python: order of AND execution

Question

If I have the following:

if a(my_var) and b(my_var):
    do something

Can I assume that b() is only evaluated if a() is True? Or might it do b() first?

Asking because evaluating b() will cause an exception when a() is False.

Answer 1

b() will only be evaluated if a(my_var) is True, yes. The and operator short-circuits if a(my_var) is falsey.

From the boolean operators documentation:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.

You can test 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 consideres empty containers and numeric 0 values as false:

>>> noisy(0) and noisy('whatever')
Called, returning 0
0
>>> noisy('') and noisy('whatever')
Called, returning ''
''
>>> noisy({}) and noisy('whatever')
Called, returning {}
{}

Custom classes can implement a __nonzero__ hook to return a boolean flag for the same test, or implement a __len__ hook if they are a container type instead; returning 0 means the container is empty and is to be considered false.

On a closely related note, the or operator does the same thing, but in reverse. If the first expression evaluates to true the second expression will not be evaluated:

>>> 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'