and
, or
:Let's first define a useful function to determine if something is executed or not. A simple function that accepts an argument, prints a message and returns the input, unchanged.
>>> def fun(i):
... print "executed"
... return i
...
One can observe the Python's short-circuiting behavior of and
, or
operators in the following example:
>>> fun(1)
executed
1
>>> 1 or fun(1) # due to short-circuiting "executed" not printed
1
>>> 1 and fun(1) # fun(1) called and "executed" printed
executed
1
>>> 0 and fun(1) # due to short-circuiting "executed" not printed
0
Note: The following values are considered by the interpreter to mean false:
False None 0 "" () [] {}
any()
, all()
:Python's any()
and all()
functions also support short-circuiting. As shown in the docs; they evaluate each element of a sequence in-order, until finding a result that allows an early exit in the evaluation. Consider examples below to understand both.
The function any()
checks if any element is True. It stops executing as soon as a True is encountered and returns True.
>>> any(fun(i) for i in [1, 2, 3, 4]) # bool(1) = True
executed
True
>>> any(fun(i) for i in [0, 2, 3, 4])
executed # bool(0) = False
executed # bool(2) = True
True
>>> any(fun(i) for i in [0, 0, 3, 4])
executed
executed
executed
True
The function all()
checks all elements are True and stops executing as soon as a False is encountered:
>>> all(fun(i) for i in [0, 0, 3, 4])
executed
False
>>> all(fun(i) for i in [1, 0, 3, 4])
executed
executed
False
Additionally, in Python
Comparisons can be chained arbitrarily; for example,
x < y <= z
is equivalent tox < y and y <= z
, except thaty
is evaluated only once (but in both casesz
is not evaluated at all whenx < y
is found to be false).
>>> 5 > 6 > fun(3) # same as: 5 > 6 and 6 > fun(3)
False # 5 > 6 is False so fun() not called and "executed" NOT printed
>>> 5 < 6 > fun(3) # 5 < 6 is True
executed # fun(3) called and "executed" printed
True
>>> 4 <= 6 > fun(7) # 4 <= 6 is True
executed # fun(3) called and "executed" printed
False
>>> 5 < fun(6) < 3 # only prints "executed" once
executed
False
>>> 5 < fun(6) and fun(6) < 3 # prints "executed" twice, because the second part executes it again
executed
executed
False
Edit:
One more interesting point to note :- Logical and
, or
operators in Python returns an operand's value instead of a Boolean (True
or False
). For example:
Operation
x and y
gives the resultif x is false, then x, else y
Unlike in other languages e.g. &&
, ||
operators in C that return either 0 or 1.
Examples:
>>> 3 and 5 # Second operand evaluated and returned
5
>>> 3 and ()
()
>>> () and 5 # Second operand NOT evaluated as first operand () is false
() # so first operand returned
Similarly or
operator return left most value for which bool(value)
== True
else right most false value (according to short-circuiting behavior), examples:
>>> 2 or 5 # left most operand bool(2) == True
2
>>> 0 or 5 # bool(0) == False and bool(5) == True
5
>>> 0 or ()
()
So, how is this useful? One example is given in Practical Python By Magnus Lie Hetland:
Let’s say a user is supposed to enter his or her name, but may opt to enter nothing, in which case you want to use the default value '<Unknown>'
.
You could use an if statement, but you could also state things very succinctly:
In [171]: name = raw_input('Enter Name: ') or '<Unknown>'
Enter Name:
In [172]: name
Out[172]: '<Unknown>'
In other words, if the return value from raw_input
is true (not an empty string), it is assigned to name (nothing changes); otherwise, the default '<Unknown>'
is assigned to name
.