In Python's pathlib module you can create a Path object from another Path and they will be equivalent:
p = Path('some/path')
p2 = Path(p)
p == p2
# True
I have a File class that I would like to implement this same behavior, where if I pass a File object to the File constructor, it will simply return the original object that was passed in. What is the correct way to do so? Do I need to override the __new__ method to achieve this?
If you really want the constructor to return the original object then you have to override __new__ method:
def __new__(cls, *args):
if isinstance(args[0], File):
return args[0]
instance = super().__new__(cls, *args)
# do initialization
return instance
Note that you cannot use __init__ because it would be called even if no new instance is created.
A cleaner solution is to use a function which return either the instance or a new instance:
def File(*args):
if isinstance (args[0], File_):
return args[0]
return File_(*args)
where File_ is the original class
As @d.b mentions in the comments, you can "extract the data and create a new instance." This happens to be exactly how Path does it, using @classmethods. Here is the key function from the source (simplified here for clarity), which is called from __new__:
@classmethod
def _parse_args(cls, args):
parts = []
for a in args:
if isinstance(a, PurePath): # <--- Here
parts += a._parts # <---
else:
a = os.fspath(a)
if isinstance(a, str):
parts.append(str(a))
else:
raise TypeError()
return cls._flavour.parse_parts(parts)
Then p == p2 returns True because of the class's definition of __eq__, though p2 is not the original object.
p is p2will evaluate toFalse. they are equal but not identical. Although, one example of where you will see this behavior is with strings, i.e.str(some_str) is some_strhappens to be true in CPython.