Do not understand why if condition evaluates to True when variable is integer

I am working with code that exhibits the following behavior. Suppose I have a variable d and I assign it to an integer 9

d = 9

why do the following statements work?

In [95]: if d:
   ....:     print d
   ....: else:
   ....:     print 'Did not print d!'
   ....:     
9

In [96]: 

when d itself is not a Boolean and does not pass the following tests:

In [96]: d is True
Out[96]: False

In [97]: d is False
Out[97]: False

I will appreciate it if someone could explain this to me and point out any misconception that I have. Thank you very much.