Example of python judging to be empty shared

In practice, we inevitably have to deal with null values, and I'm sure many beginners will write the following code:.

if a is None:
    do :
    do the other thing.

Writing it this way looks good, but it can actually be problematic. In general, Python treats the following cases as nulls.

None
False
0,0.0,0L
'',(),[],{}

One of the special things about None is that it is neither the value 0 nor the null value of some data structure; it is itself a null object. Its type is NoneType and it follows the single

example pattern, that is, all None in the same namespace are essentially the same null-valued object.

>>> id(None)
1795884240
>>> None == 0
False
>>> None == ''
False
>>> a = None
>>> id(a)
1795884240
>>> a == None
True

The above judgment clearly does not meet our expectations: a==None is True only if a is shown to be assigned to None.

So what should we do about the more generalized None value judgment in Python?

>>> a = ''    # Here is an example of an empty string only, the same applies to other null values
>>> if a:
...        print 'a is not empty'
...    else:
...        print 'a is a empty string'
'a is a empty string.'

As you can see, the if a judgment approach yields the result we want, so what exactly is the process of if a judgment approach?

if a will first go to call a's __nonzero__() to determine whether a is empty, and return True/False, if an object does not define __nonzero__(), go to call its __len__() to

Make a judgment (here the return value is 0 for null), if an object does not define the above two methods, then the result of if a will always be True

Next, to verify the above statement.

>>>class A(object):
...        def __nonzero__(self):
...            print 'running on the __nonzero__'
...            return True
>>>class B(object):
...        def __len__(self):
...            print 'running on the __len__'
...            return False
>>> a, b = A(), B()
>>>if a:
...        print 'Yep'
...    else:
...        print 'Nop'
running on the __nonzero__
Yep
>>>if b:
...        print 'Yep'
...    else:
...        print 'Nop'
running on the __len__
Nop

Content Extension:

How to determine if a python function returns null

I don't know what you mean by empty here, is it None or ''?

I'll tell you all about DU:

None is a zhi an empty dao object that represents nothing.

And '', a string object, represents an empty string

If the return value is None, you use the if None: judgment

If it returns '', you use if len('') == 0: Judgment

Netflix Share:

You can directly bai put the function into the if after the du as a condition, if it is empty then zhi judgment result dao is false, for example:

def test():
    return None
     
if test():
    print True
else:
    print False

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