Not None Test in Python

not None test in Python

if val is not None:
    # ...

is the Pythonic idiom for testing that a variable is not set to None. This idiom has particular uses in the case of declaring keyword functions with default parameters. is tests identity in Python. Because there is one and only one instance of None present in a running Python script/program, is is the optimal test for this. As Johnsyweb points out, this is discussed in PEP 8 under "Programming Recommendations".

As for why this is preferred to

if not (val is None):
    # ...

this is simply part of the Zen of Python: "Readability counts." Good Python is often close to good pseudocode.

How to check in python if variable is not None and greater than something in one line?

You can also use python ternary operator. In your example, this might help you. You can extend the same further too.

#if X is None, do nothing
>>> x = ''
>>> x if x and x>0 else None
#if x is not None, print it
>>> x = 1
>>> x if x and x>0 else None
1

Dealing with string values

>>> x = 'hello'
>>> x if x and len(x)>0 else None
'hello'
>>> x = ''
>>> x if x and len(x)>0 else None
>>>

Python `if x is not None` or `if not x is None`?

There's no performance difference, as they compile to the same bytecode:

>>> import dis
>>> dis.dis("not x is None")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE
>>> dis.dis("x is not None")
  1           0 LOAD_NAME                0 (x)
              2 LOAD_CONST               0 (None)
              4 COMPARE_OP               9 (is not)
              6 RETURN_VALUE

Stylistically, I try to avoid not x is y, a human reader might misunderstand it as (not x) is y. If I write x is not y then there is no ambiguity.

In Python how should I test if a variable is None, True or False

Don't fear the Exception! Having your program just log and continue is as easy as:

try:
    result = simulate(open("myfile"))
except SimulationException as sim_exc:
    print "error parsing stream", sim_exc
else:
    if result:
        print "result pass"
    else:
        print "result fail"

# execution continues from here, regardless of exception or not

And now you can have a much richer type of notification from the simulate method as to what exactly went wrong, in case you find error/no-error not to be informative enough.

How to test NoneType in python?

So how can I question a variable that is a NoneType?

Use is operator, like this

if variable is None:

Why this works?

Since None is the sole singleton object of NoneType in Python, we can use is operator to check if a variable has None in it or not.

Quoting from is docs,

The operators is and is not test for object identity: x is y is true if and only if x and y are the same object. x is not y yields the inverse truth value.

Since there can be only one instance of None, is would be the preferred way to check None.

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Hear it from the horse's mouth

Quoting Python's Coding Style Guidelines - PEP-008 (jointly defined by Guido himself),

Comparisons to singletons like None should always be done with is or is not, never the equality operators.

if A vs if A is not None:

The statement

if A:

will call A.__bool__() (see Special method names documentation), which was called __nonzero__ in Python 2, and use the return value of that function. Here's the summary:

object.__bool__(self)

Called to implement truth value testing and the built-in operation bool(); should return False or True. When this method is not defined, __len__() is called, if it is defined, and the object is considered true if its result is nonzero. If a class defines neither __len__() nor __bool__(), all its instances are considered true.

On the other hand,

if A is not None:

compares only the reference A with None to see whether it is the same or not.

What is the most pythonic way to check if multiple variables are not None?

There's nothing wrong with the way you're doing it.

If you have a lot of variables, you could put them in a list and use all:

if all(v is not None for v in [A, B, C, D, E]):

Best way to do a not None test in Python for a normal and Unicode empty string?

Empty strings are considered false.

if string:
    # String is not empty.
else:
    # String is empty.

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