Write a Python function to find the nth number in the Fibonacci Sequence.

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# The function should return the nth Fibonacci number.
    # As an example, if n = 4, the function should return 3.
    # As another example, if n = 5, the function should return 5.
    # Hint: You will need to create a list of Fibonacci numbers up to the nth number.
    # You can use a for loop to create the list.
    # The function definition should include a parameter n, which is the nth number.
    # The function definition should also include a local variable result, which is the nth Fibonacci number.
    # The function should return result.

    # TODO: WRITE YOUR FUNCTION HERE

    # TODO: TEST YOUR FUNCTION HERE


def main():
    """
    This main function is here to test your code.
    Do not modify this function.
    """
    print(fibonacci(1))
    print(fibonacci(2))
    print(fibonacci

Write a Python function that prints the next 20 leap years. Reply with only the function.

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# (Hint: 2016, 2020, 2024, 2028, 2032)
    #
    # def leap_years():
    #     year = 2016
    #     while year <= 2032:
    #         if year % 4 == 0:
    #             print(year)
    #         year += 1
    #
    # leap_years()

    # 10. Write a Python program to find the number of days between two dates.
    #
    # def days_between(date1, date2):
    #     return abs((date1 - date2).days)
    #
    # print(days_between(date(2013, 12, 20), date(2013, 12, 26)))

    # 11. Write a Python program to get the volume of a sphere with radius 6.
    #
    #

Explain the bug in the following code:

```
from time import sleep
from multiprocessing.pool import ThreadPool
 
def task():
    sleep(1)
    return 'all done'

if __name__ == '__main__':
    with ThreadPool() as pool:
        result = pool.apply_async(task())
        value = result.get()
        print(value)
```

all answers

### Answer

The `apply_async` method takes a function and its arguments as parameters.

```
result = pool.apply_async(task())
```

The `task()` function is called immediately, and the return value is passed to `apply_async`.

```
result = pool.apply_async('all done')
```

The `apply_async` method is called with a single argument, the string `'all done'`.

```
result = pool.apply_async(args=('all done',))
```

The `apply_async` method is called with the `args` parameter set to a tuple containing the string `'all done'`.

```
result = pool.apply_async(func=print, args=('all done',))
```

The `apply_async` method is called with the `func` parameter set to the built-in `print` function, and the `args` parameter set to a tuple containing the string `'all done'`.

```
result = pool.apply_async(func=print, args

Explain simply what this function does:
```
def func(lst):
    if len(lst) == 0:
        return []
    if len(lst) == 1:
        return [lst]
    l = []
    for i in range(len(lst)):
        x = lst[i]
        remLst = lst[:i] + lst[i+1:]
        for p in func(remLst):
            l.append([x] + p)
    return l
```

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### 2.2.2.2. 递归函数

#### 2.2.2.2.1. 递归函数的定义

递归函数是一种特殊的函数，它的返回值依赖于它自身。

#### 2.2.2.2.2. 递归函数的特点

递归函数的特点是：

- 递归函数的返回值依赖于它自身
- 递归函数的参数列表中必须包含一个或多个用于终止递归的基本条件

#### 2.2.2.2.3. 递归函数的应用

递归函