python中的bool是什么意思

>>> bool(0)
False
>>> bool(1)
True
>>> bool(-1)
True
>>> bool(21334)
True

>>> bool('')
False
>>> bool(None)
False
>>> bool('asd')
True
>>> bool('hello')
True

>>> a = []
>>> bool(a)
False
>>> a.append(1)
>>> bool(a)
True

>>> x = raw_input('Please enter a number :')
Please enter a number :
>>> bool(x.strip())
False
>>> x = raw_input('Please enter a number :')
Please enter a number :4
>>> bool(x.strip())
True