PEP 810: Updates from discussion feedback

Author: pablogsal

peps/pep-0810.rst

@@ -155,12 +155,15 @@ the brainstorming we have already completed in preparation for this proposal
 as reference.
 
 The choice to introduce a new ``lazy`` keyword reflects the need for explicit
-syntax. Import behavior is too fundamental to be left implicit or hidden
-behind global flags or environment variables. By marking laziness directly at
-the import site, the intent is immediately visible to both readers and tools.
-This avoids surprises, reduces the cognitive burden of reasoning about
-imports, and keeps lazy import semantics in line with Python's tradition of
-explicitness.
+syntax. Lazy imports have different semantics from normal imports: errors and
+side effects occur at first use rather than at the import statement. This
+semantic difference makes it critical that laziness is visible at the import
+site itself, not hidden in global configuration or distant module-level
+declarations. The ``lazy`` keyword provides local reasoning about import
+behavior, avoiding the need to search elsewhere in the code to understand
+whether an import is deferred. The rest of the import semantics remain
+unchanged: the same import machinery, module finding, and loading mechanisms
+are used.
 
 Another important decision is to represent lazy imports with proxy objects in
 the module's namespace, rather than by modifying dictionary lookup. Earlier
@@ -280,13 +283,19 @@ Examples of syntax errors:
 Semantics
 ---------
 
-When the ``lazy`` keyword is used, the import becomes *potentially lazy*.
-Unless lazy imports are disabled or suppressed (see below), the module is not
+When the ``lazy`` keyword is used, the import becomes *potentially lazy*
+(see `Lazy imports filter`_ for advanced override mechanisms). The module is not
 loaded immediately at the import statement; instead, a lazy proxy object is
 created and bound to the name. The actual module is loaded on first use of
 that name.
 
-Example:
+When using ``lazy from ... import``, **each imported name** is bound to a lazy
+proxy object. The first access to **any** of these names triggers loading of
+the entire module and reifies **only that specific name** to its actual value.
+Other names remain as lazy proxies until they are accessed. The interpreter's
+adaptive specialization will optimize away the lazy checks after a few accesses.
+
+Example with ``lazy import``:
 
 .. code-block:: python
 
@@ -301,6 +310,24 @@ Example:
 
   print('json' in sys.modules)  # True - now loaded
 
+Example with ``lazy from ... import``:
+
+.. code-block:: python
+
+  import sys
+
+  lazy from json import dumps, loads
+
+  print('json' in sys.modules)           # False - module not loaded yet
+
+  # First use of 'dumps' triggers loading json and reifies ONLY 'dumps'
+  result = dumps({"hello": "world"})
+
+  print('json' in sys.modules)           # True - module now loaded
+
+  # Accessing 'loads' now reifies it (json already loaded, no re-import)
+  data = loads(result)
+
 A module may contain a :data:`!__lazy_modules__` attribute, which is a
 sequence of fully qualified module names (strings) to make *potentially lazy*
 (as if the ``lazy`` keyword was used). This attribute is checked on each
@@ -327,13 +354,12 @@ import is ever imported lazily, and the behavior is equivalent to a regular
 import statement: the import is *eager* (as if the lazy keyword was not used).
 
 Finally, the application may use a custom filter function on all *potentially
-lazy* imports to determine if they should be lazy or not.
-If a filter function is set, it will be called with the name of the module
-doing the import, the name of the module being imported, and (if applicable)
-the fromlist.
-An import remains lazy only if the filter function returns ``True``.
-
-If no lazy import filter is set, all *potentially lazy* imports are lazy.
+lazy* imports to determine if they should be lazy or not (this is an advanced
+feature, see `Lazy imports filter`_). If a filter function is set, it will be
+called with the name of the module doing the import, the name of the module
+being imported, and (if applicable) the fromlist. An import remains lazy only
+if the filter function returns ``True``. If no lazy import filter is set, all
+*potentially lazy* imports are lazy.
 
 Lazy objects
 ------------
@@ -586,19 +612,25 @@ After several calls, ``LOAD_GLOBAL`` specializes to ``LOAD_GLOBAL_MODULE``:
 Lazy imports filter
 -------------------
 
-This PEP adds the following new functions to manage the lazy imports filter:
+*Note: This is an advanced feature. Library developers should NOT call these
+functions. These are intended for specialized/advanced users who need
+fine-grained control over lazy import behavior when using the global flags (see
+rejected ideas section for details).*
+
+This PEP adds the following new functions to the ``sys`` module to manage the
+lazy imports filter:
 
-* ``importlib.set_lazy_imports_filter(func)`` - Sets the filter function. If
+* ``sys.set_lazy_imports_filter(func)`` - Sets the filter function. If
   ``func=None`` then the import filter is removed. The ``func`` parameter must
   have the signature: ``func(importer: str, name: str, fromlist: tuple[str, ...] | None) -> bool``
 
-* ``importlib.get_lazy_imports_filter()`` - Returns the currently installed
+* ``sys.get_lazy_imports_filter()`` - Returns the currently installed
   filter function, or ``None`` if no filter is set.
 
