为什么 itertools.repeat 总是生成相同的随机数？

Question

Compare the outputs of these two functions:

from itertools import repeat
def rand_list1():
    l = lambda: np.random.rand(3)
    return list(repeat(l(), 5))

def rand_list2():
    return [np.random.rand(3) for i in range(5)]

We see that rand_list1 who uses itetools.repeat always generates the same 3 numbers. why is this? Can it be avoided, so each call of rand_list() will generate new numbers?

For example, the output of rand_list1() :

[[0.07678796 0.22623777 0.07533145]
 [0.07678796 0.22623777 0.07533145]
 [0.07678796 0.22623777 0.07533145]
 [0.07678796 0.22623777 0.07533145]
 [0.07678796 0.22623777 0.07533145]]

and the output of rand_list2() :

[[0.77863856 0.30345662 0.7007517 ]
 [0.56422447 0.97138115 0.47976387]
 [0.20576279 0.92875791 0.06518335]
 [0.2992384  0.89726684 0.16917078]
 [0.8440534  0.38016789 0.51691172]]
    

Answer 1

There is a basic miscomprehension on how the language works in your question.

With the lambda expression, you simply create a new function named l .

At the moment you do l() Python will call the function - and it will return a value: it is the returned value that will be used in place of the expression l() in the remaining of the larger expression. So, in this case, you are actually calling repeat with a single, already generated, number as the first parameter.

FUnctions that are passed as arguments to be called on their destination, and then are run anew each time are an allowed construct in Python, but (1) they depend on the receiving function being able to use functions as arguments, and that is not the case of repeat , and (2) more importanty one has to pass the function name without typing in the parentheses .

In this case, repeat is redundant, as the universal syntax that allows one to call a function multiple times to create an iterator already does the repetition you thought repeat would create for you.

Just do:

return [l() for _ in range(5)]

This will call l() for each interaction of the loop.

(btw, one should strongly avoid l as a single variable or function name in any context, as in many fonts it ishard to distinguish l from 1 )

Answer 2

The reason why list(repeat(l(), 5)) repeats the same value.

itertools.repeat() will just iterate the same result.

Because l() is assigned the result of l() in list(repeat(l(), 5)) when the l is called (which means l() ).

itertools.repeat() document

repeat(10, 3) --> 10 10 10

So what is exactly going on?

stage 1

 list(repeat(l(), 5))

stage 2

 list(repeat([some numbers], 5))

stage 3

 list(repeat([some numbers], 5))   --> [some numbers], [some numbers], [some numbers], [some numbers], [some numbers]