b() will only be evaluated if a(my_var) True , yes. Short circuit the and operator if a(my_var) false.
From the documentation of Boolean operators :
The expression x and y first evaluates x ; if x is false, its value is returned; otherwise, y is evaluated and the return value is returned.
You can verify this yourself with a function that prints something when called:
>>> def noisy(retval): ... print "Called, returning {!r}".format(retval) ... return retval ... >>> noisy(True) and noisy('whatever') Called, returning True Called, returning 'whatever' 'whatever' >>> noisy(False) and noisy('whatever') Called, returning False False
Python considers empty containers and numeric values 0 false:
>>> noisy(0) and noisy('whatever') Called, returning 0 0 >>> noisy('') and noisy('whatever') Called, returning '' '' >>> noisy({}) and noisy('whatever') Called, returning {} {}
User classes can implement __nonzero__ hook to return a Boolean flag for the same test or implement __len__ hook if they are a container type; return 0 means that the container is empty and should be considered false.
In a close note, the or operator does the same, but vice versa. If the first expression evaluates to true, the second expression will not evaluate:
>>> noisy('Non-empty string is true') or noisy('whatever') Called, returning 'Non-empty string is true' 'Non-empty string is true' >>> noisy('') or noisy('But an empty string is false') Called, returning '' Called, returning 'But an empty string is false' 'But an empty string is false'