What is the relationship between the Python datamodel and builtin functions? The builtins and operators use the underlying datamodel methods or attributes. The builtins and operators have more elegant behavior and are in general more forward compatible. The special methods of the datamodel are semantically non-public interfaces. The builtins and language operators are specifically intended … Read more
def isbound(method): return method.im_self is not None def instance(bounded_method): return bounded_method.im_self User-defined methods: When a user-defined method object is created by retrieving a user-defined function object from a class, its im_self attribute is None and the method object is said to be unbound. When one is created by retrieving a user-defined function object from a … Read more
Testing strings with is only works when the strings are interned. Unless you really know what you’re doing and explicitly interned the strings you should never use is on strings. is tests for identity, not equality. That means Python simply compares the memory address a object resides in. is basically answers the question “Do I … Read more
None is always less than any datatype in Python 2 (see object.c). In Python 3, this was changed; now doing comparisons on things without a sensible natural ordering results in a TypeError. From the 3.0 “what’s new” updates: Python 3.0 has simplified the rules for ordering comparisons: The ordering comparison operators (<, <=, >=, >) … Read more
I wouldn’t use __dict__ directly, but you can add a function to explicitly “freeze” a instance: class FrozenClass(object): __isfrozen = False def __setattr__(self, key, value): if self.__isfrozen and not hasattr(self, key): raise TypeError( “%r is a frozen class” % self ) object.__setattr__(self, key, value) def _freeze(self): self.__isfrozen = True class Test(FrozenClass): def __init__(self): self.x = … Read more
import inspect def get_class_that_defined_method(meth): for cls in inspect.getmro(meth.im_class): if meth.__name__ in cls.__dict__: return cls return None
Python, should I implement __ne__() operator based on __eq__? Short Answer: Don’t implement it, but if you must, use ==, not __eq__ In Python 3, != is the negation of == by default, so you are not even required to write a __ne__, and the documentation is no longer opinionated on writing one. Generally speaking, … Read more
The __getitem__() method will receive a slice object when the object is sliced. Simply look at the start, stop, and step members of the slice object in order to get the components for the slice. >>> class C(object): … def __getitem__(self, val): … print val … >>> c = C() >>> c[3] 3 >>> c[3:4] … Read more
With the following program #!/usr/bin/env python import foo def fullname(o): klass = o.__class__ module = klass.__module__ if module == ‘builtins’: return klass.__qualname__ # avoid outputs like ‘builtins.str’ return module + ‘.’ + klass.__qualname__ bar = foo.Bar() print(fullname(bar)) and Bar defined as class Bar(object): def __init__(self, v=42): self.val = v the output is $ ./prog.py foo.Bar … Read more
Take a look at the inspect module – this will do the inspection of the various code object properties for you. >>> inspect.getfullargspec(a_method) ([‘arg1’, ‘arg2’], None, None, None) The other results are the name of the *args and **kwargs variables, and the defaults provided. ie. >>> def foo(a, b, c=4, *arglist, **keywords): pass >>> inspect.getfullargspec(foo) … Read more