# encoding: utf-8 """Utility functions and classes.""" from __future__ import division import functools class Length(int): """ Base class for length classes Inches, Emu, Cm, Mm, Pt, and Px. 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): return int.__new__(cls, emu) @property def inches(self): """ Floating point length in inches """ return self / float(self._EMUS_PER_INCH) @property def centipoints(self): """ 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): """ Floating point length in centimeters """ return self / float(self._EMUS_PER_CM) @property def emu(self): """ Integer length in English Metric Units """ return self @property def mm(self): """ Floating point length in millimeters """ return self / float(self._EMUS_PER_MM) @property def pt(self): """ 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): 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): 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): 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): return Length.__new__(cls, int(emu)) class Mm(Length): """ Convenience constructor for length in millimeters """ def __new__(cls, mm): 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): emu = int(points * Length._EMUS_PER_PT) return Length.__new__(cls, emu) class lazyproperty(object): """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): """*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 # ---adopt fget's __name__, __doc__, and other attributes functools.update_wrapper(self, fget) def __get__(self, obj, type=None): """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 # ---when accessed on instance, start by checking instance __dict__ value = obj.__dict__.get(self.__name__) if value is None: # ---on first access, __dict__ item will 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 value def __set__(self, obj, value): """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") # pragma: no cover