-* ``importlib.set_lazy_imports(enabled=None, /)`` - Programmatic API for
-  controlling lazy imports at runtime. The ``enabled`` parameter can be
-  ``None`` (respect ``lazy`` keyword only), ``True`` (force all imports to be
-  potentially lazy), or ``False`` (force all imports to be eager).
+* ``sys.set_lazy_imports(mode, /)`` - Programmatic API for
+  controlling lazy imports at runtime. The ``mode`` parameter can be
+  ``"normal"`` (respect ``lazy`` keyword only), ``"all"`` (force all imports to be
+  potentially lazy), or ``"none"`` (force all imports to be eager).
 
 The filter function is called for every potentially lazy import, and must
 return ``True`` if the import should be lazy. This allows for fine-grained
@@ -646,7 +678,7 @@ Example:
       return True  # Allow lazy import
 
   # Install the filter
-  importlib.set_lazy_imports_filter(exclude_side_effect_modules)
+  sys.set_lazy_imports_filter(exclude_side_effect_modules)
 
   # These imports are checked by the filter
   lazy import data_processor        # Filter returns True -> stays lazy
@@ -662,11 +694,15 @@ Example:
 Global lazy imports control
 ----------------------------
 
+*Note: This is an advanced feature. Library developers should NOT use the global
+activation mechanism. This is intended for application developers and framework
+authors who need to control lazy imports across their entire application.*
+
 The global lazy imports flag can be controlled through:
 
 * The ``-X lazy_imports=<mode>`` command-line option
 * The ``PYTHON_LAZY_IMPORTS=<mode>`` environment variable
-* The ``importlib.set_lazy_imports(mode)`` function (primarily for testing)
+* The ``sys.set_lazy_imports(mode)`` function (primarily for testing)
 
 Where ``<mode>`` can be:
 
@@ -687,12 +723,10 @@ lazy* import is ever imported lazily, the import filter is never called, and
 the behavior is equivalent to a regular ``import`` statement: the import is
 *eager* (as if the lazy keyword was not used).
 
-Python code can run the :func:`!importlib.set_lazy_imports` function to override
+Python code can run the :func:`!sys.set_lazy_imports` function to override
 the state of the global lazy imports flag inherited from the environment or CLI.
 This is especially useful if an application needs to ensure that all imports
-are evaluated eagerly, via ``importlib.set_lazy_imports('none')``.
-Alternatively, :func:`!importlib.set_lazy_imports` can be used with boolean
-values for programmatic control.
+are evaluated eagerly, via ``sys.set_lazy_imports("none")``.
 
 
 Backwards Compatibility
@@ -790,7 +824,7 @@ The `pyperformance suite`_ confirms the implementation is performance-neutral.
 Filter function performance
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-The filter function (set via ``importlib.set_lazy_imports_filter()``) is called for
+The filter function (set via ``sys.set_lazy_imports_filter()``) is called for
 every *potentially lazy* import to determine whether it should actually be
 lazy. When no filter is set, this is simply a NULL check (testing whether a
 filter function has been registered), which is a highly predictable branch that
@@ -914,8 +948,8 @@ Security Implications
 
 There are no known security vulnerabilities introduced by lazy imports.
 Security-sensitive tools that need to ensure all imports are evaluated eagerly
-can use :func:`!importlib.set_lazy_imports` with ``enabled=False`` to force
-eager evaluation, or use :func:`!importlib.set_lazy_imports_filter` for fine-grained
+can use :func:`!sys.set_lazy_imports` with ``"none"`` to force
+eager evaluation, or use :func:`!sys.set_lazy_imports_filter` for fine-grained
 control.
 
 How to Teach This
@@ -1020,6 +1054,31 @@ global approach. The key differences are:
 - **Simpler implementation**: Uses proxy objects instead of modifying core
   dictionary behavior.
 
