I'm somewhat new at programming and just for fun, tried to figure out how the django library works. I went to see how to ORM functions are implemented. I followed the functions to BaseManger, which inherits from Generic[_T]. But then I got stuck.
Here's what is in BaseManager for something like get()
def get(self, *args: Any, **kwargs: Any) -> _T: ...
I assumed something is happening in the parent class Generic, but this is what I got:
class Generic:
"""Abstract base class for generic types.
A generic type is typically declared by inheriting from
this class parameterized with one or more type variables.
For example, a generic mapping type might be defined as::
class Mapping(Generic[KT, VT]):
def __getitem__(self, key: KT) -> VT:
...
# Etc.
This class can then be used as follows::
def lookup_name(mapping: Mapping[KT, VT], key: KT, default: VT) -> VT:
try:
return mapping[key]
except KeyError:
return default
"""
__slots__ = ()
_is_protocol = False
@_tp_cache
def __class_getitem__(cls, params):
if not isinstance(params, tuple):
params = (params,)
if not params and cls is not Tuple:
raise TypeError(
f"Parameter list to {cls.__qualname__}[...] cannot be empty")
msg = "Parameters to generic types must be types."
params = tuple(_type_check(p, msg) for p in params)
if cls in (Generic, Protocol):
# Generic and Protocol can only be subscripted with unique type variables.
if not all(isinstance(p, TypeVar) for p in params):
raise TypeError(
f"Parameters to {cls.__name__}[...] must all be type variables")
if len(set(params)) != len(params):
raise TypeError(
f"Parameters to {cls.__name__}[...] must all be unique")
else:
# Subscripting a regular Generic subclass.
_check_generic(cls, params, len(cls.__parameters__))
return _GenericAlias(cls, params)
def __init_subclass__(cls, *args, **kwargs):
super().__init_subclass__(*args, **kwargs)
tvars = []
if '__orig_bases__' in cls.__dict__:
error = Generic in cls.__orig_bases__
else:
error = Generic in cls.__bases__ and cls.__name__ != 'Protocol'
if error:
raise TypeError("Cannot inherit from plain Generic")
if '__orig_bases__' in cls.__dict__:
tvars = _collect_type_vars(cls.__orig_bases__)
# Look for Generic[T1, ..., Tn].
# If found, tvars must be a subset of it.
# If not found, tvars is it.
# Also check for and reject plain Generic,
# and reject multiple Generic[...].
gvars = None
for base in cls.__orig_bases__:
if (isinstance(base, _GenericAlias) and
base.__origin__ is Generic):
if gvars is not None:
raise TypeError(
"Cannot inherit from Generic[...] multiple types.")
gvars = base.__parameters__
if gvars is not None:
tvarset = set(tvars)
gvarset = set(gvars)
if not tvarset <= gvarset:
s_vars = ', '.join(str(t) for t in tvars if t not in gvarset)
s_args = ', '.join(str(g) for g in gvars)
raise TypeError(f"Some type variables ({s_vars}) are"
f" not listed in Generic[{s_args}]")
tvars = gvars
cls.__parameters__ = tuple(tvars)
I don't understand this class and dont get how the ORM get() works in the end. can someone enlighten me to how it works?
