Lazy attributes to delay initialization
Project description
Lazy
lazyfields
has utilities to allow for lazily defined attributes so that you can avoid some patterns like the one below or avoid calculating a property twice.
class MyClass:
def __init__(self):
self._val = None
def get_val(self):
if val is None:
self._do_something()
return self._val
lazyfield
The lazyfields.lazyfield
descriptor streamlines the process of implementing lazy attributes. Here's how you can use it:
from lazyfields import lazyfield
class MyClass:
@lazyfield
def expensive_value(self):
return do_expensive_operation()
instance = MyClass()
instance.expensive_value # Does the expensive operation, saves the result and returns the value
instance.expensive_value # Returns directly the cached value
del instance.expensive_value # Cleans the cache
instance.expensive_value # redo the expensive operation
instance.expensive_value = "Other" # Overwrites the cached value with the value assigned
lazyfield
saves the value directly in the class as a hidden attribute so you don't have to worry about garbage collection or weakrefs
asynclazyfield
The lazyfields.asynclazyfield descriptor tackles the same issue as lazyfield, but while preserving the asynchronous API
from lazyfields import asynclazyfield
class MyClass:
@asynclazyfield
async def expensive_value(self):
return await do_expensive_operation()
instance = MyClass()
await instance.expensive_value() # you still call it as a function, but it will do the same thing, call the function, store the result and return the value
await instance.expensive_value() # now it only returns the value
dellazy(instance, "expensive_value") # clear the stored value
await instance.expensive_value() # here the calculation is done again
setlazy(instance, "expensive_value", "Other") # overwrite the stored value with "Other"
Helpers
lazyfields
provides helpers to work with frozen classes or when using the asynclazyfield and need to set or reset the field manually
setlazy(instance: Any, attribute: str, value: Any, bypass_setattr: bool = False)
setlazy allows you to directly change the hidden attribute behind any lazy field, and the bypass_setattr parameter will instead of using thesetattr
function, useobject.__setattr__
.dellazy(instance: Any, attribute: str, bypass_delattr: bool = False)
dellazy allows you to clear the value stored in the hidden attribute to allow for recalculation. Similar to setlazy the bypass_delattr parameter will useobject.__delattr__
instead ofdelattr
force_set(instance: Any, attribute: str, value: Any)
force_set is just a shortcut for setlazy(..., bypass_setattr=True)force_del(instance: Any, attribute: str)
force_del is just a shortcut for dellazy(..., bypass_delattr=True)is_initialized(instance: Any, attribute: str)
Returns whether the lazyfield has stored a value yet, without triggering the routine inadvertently.
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