+What changes at reification time? What stays the same?
+------------------------------------------------------
+
+**What changes (the timing):**
+
+* **When** the module is imported - deferred to first use instead of at the
+  import statement
+* **When** import errors occur - at first use rather than at import time
+* **When** module-level side effects execute - at first use rather than at
+  import time
+
+**What stays the same (everything else):**
+
+* The import machinery used - same ``__import__``, same hooks, same loaders
+* The module object created - identical to an eagerly imported module
+* Import state consulted - ``sys.path``, ``sys.meta_path``, etc. at
+  **reification time** (not at import statement time)
+* Module attributes and behavior - completely indistinguishable after
+  reification
+* Thread safety - same import lock discipline as normal imports
+
+In other words: lazy imports only change **when** something happens, not
+**what** happens. After reification, a lazy-imported module is
+indistinguishable from an eagerly imported one.
+
 What happens when lazy imports encounter errors?
 ------------------------------------------------
 
@@ -1269,7 +1328,7 @@ exclude specific modules that are known to have problematic side effects:
           return False  # Import eagerly
       return True  # Allow lazy import
 
-  importlib.set_lazy_imports_filter(my_filter)
+  sys.set_lazy_imports_filter(my_filter)
 
 The filter function receives the importer module name, the module being
 imported, and the fromlist (if using ``from ... import``). Returning ``False``
@@ -1638,18 +1697,36 @@ Making ``lazy`` imports find the module without loading it
 
 The Python ``import`` machinery separates out finding a module and loading
 it, and the lazy import implementation could technically defer only the
-loading part. However:
-
-- Finding the module does not guarantee the import will succeed, nor even
-  that it will not raise ImportError.
-- Finding modules in packages requires that those packages are loaded, so
-  it would only help with lazy loading one level of a package hierarchy.
-- Since "finding" attributes in modules *requires* loading them, this would
-  create a hard to explain difference between
-  ``from package import module`` and ``from module import function``.
-- A significant part of the performance win is skipping the finding part
-  (which may involve filesystem searches and consulting multiple importers
-  and meta-importers).
+loading part. However, this approach was rejected for several critical reasons.
+
+A significant part of the performance win comes from skipping the finding phase.
+The issue is particularly acute on NFS-backed filesystems and distributed
+storage, where each ``stat()`` call incurs network latency. In these kinds of
+environments, ``stat()`` calls can take tens to hundreds of milliseconds
+depending on network conditions. With dozens of imports each doing multiple
+filesystem checks traversing ``sys.path``, the time spent finding modules
+before executing any Python code can become substantial. In some measurements,
+spec finding accounts for the majority of total import time. Skipping only the
+loading phase would leave most of the performance problem unsolved.
+
+More critically, separating finding from loading creates the worst of both
+worlds for error handling. Some exceptions from the import machinery (e.g.,
+``ImportError`` from a missing module, path resolution failures,
+``ModuleNotFoundError``) would be raised at the ``lazy import`` statement, while
+others (e.g., ``SyntaxError``, ``ImportError`` from circular imports, attribute
+errors from ``from module import name``) would be raised later at first use.
+This split is both confusing and unpredictable: developers would need to
+understand the internal import machinery to know which errors happen when. The
+current design is simpler: with full lazy imports, all import-related errors
+occur at first use, making the behavior consistent and predictable.
+
+Additionally, there are technical limitations: finding the module does not
+guarantee the import will succeed, nor even that it will not raise ImportError.
+Finding modules in packages requires that those packages are loaded, so it
+would only help with lazy loading one level of a package hierarchy. Since
+"finding" attributes in modules *requires* loading them, this would create a
+hard to explain difference between ``from package import module`` and
+``from module import function``.
 
 Placing the ``lazy`` keyword in the middle of from imports
 ----------------------------------------------------------
@@ -1706,6 +1783,23 @@ to add more specific re-enabling mechanisms later, when we have a clearer
 picture of real-world use and patterns, than it is to remove a hastily added
 mechanism that isn't quite right.
 
+Using underscore-prefixed names for advanced features
+------------------------------------------------------
+
+The global activation and filter functions (``sys.set_lazy_imports``,
+``sys.set_lazy_imports_filter``, ``sys.get_lazy_imports_filter``) could be
+marked as "private" or "advanced" by using underscore prefixes (e.g.,
+``sys._set_lazy_imports_filter``). This was rejected because branding as
+advanced features through documentation is sufficient. These functions have
+legitimate use cases for advanced users, particularly operators of large
+deployments. Providing an official mechanism prevents divergence from upstream
+CPython. The global mode is intentionally documented as an advanced feature for
+operators running huge fleets, not for day-to-day users or libraries. Python
+has precedent for advanced features that remain public APIs without underscore
+prefixes - for example, ``gc.disable()``, ``gc.get_objects()``, and
+``gc.set_threshold()`` are advanced features that can cause issues if misused,
+yet they are not underscore-prefixed.
+
 Using a decorator syntax for lazy imports
 ------------------------------------------