What would be a way to check if a variable or class existed in an if?

[wavic]

In [1]: my_var = 'I am here!'

In [2]: dir()
Out[2]: 
['In',
 'Out',
 '_',
 '__',
 '___',
 '__builtin__',
 '__builtins__',
 '__doc__',
 '__loader__',
 '__name__',
 '__package__',
 '__spec__',
 '_dh',
 '_i',
 '_i1',
 '_i2',
 '_ih',
 '_ii',
 '_iii',
 '_oh',
 'exit',
 'get_ipython',
 'my_var',
 'quit']

In [3]: if 'my_var' in dir():
   ...:     print("aha")
   ...:     
aha