Python generator 類型

場景：

使用gurobi求解優化問題時，遇到quicksum()函數用法以下：

quicksum(mu[i] for i in range(n))

讀着很流暢並且好像並沒什麼問題欸，但

mu[i] for i in range(n)

返回的又是什麼？

 

看了下quicksum()函數的介紹：

def quicksum(p_list): # real signature unknown; restored from __doc__
    """
    ROUTINE:
        quicksum(list)
    
      PURPOSE:
        A quicker version of the Python built-in 'sum' function for building
        Gurobi expressions.
    
      ARGUMENTS:
        list: A list of terms.
    
      RETURN VALUE:
        An expression that represents the sum of the input arguments.
    
      EXAMPLE:
        expr = quicksum([x, y, z])
        expr = quicksum([1.0, 2*y, 3*z*z])
    """
    pass

因此，上述代碼返回的是個list？

 

python console中試了下：

x = [1,2,3]
print (x[i] for i in range(2))
<generator object <genexpr> at 0x000000000449A750>

並非list欸，是個generator object。

難道說generator object能夠賦值給list變量？

 

查了下generator的相關文章(其中的yeild是關鍵 ，參考yeild介紹)

 

而後是generator和list~迭代器的關係

Python關鍵字yield詳解以及Iterable 和Iterator區別

 

A generator expression can be used whenever a method accepts an Iterable argument (something that can be iterated over). For example, most Python methods that accept a list argument (the most common type of Iterable) will also accept a generator expression.

 

In:(x*x for x in range(3))
Out:<generator object <genexpr> at 0x00000000045E8AF8>
In:[x*x for x in range(3)]
Out:[0, 1, 4]

 

 

其餘的一些補充，關於與for語句的結合：

List comprehension and generator expressions can both contain more than one for clause, and one or more if clauses. The following example builds a list of tuples containing all x,y pairs where x and y are both less than 4 and x is less than y:

gurobi> [(x,y) for x in range(4) for y in range(4) if x < y]
[(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)]

Note that the for statements are executed left-to-right, and values from one can be used in the next, so a more efficient way to write the above is:

gurobi> [(x,y) for x in range(4) for y in range(x+1, 4)]