Package: lazy-load
A minimalistic interface that allows lazy evaluation of expressions and function calls.Note: This small library is heavily based onpython-lazy-object-proxy.Why using ℒazy-ℒoad? Lazy loading in general may make some software implementations much more efficient.
Especially if it is not known if some data has to be loaded or not. Often the resulting code is less efficient,
because eager loading is used or the code is not elegant, because one has to program (somehow) lazy loading.Advantages of this library are that lazy-loading may be used quite elegant and effective.ExamplesIn a loop it might happen that a special condition appears once or even more often. If this is the case,
an expensive functionexpensive_functionis called and on the resulting object an operation has
to be done. If the expensive function had to called more than once, than the result object may be reused.Possible implementation:defexpensive_function():print("function evaluation")...returnresultobj=Noneforx,y,pinget_coordinates():iftest_for_something(x,y,p):ifobjisNone:obj=expensive_function()obj.do_something(x,y)Given this library, it might be done like this:fromlazy_loadimportlazydefexpensive_function():print("function evaluation")...returnresultobj=lazy(expensive_function)forx,y,pinget_coordinates():iftest_for_something(x,y,p):obj.do_something(x,y)There are similar situations outside of loops. The implementation withoutlazy-loadmight look like this:defexpensive_function():print("function evaluation")...returnresultobj=Nonedefget_obj():globalobjifobjisNone:obj=expensive_function()returnobjifcondition_a:get_obj().xyz()ifcondition_b:do_something()ifcondition_c:get_obj().abc()This code can be realized much easier withlazy-load. Not only is the code shorter, but it is also more readable:fromlazy_loadimportlazydefexpensive_function():print("function evaluation")...returnresultobj=lazy(expensive_function)ifcondition_a:obj.xyz()ifcondition_b:do_something()ifcondition_c:obj.abc()It might be the case that the expensive function is used more often and always a lazy evaluation is done.
In this case, a decorator might be used to indicate that all function calls to this function shall be lazily
evaluated. This makes it possible to normally use the function. The behaviour is still the same like in the first example:fromlazy_loadimportlazy_func@lazy_funcdefexpensive_function():print("function evaluation")...returnresultobj=expensive_function()forx,y,pinget_coordinates():iftest_for_something(x,y,p):obj.do_something(x,y)A lazy evaluation of functions / methods calls might be done with the@lazy_funcdecorator of with thelazy-call. This was already
shown, therefore the following examples show how to do a one-shot lazy evaluation of a function call:fromlazy_loadimportlazy,lzdefexpensive_func(x,y):print(f"function evaluation with arguments x={x}, y={y}")...returnresult# Possibility 1: Use `lazy` with a callableobj=lazy(lambda:expensive_func(a,b))# Possibility 2: If it doesn't matter if the argument expressions for the expensive-function are eager evaluated, the call may be simplified:obj=lazy(expensive_func,a,b)# Possibility 3: `lazy` has a short version / alias: `lz`obj=lz(expensive_func,a,b)Python allows it to pass functions around: This is often used for callbacks, but also for other use cases.
Assuming an expensive function is passed to an object which calls this function and stores the result of
the function call in an attribute. Later it might happen that this attribute is used. Depending on the
program flow it also might happen that this attribute is not used. With a lazily evaluated function the
expensive function call is only executed if the result is really used. The lazily evaluated version of
a function has the exact same signature as the original function.One might now like to have the possibility to on-the-fly convert a callable to a lazily evaluated callable.
This might be done in the following way:fromlazy_loadimportlazy_func,lfdefexpensive_func(x):print(f"function evaluation with argument x={x}")...returnresult# Possibility 1: Use `lazy_func`.my_obj.do_something(f=lazy_func(expensive_func))# Possibility 2: Use `lf` which is an alias of `lazy_func`my_obj.do_something(f=lf(expensive_func))# Possibility 3: Use the `ℒ`-"operator"my_obj.do_something(f=ℒ[expensive_func])Actually, I want to go deeper into theℒ`azy- or `ℒ-“operator”. This operator converts on-the-fly a function
to a lazily evaluated function. Another example:fromlazy_loadimportℒdeftest(name):print(f"hey{name}")returnTrueres=test("peter")# prints "hey peter"test_l=ℒ[test]res=test_l("hans")# prints nothingifres:print("res is True")# prints "hey hans\nres is True"It is also possible to convert multiple functions to lazily evaluated functions usingℒ:fromlazy_loadimportℒdeff1(x):print(f"f1{x}")returnTruedeff2(x):print(f"f1{x}")returnTruef1_l,f2_l,f3_l=ℒ[f1,f2,lambdax:x==1]# This is equal to:f1_l=ℒ[f1]f2_l=ℒ[f2]f3_l=ℒ[lambdax:x==1]Finally, one might like to decorate a class in a way that all its public methods which have a return
value are lazily evaluated. Public methods are all methods that have a name not starting with_.
Methods with a return value are identificated by the given return type hint which must not beNone.
This behaviour might be done with the@lazy_class-decorator (alias:lc):fromlazy_loadimportlazy_class@lazy_classclassMyClass:def__init__(self):# Method name starts with "_" => is not public; therefore it is eager evaluatedpassdefsetX(x)->None:# Method does not return a value => therefore it is eager evaluated...defdo():# Method does not hav a return value type hint =>  therefore it is eager evaluated...defcompute()->int:# Method will always be lazily evaluated...returnresultFinally, it is also possible to force the evaluation of a lazy loading object by usingforce_eval(aliasfe).
This function can safely to used to non-lazy loading objects: It is then just equal to the identity function.fromlazy_loadimportlazy,force_evaldeff1(x):print(f"f1{x}")returnTruelazy_obj=lazy(f1,1)# The following expression prints "f1 1" and returns "True"force_eval(lazy_obj)Theforce_evalfunction may also be applied to lazy-functions (which are created withlazy_func(x),@lazy_funcor withℒ). This restores the original non-lazy / eager function. For non-lazy functions this call has no effect:fromlazy_loadimportlazy_func,force_eval@lazy_funcdeff(x):print("hey")returnx**2# The following line prints nothingobj=f(2)f_eager=force_eval(f)# The following line prints "hey" and "obj" has immediatly the value "4"obj=f_eager(2)Installationpip install lazy-loadRequirementsPython 3.6 or Python 3.7.LicenceMITAuthorslazy_loadwas written byBenjamin Bruno Meier.