python - 具有命名字段访问的python张量

Question

我想在 Python 中使用类似于或优于 R 数组的东西。R 数组是具有 dimnames 属性的类张量对象，它允许直接允许基于名称（字符串）对张量进行子集化。在 numpy 中，recarrays 允许列名，pandas 允许灵活高效的二维数组子集。Python 中是否有一些东西允许通过使用名称（或者更好的是，在 Python 中可散列且不可变的对象）来进行与 ndarray 的切片和子集类似的操作？

Answer 1

How about this quick and dirty mapping from lists of strings to indices? You could clean up the notation with callable classes.

def make_dimnames(names):
    return [{n:i for i,n in enumerate(name)} for name in names]
def foo(d, *args):
    return [d[x] for x in args]

A = np.arange(9).reshape(3,3)
dimnames = [('x','y','z'),('a','b','c')]
Adims = make_dimnames(dimnames)
A[foo(Adims[0],'x','z'),foo(Adims[1],'b')]  # A[[0,2],[1]]
A[foo(Adims[0],'x','z'),slice(*foo(Adims[1],'b','c'))]  # A[[0,2],slice(1,2)]

Or does R do something more significant with the dimnames?

A class compresses the syntax a bit:

class bar(object):
    def __init__(self,dimnames):
        self.dd = {n:i for i,n in enumerate(dimnames)}
    def __call__(self,*args):
        return [self.dd[x] for x in args]
    def __getitem__(self,key):
        return self.dd[key]
d0, d1 = bar(['x','y','z']), bar(['a','b','c'])
A[d0('x','z'),slice(*d1('a','c'))]

http://docs.scipy.org/doc/numpy/user/basics.subclassing.html sublassing ndarray, with simple example of adding an attribute (which could be dinnames). Presumably extending the indexing to use that attribute shouldn't be hard.

Inspired by the use of __getitem__ in numpy/index_tricks, I've generalized the indexing:

class DimNames(object):
    def __init__(self, dimnames):
        self.dd = [{n:i for i,n in enumerate(names)} for names in dimnames]
    def __getitem__(self,key):
        # print key
        if isinstance(key, tuple):
            return tuple([self.parse_key(key, self.dd[i]) for i,key in enumerate(key)])
        else:
            return self.parse_key(key, self.dd[0])
    def parse_key(self,key, dd):
        if key is None:
            return key
        if isinstance(key,int):
            return key
        if isinstance(key,str):
            return dd[key]
        if isinstance(key,tuple):
            return tuple([self.parse_key(k, dd) for k in key])
        if isinstance(key,list):
            return [self.parse_key(k, dd) for k in key]
        if isinstance(key,slice):
            return slice(self.parse_key(key.start, dd),
                         self.parse_key(key.stop, dd),
                         self.parse_key(key.step, dd))
        raise KeyError

dd = DimNames([['x','y','z'], ['a','b','c']])

print A[dd['x']]              # A[0]
print A[dd['x','c']]          # A[0,2]
print A[dd['x':'z':2]]        # A[0:2:2]
print A[dd[['x','z'],:]]      # A[[0,2],:]
print A[dd[['x','y'],'b':]]   # A[[0,1], 1:]
print A[dd[:'z', :2]]         # A[:2,:2]

I suppose further steps would be to subclass A, add dd as attribute, and change its __getitem__, simplifying the notation to A[['x','z'],'b':].