PyTricks:Functions are first-class citizens in Python

函數在python中是first-class citizens:
1.能作爲其他函數的參數被傳遞
2.能從其他函數中作爲值返回
3.能夠分配成變量且存儲在數據結構中

示例：

Python 3.6.0 (default, Oct 21 2017, 01:22:56)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> def myfunc(a,b):
...     return a+b
...
>>> funcs=[myfunc]
>>> funcs[0]
<function myfunc at 0x7fdd6a27ae18>
>>> funcs[0](2,3)
5

【什麼是First-class citizen?】–摘自first-class citizens wiki百科
　　In programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, and assigned to a variable.
　　在編程語言設計中，在一個給定的編程語言裏first-class citizen（也可以是類型，對象，實體或值）是一個實體（支持所有通常可用於其他實體的操作）。這些操作典型地包括作爲參數被傳遞，從函數返回，並將其分配給一個變量。