Assessing a lazy Python dictionary

Question

Python evangelists will say why Python does not have a switch statement because it has dictionaries. So ... how can I use a dictionary to solve this problem here? The problem is that all values ​​are evaluated differently and throw exceptions depending on the input.

This is just a dumb example of a class that stores a number or list of numbers and provides multiplication.

class MyClass(object):

    def __init__(self, value):
        self._value = value

    def __mul__(self, other):
        return {
            (False, False): self._value * other._value                        ,
            (False, True ): [self._value * o for o in other._value]           ,
            (True , False): [v * other._value for v in self._value]           ,
            (True , True ): [v * o for v, o in zip(self._value, other._value)],
        }[(isinstance(self._value, (tuple, list)), isinstance(other._value, (tuple, list)))]

    def __str__(self):
        return repr(self._value)
    __repr__ = __str__



>>> x = MyClass(2.0)
>>> y = MyClass([3.0, 4.0, 5.0])
>>> print x
2.0
>>> print y
[3.0, 4.0, 5.0]
>>> print x * y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in __mul__
TypeError: can't multiply sequence by non-int of type 'float'

One way I could solve this is to prefix each value with "lambda:" and after searching the dictionary will call the lambda function ... "} (isinsta ...)"

Is there a better way?

Answer 1

, :

    def __mul__(self, other): 
        scalar_times_scalar = lambda x,y: x*y
        scalar_times_seq    = lambda x,y: [x*y_i for y_i in y]
        seq_times_scalar    = lambda x,y: scalar_times_seq(y,x)
        seq_times_seq       = lambda x,y: [x_i*y_i for x_i,y_i in zip(x,y)]
        self_is_seq, other_is_seq = (isinstance(ob._value,(tuple, list)) 
                                                    for ob in (self, other))
        fn = {
            (False, False): scalar_times_scalar, 
            (False, True ): scalar_times_seq, 
            (True , False): seq_times_scalar, 
            (True , True ): seq_times_seq, 
            }[(self_is_seq, other_is_seq)] 
        return fn(self._value, other._value)

, , . __mul__ .

Answer 2

:

• if. True False, . if... elif... elif... , , , Python.

• dict ( , ) () . , , ( "" ).

Answer 3

, - .
, , , , , .

In my opinion, the main goal when writing software should be readable; for this reason, I would go for a set of if / elif that explicitly compares two values ​​(instead of having a mapping for types); then, if the measurements show performance problems, you can study other solutions (for example, searching for a dictionary with functions).