import inside the function prevents, in this case,
+    `numpy`, from being imported at function definition time.
+    That saves time if `myfunc` ends up not being called.
 
-    This function provides a proxy module that, upon access, imports
+    This `load` function returns a proxy module that, upon access, imports
     the actual module.  So the idiom equivalent to the above example is::
 
-      la = lazy.load("numpy.linalg")
+      np = lazy.load("numpy")
 
       def myfunc():
-          la.norm(...)
+          np.norm(...)
           ....
 
     The initial import time is fast because the actual import is delayed
     until the first attribute is requested. The overall import time may
     decrease as well for users that don't make use of large portions
-    of the library.
+    of your library.
+
+    Warning
+    -------
+    While lazily loading *sub*packages technically works, it causes the
+    package (that contains the subpackage) to be eagerly loaded even
+    if the package is already lazily loaded.
+    So, you probably shouldn't use subpackages with this `load` feature.
+    Instead you should encourage the package maintainers to use the
+    `lazy_loader.attach` to make their subpackages load lazily.
 
     Parameters
     ----------
     fullname : str
         The full name of the module or submodule to import.  For example::
 
           sp = lazy.load('scipy')  # import scipy as sp
-          spla = lazy.load('scipy.linalg')  # import scipy.linalg as spla
+
     error_on_import : bool
         Whether to postpone raising import errors until the module is accessed.
         If set to `True`, import errors are raised as soon as `load` is called.
@@ -165,6 +176,14 @@ def myfunc():
     except KeyError:
         pass
 
+    if "." in fullname:
+        msg = (
+            "subpackages can technically be lazily loaded, but it causes the "
+            "package to be eagerly loaded even if it is already lazily loaded."
+            "So, you probably shouldn't use subpackages with this lazy feature."
+        )
+        warnings.warn(msg, RuntimeWarning)
+
     spec = importlib.util.find_spec(fullname)
     if spec is None:
         if error_on_import:

lazy_loader/tests/test_lazy_loader.py

@@ -1,3 +1,4 @@
+import importlib
 import sys
 import types
 
@@ -27,6 +28,16 @@ def test_lazy_import_basics():
         pass
 
 
+def test_lazy_import_subpackages():
+    with pytest.warns(RuntimeWarning):
+        hp = lazy.load("html.parser")
+    assert "html" in sys.modules
+    assert type(sys.modules["html"]) == type(pytest)
+    assert isinstance(hp, importlib.util._LazyModule)
+    assert "html.parser" in sys.modules
+    assert sys.modules["html.parser"] == hp
+
+
 def test_lazy_import_impact_on_sys_modules():
     math = lazy.load("math")
     anything_not_real = lazy.load("anything_not_real")