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"""Utility functions and classes."""
from __future__ import annotations
import functools
from typing import Any, Callable, Generic, TypeVar, cast
class Length(int):
"""Base class for length classes Inches, Emu, Cm, Mm, and Pt.
Provides properties for converting length values to convenient units.
"""
_EMUS_PER_INCH = 914400
_EMUS_PER_CENTIPOINT = 127
_EMUS_PER_CM = 360000
_EMUS_PER_MM = 36000
_EMUS_PER_PT = 12700
def __new__(cls, emu: int):
return int.__new__(cls, emu)
@property
def inches(self) -> float:
"""Floating point length in inches."""
return self / float(self._EMUS_PER_INCH)
@property
def centipoints(self) -> int:
"""Integer length in hundredths of a point (1/7200 inch).
Used internally because PowerPoint stores font size in centipoints.
"""
return self // self._EMUS_PER_CENTIPOINT
@property
def cm(self) -> float:
"""Floating point length in centimeters."""
return self / float(self._EMUS_PER_CM)
@property
def emu(self) -> int:
"""Integer length in English Metric Units."""
return self
@property
def mm(self) -> float:
"""Floating point length in millimeters."""
return self / float(self._EMUS_PER_MM)
@property
def pt(self) -> float:
"""Floating point length in points."""
return self / float(self._EMUS_PER_PT)
class Inches(Length):
"""Convenience constructor for length in inches."""
def __new__(cls, inches: float):
emu = int(inches * Length._EMUS_PER_INCH)
return Length.__new__(cls, emu)
class Centipoints(Length):
"""Convenience constructor for length in hundredths of a point."""
def __new__(cls, centipoints: int):
emu = int(centipoints * Length._EMUS_PER_CENTIPOINT)
return Length.__new__(cls, emu)
class Cm(Length):
"""Convenience constructor for length in centimeters."""
def __new__(cls, cm: float):
emu = int(cm * Length._EMUS_PER_CM)
return Length.__new__(cls, emu)
class Emu(Length):
"""Convenience constructor for length in english metric units."""
def __new__(cls, emu: int):
return Length.__new__(cls, int(emu))
class Mm(Length):
"""Convenience constructor for length in millimeters."""
def __new__(cls, mm: float):
emu = int(mm * Length._EMUS_PER_MM)
return Length.__new__(cls, emu)
class Pt(Length):
"""Convenience value class for specifying a length in points."""
def __new__(cls, points: float):
emu = int(points * Length._EMUS_PER_PT)
return Length.__new__(cls, emu)
_T = TypeVar("_T")
class lazyproperty(Generic[_T]):
"""Decorator like @property, but evaluated only on first access.
Like @property, this can only be used to decorate methods having only a `self` parameter, and
is accessed like an attribute on an instance, i.e. trailing parentheses are not used. Unlike
@property, the decorated method is only evaluated on first access; the resulting value is
cached and that same value returned on second and later access without re-evaluation of the
method.
Like @property, this class produces a *data descriptor* object, which is stored in the __dict__
of the *class* under the name of the decorated method ('fget' nominally). The cached value is
stored in the __dict__ of the *instance* under that same name.
Because it is a data descriptor (as opposed to a *non-data descriptor*), its `__get__()` method
is executed on each access of the decorated attribute; the __dict__ item of the same name is
"shadowed" by the descriptor.
While this may represent a performance improvement over a property, its greater benefit may be
its other characteristics. One common use is to construct collaborator objects, removing that
"real work" from the constructor, while still only executing once. It also de-couples client
code from any sequencing considerations; if it's accessed from more than one location, it's
assured it will be ready whenever needed.
Loosely based on: https://stackoverflow.com/a/6849299/1902513.
A lazyproperty is read-only. There is no counterpart to the optional "setter" (or deleter)
behavior of an @property. This is critically important to maintaining its immutability and
idempotence guarantees. Attempting to assign to a lazyproperty raises AttributeError
unconditionally.
The parameter names in the methods below correspond to this usage example::
class Obj(object)
@lazyproperty
def fget(self):
return 'some result'
obj = Obj()
Not suitable for wrapping a function (as opposed to a method) because it is not callable.
"""
def __init__(self, fget: Callable[..., _T]) -> None:
"""*fget* is the decorated method (a "getter" function).
A lazyproperty is read-only, so there is only an *fget* function (a regular
@property can also have an fset and fdel function). This name was chosen for
consistency with Python's `property` class which uses this name for the
corresponding parameter.
"""
# --- maintain a reference to the wrapped getter method
self._fget = fget
# --- and store the name of that decorated method
self._name = fget.__name__
# --- adopt fget's __name__, __doc__, and other attributes
functools.update_wrapper(self, fget) # pyright: ignore
def __get__(self, obj: Any, type: Any = None) -> _T:
"""Called on each access of 'fget' attribute on class or instance.
*self* is this instance of a lazyproperty descriptor "wrapping" the property
method it decorates (`fget`, nominally).
*obj* is the "host" object instance when the attribute is accessed from an
object instance, e.g. `obj = Obj(); obj.fget`. *obj* is None when accessed on
the class, e.g. `Obj.fget`.
*type* is the class hosting the decorated getter method (`fget`) on both class
and instance attribute access.
"""
# --- when accessed on class, e.g. Obj.fget, just return this descriptor
# --- instance (patched above to look like fget).
if obj is None:
return self # type: ignore
# --- when accessed on instance, start by checking instance __dict__ for
# --- item with key matching the wrapped function's name
value = obj.__dict__.get(self._name)
if value is None:
# --- on first access, the __dict__ item will be absent. Evaluate fget()
# --- and store that value in the (otherwise unused) host-object
# --- __dict__ value of same name ('fget' nominally)
value = self._fget(obj)
obj.__dict__[self._name] = value
return cast(_T, value)
def __set__(self, obj: Any, value: Any) -> None:
"""Raises unconditionally, to preserve read-only behavior.
This decorator is intended to implement immutable (and idempotent) object
attributes. For that reason, assignment to this property must be explicitly
prevented.
If this __set__ method was not present, this descriptor would become a
*non-data descriptor*. That would be nice because the cached value would be
accessed directly once set (__dict__ attrs have precedence over non-data
descriptors on instance attribute lookup). The problem is, there would be
nothing to stop assignment to the cached value, which would overwrite the result
of `fget()` and break both the immutability and idempotence guarantees of this
decorator.
The performance with this __set__() method in place was roughly 0.4 usec per
access when measured on a 2.8GHz development machine; so quite snappy and
probably not a rich target for optimization efforts.
"""
raise AttributeError("can't set attribute")
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