是否有内置Python可以从字典或对象中进行不可知的选择？

Question

I'm looking for a function f(x,selector) that acts like x.get(selector) when x is a dictionary and getattr(x, selector) when x is a class instance. Easy enough to write, but is there one built in?

Answer 1

No, there is no built-in.

Just could just exception handling:

def f(x, selector):
    try:
        # Assume dictionary and has .get() method
        return x.get(selector)
    except AttributeError:
        # Assumption incorrect, no .get() method present
        return getattr(x, selector)

or swap the two lines if you want attributes to be checked for first.

Another variant, supporting a default:

_sentinel = object()

def f(x, selector, default=_sentinel):
    try:
        # Assume mapping access is supported
        result = x[selector]
    except KeyError:
        # key missing, fall through to default handling
        pass
    except TypeError:
        # Assumption incorrect, not a mapping, fall back to attributes
        result = getattr(x, selector, default)
    if result is _sentinel:
        raise KeyError(selector)
    return default

but you could also collapse the KeyError handling into the second TypeError handler and fall back to getattr() first.

Take into account however that you are trying to mix two namespaces here; keys and attributes can collide, and attribute literals are more limited than keys, etc. Most Python code doesn't try to mix the two, which is why there is no built-in for this task.

Answer 2

Another way is to check type of object

def f(x, selector):
    if isinstance(x, dict):
       return x.get(selector)
    return getattr(x, selector)

Answer 3

In [297]: class Foo:
   .....:     def __init__(self):
   .....:         self.bar=5
   .....:         


In [298]: def f(x, selector):
   .....:     if hasattr(x, 'get'):
   .....:         g = operator.itemgetter(selector)
   .....:     else:  # x is an object
   .....:         g = operator.attrgetter(selector)
   .....:     return g(x)
   .....: 

In [299]: x = Foo()

In [300]: f(x, 'bar')
Out[300]: 5

In [301]: d = {char:i for i,char in enumerate('asdf')}

In [302]: f(d, 'a')
Out[302]: 0

In [303]: d
Out[303]: {'a': 0, 's': 1, 'd': 2, 'f': 3}