提问：UnboundLocalError 是如何来的？

背景：

python2.7
windows7
pycharm

错误内容：

coding:utf-8
import random

def func(ok):
    if ok:
        a = random.randint(1,10)
    else:
        import random
        a = random.randint(1, 10)
        print a
    return a

if __name__ == '__main__':
    func(True)

错误提示：

Traceback (most recent call last):
  File "F:/***/test_two.py", line 15, in <module>
    func(True)
  File "F:/***/test_two.py", line 6, in func
    a = random.randint(1,10)
UnboundLocalError: local variable 'random' referenced before assignment

个人的尝试：

这是来源于一篇文章，在解释关于命名空间和作用域时举的例子。戳这里，但是，并没有解释为什么。而我也不能明白出现错误的原理。在我的认知里面，如果local()中找不到random,那就到global中寻找random，那么肯定是能够找到顶部的random的，为何会出现UnboundLocalError的错误？然后我进行了如下的尝试：

test_1

# coding:utf-8
import random

def func(ok):
    if ok:
        try:
            a = random.randint()
        except:
            global random
            a = random.randint(1,10)
    else:
        import random
        a = random.randint(1, 10)
        print a
    return a

if __name__ == '__main__':
    func(True)

结果显示

F:/****/test_two.py:9: SyntaxWarning: name 'random' is used prior to global declaration
  global random

test_2

# coding:utf-8

def func(ok):
    if ok:
        try:
            a = random.randint(1,10)
        except:
            global random
            a = random.randint(1,10)
    else:
        import random
        a = random.randint(1, 10)
    return a

if __name__ == '__main__':
    func(True)

错误：

F:/***/test_two.py:8: SyntaxWarning: name 'random' is used prior to global declaration
  global random
Traceback (most recent call last):
  File "F:/***/test_two.py", line 16, in <module>
    func(True)
  File "F:/***/test_two.py", line 9, in func
    a = random.randint(1,10)
NameError: global name 'random' is not defined

test_3

# coding:utf-8

def func(ok):
    if ok:
        try:
            a = random.randint(1,10)
        except:
            global random
            a = random.randint(1,10)
    else:
        import random
        a = random.randint(1, 10)
    return a

if __name__ == '__main__':
    func(False)
    func(True)

F:/***/test_two.py:8: SyntaxWarning: name 'random' is used prior to global declaration
  global random

test_4

# coding:utf-8

def func(ok):
    if ok:
        a = random.randint(0,10)
    else:
        import random
        a = random.randint(1, 10)
    return a

if __name__ == '__main__':
    func(False)
    func(True)




    
Traceback (most recent call last):
  File "F:/***/test_two.py", line 13, in <module>
    func(True)
  File "F:/***/test_two.py", line 5, in func
    a = random.randint(0,10)
UnboundLocalError: local variable 'random' referenced before assignment

有人可以帮我解释一下，为什么在例子中，会出现UnboundLocalError么？同时，为何在函数中执行到import random，后，global 能够得到random？这是python的什么原理或者规则？