LazyImporter class to manage imports on attribute access.
Project description
ducktools: lazyimporter
Create an object to handle lazily importing from other modules.
Nearly every form of "lazyimporter" module name is taken on PyPI so this is namespaced.
Intended to help save on start time where some modules are only needed for specific functions while allowing information showing the import information to appear at the top of a module where expected.
This form of import works by creating a specific LazyImporter object that lazily imports modules or module attributes when the module or attribute is accessed on the object.
How to download
Download from PyPI:
python -m pip install ducktools-lazyimporter
Example
Example using the packaging module.
__version__ = "v0.1.5"
from ducktools.lazyimporter import LazyImporter, FromImport
laz = LazyImporter([
FromImport("packaging.version", "Version")
])
def is_newer_version(version_no: str) -> bool:
"""Check if a version number given indicates
a newer version than this package."""
this_ver = laz.Version(__version__)
new_ver = laz.Version(version_no)
return new_ver > this_ver
# Import will only occur when the function is called and
# laz.Version is accessed
print(is_newer_version("v0.2.0"))
Hasn't this already been done
Yes.
But...
Most implementations rely on stdlib modules that are themselves slow to import
(for example: typing, importlib.util, logging, inspect, ast).
By contrast lazyimporter
only uses modules that python imports on launch
as part of site
.
lazyimporter
does not attempt to propagate laziness, only the modules provided
to lazyimporter
directly will be imported lazily. Any subdependencies of those
modules will be imported eagerly as if the import statement is placed where the
importer attribute is first accessed.
Use Case
There are two main use cases this is designed for.
Replacing in-line imports used in a module
Sometimes it is useful to use tools from a module that has a significant import time. If this is part of a function/method that won't necessarily always be used it is common to delay the import and place it inside the function/method.
Regular import within function:
def get_copy(obj):
from copy import deepcopy
return deepcopy(obj)
With a LazyImporter:
from ducktools.lazyimporter import LazyImporter, FromImport
laz = LazyImporter([FromImport("copy", "deepcopy")])
def get_copy(obj):
return laz.deepcopy(obj)
While the LazyImporter is more verbose, it only invokes the import mechanism once when first accessed, while placing the import within the function invokes it every time the function is called. This can be a significant overhead if the function ends up used in a loop.
This also means that if the attribute is accessed anywhere it will be imported and in place wherever it is used.
Delaying the import of parts of a module's public API
Eager import:
from .submodule import useful_tool
__all__ = [..., "useful_tool"]
Lazy import:
from ducktools.lazyimporter import LazyImporter, FromImport, get_module_funcs
__all__ = [..., "useful_tool"]
laz = LazyImporter(
[FromImport(".submodule", "useful_tool")],
globs=globals(), # If relative imports are used, globals() must be provided.
)
__getattr__, __dir__ = get_module_funcs(laz, __name__)
The import classes
In all of these instances modules
is intended as the first argument
to LazyImporter
and all attributes would be accessed from the
LazyImporter
instance and not in the global namespace.
eg:
from ducktools.lazyimporter import LazyImporter, ModuleImport
modules = [ModuleImport("functools")]
laz = LazyImporter(modules)
laz.functools # provides access to the module "functools"
ModuleImport
ModuleImport
is used for your basic module style imports.
from ducktools.lazyimporter import ModuleImport
modules = [
ModuleImport("module"),
ModuleImport("other_module", "other_name"),
ModuleImport("base_module.submodule", asname="short_name"),
]
is equivalent to
import module
import other_module as other_name
import base_module.submodule as short_name
when provided to a LazyImporter.
FromImport and MultiFromImport
FromImport
is used for standard 'from' imports, MultiFromImport
for importing
multiple items from the same module. By using a MultiFromImport
, when the first
attribute is accessed, all will be assigned on the LazyImporter.
from ducktools.lazyimporter import FromImport, MultiFromImport
modules = [
FromImport("dataclasses", "dataclass"),
FromImport("functools", "partial", "partfunc"),
MultiFromImport("collections", ["namedtuple", ("defaultdict", "dd")]),
]
is equivalent to
from dataclasses import dataclass
from functools import partial as partfunc
from collections import namedtuple, defaultdict as dd
when provided to a LazyImporter.
TryExceptImport
TryExceptImport
is used for compatibility where a module may not be available
and so a fallback module providing the same functionality should be used. For
example when a newer version of python has a stdlib module that has replaced
a third party module that was used previously.
from ducktools.lazyimporter import TryExceptImport
modules = [
TryExceptImport("tomllib", "tomli", "tomllib"),
]
is equivalent to
try:
import tomllib as tomllib
except ImportError:
import tomli as tomllib
when provided to a LazyImporter.
How does it work
The following lazy importer:
from ducktools.lazyimporter import LazyImporter, FromImport
laz = LazyImporter([FromImport("functools", "partial")])
Generates an object that's roughly equivalent to this:
class SpecificLazyImporter:
def __getattr__(self, name):
if name == "partial":
from functools import partial
setattr(self, name, partial)
return partial
raise AttributeError(...)
laz = SpecificLazyImporter()
The first time the attribute is accessed the import is done and the output is stored on the instance, so repeated access immediately gets the desired object and the import mechanism is only invoked once.
(The actual __getattr__
function uses a dictionary lookup and delegates importing
to the FromImport class. Names are all dynamic and imports are done through
the __import__
function.)
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