Python'和'可以返回无？

Question

I am looking at some code which apparently runs, as nobody has complained about it, but am well confused by what they have written:

if a and b is not None:
    # do something

I have always thought of the 'and' operator as something which returns True or False, now am starting to doubt myself.. What else would it return, a number.. It is probably not pythonic, but am I missing something - how can someone write something like that?

Answer 1

It means if a is Truthy and b is not None and not what you thought it meant ie a and b is Truthy

a = 999
b = None

if a and b is not None:
    print("a is True but b is None")
else:
    print("a is True and b is not None")

Answer 2

The code above means:

If a (is truthy), AND b isn't None , then #do something .

Answer 3

According to [Python 3]: Operator precedence ( emphasis is mine):

The following table summarizes the operator precedence in Python, from lowest precedence (least binding) to highest precedence (most binding) .

 ... and Boolean AND not x Boolean NOT in, not in, is, is not, <, <=, >, >=, !=, == Comparisons, including membership tests and identity tests ... 

The fact that is not comes after and , means that it will be evaluated before and (both might not be evaluated at all, due to lazy evaluation - thanks @NickA for the comment), so the expression is equivalent to (adding parentheses for clarity):

if a and (b is not None):

Also, according to [Python 3]: Truth Value Testing :

Any object can be tested for truth value, for use in an if or while condition or as operand of the Boolean operations below.

your if statement is perfectly OK (produces a bool ).

Examples (using [Python 3]: class bool ( [x] ) ):

 >>> bool(0) False >>> bool(100) True >>> bool([]) False >>> bool([0]) True >>> bool(None) False >>> bool({}) False >>> bool({1: 1}) True >>> bool(None is not None) False >>> bool(1 is not None) True >>> bool(2 and 1 is not None) True 

Answer 4

In this code, you evaluate b is not None first, which is a boolean.

And then a is implicitly converted into a boolean (For a list / dict , False if it is empty. For a number, False if it is 0)

then and is evaluated which always return a boolean.

Answer 5

It is most certainly valid.

When we take a variable which has been assigned a value and perform the AND operation with another variable with a null (in python - None), the AND operation results in a None . Hence, the check can be made. To make my point clear, please refer below.

a=2;c=None; b=11
if a and c is not None:
    print("T")
else:
    print("F")

and

print(a and c)

results in None

and

print(a and b)

results in Non-null value - in our case 11