if a != None:

>>> x = 1

>>> not x

False

>>> x = [ 1 ]

>>> not x

False

>>> x = 0

>>> not x

True

>>> x = [ 0 ]     # You don't want to fall in this one.

>>> not x

False

 

在python中 None, False, 空字符串"", 0, 空列表[], 空字典{}, 空元组()都相当于False ，即：
not None == not False == not '' == not 0 == not [] == not {} == not ()

 if a != None:

其意思是a!=none,则执行:后面的语句