Python code if not x: And if x are not none: And if not X are none: Use introduction _python

Source: Internet
Author: User
Tags in python

There are three main ways in which a variable is always judged to be none:
The first is "if X is None";
The second type is ' if not x: ';
The third is ' if not ' (this is a clearer understanding of ' if not ' (x is None) ').
If you think it makes no difference, then you should be careful, there is a hole in it. Let's take a look at the code:

>>> x = 1
>>> not x
>>> x = [1]
>>> not x
>& gt;> x = 0
>>> not x
>>> x = [0]     # you don ' t want to fall
in this one. >>> not x

None in Python, false, empty string "", 0, empty list [], empty Dictionary {}, empty tuple () is equal to false, namely:

Copy Code code as follows:

Not None = = not False = = not ' = = not 0 = not [] = = not {} = Not ()

So when using a list, if you want to differentiate between x==[] and x==none two situations, then ' if not x: ' There will be a problem:

>>> x = []
>>> y = none
>>> x is None
>>> y None
>>> not x
>>> not y
>>> not X are None
>>> True
>>> not y is None

Perhaps you are trying to determine if X is None, but you are also judging the situation of ' x==[', in which case it will be indistinguishable.
For Pythoner accustomed to using if not X, it must be clear that x equals None, False, empty string "", 0, empty list [], empty Dictionary {}, empty tuple () does not affect your judgment.
For the ' if ' not none ' and ' if not ' ' none ' notation, it is obvious that the former is clearer, and the latter may misinterpret the reader as ' if (not X ') ' None ', and therefore recommend the former, which is also the recommended style of Google

' If ' isn't None ' is the best way to spell, clear, no mistakes, and later insist on using this notation.
The premise of using if not X is that it must be clear that x equals None, False, empty string "", 0, empty list [], empty Dictionary {}, empty tuple () has no effect on your judgment.

But this does not apply to the case where the variable is a function, the following is reproduced from:

The difference between foo is none and foo = = None

Problem links

If Foo is None:pass
if foo = = None:pass

If you compare the same object instance, the is always returns true and = = ultimately depends on the EQ ()

>>> class Foo (object):
  def __eq__ (self, Other): Return

>>> f = foo ()
>> > f = = None
>>> f
is none False

>>> list1 = [1, 2, 3]
>>> list2 = [1, 2, 3]
>>> list1==list2
>>> list1 is List2

Other than that

(Ob1 is OB2) equivalent to (ID (ob1) = = ID (ob2))

Added, 2013.10.09
Transferred from

What is not specifically in Python, for example, is heartfelt thanks

In Python, not is a logical word, used for Boolean true and False,not true to false,not false to True, and the following are several common uses of not:
(1) Not and the logical judgment sentence if used to execute the statement after the colon, when the expression after not is false. Like what:
A = False
If not a: (here because A is false, not A is true)
print "Hello"
Here we can output the results hello
(2) to determine whether an element is in a list or a dictionary, if a isn't in B,a is an element, B is a list or a dictionary, meaning that if a is not in list B, then execute the statement following the colon, such as:
A = 5
b = [1, 2, 3]
If a isn't in B:
print "Hello"
We can also output the results here Hello

Not x means equivalent to if X is false, then True, else false

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