if A vs if A is not None

Question

Can I use   if A    instead of   if A is not None    The latter seems so verbose  Is there a difference

User · Answer

The other answers have all given good information, but I feel this needs to be made clear.

No, you should never use:

if A:

if what you need to test is:

if A is not None:

The first will often work as a replacement for the second but the first is a common source of hard to find bugs. Even if all you are doing is writing some quick throw-away code, you should not allow yourself to get into the habit of writing buggy code. Train your fingers and your mind to write and read the correct verbose form of the test:

if A is not None:

The common use of None is to define optional parameter values and to give a variable a default starting value which means "no value yet", and functions a meaning of "no value retured". If you write a function such as:

def my_func(a_list, obj=None):
    if obj:          # Trying to test if obj was passed
        a_list.append(obj)
    # do something with a_list

This will work fine for many real world uses like:

my_func(user_names, "bob")

But if any of this happens:

my_func(user_names, "")
my_func(user_names, [])
my_func(user_names, 0)

Then the zero-length objects will not be added to the list. When the first code was written, you might know that zero-length user names are not allowed. So the short code works fine. But then you try to modify the code to use an empty string to mean something like an anonymous user with no name, and suddenly this function stops doing what it is expected to do (adding the annonomous user to the list).

For example, lets say you have logic to define when a user is allowed to log in written like:

new_user = get_user_name()

if user_list_ok(my_func(current_users, new_user)):
    # user_list_ok() tests that no more than 10 users can
    # log in at the same time, and that no sigle user can
    # can log in more than 3 times at once.

    # This user is allowed to log in!

    log_user_in(new_user)

This now creates subtle and complex bugs in your code. The anonmous user of "" will not be added to the list for the test, so when the system has hit the limit of 10 users, the test will still allow the anonomous user to log in, pushing the user count to 11. This then might trigger a bug in another part of the system that requires there only be 10 users max.

When you need to test for "var does not have a value" you should always use the longer "is not None" test even though it feels ugly and verbose.

User · Answer

They do very different things  The below checks if A has anything except the values False      None     and 0  It checks the value of A  if A   The below checks if A is a different object than None  It checks and compares the reference  memory address  of A and None  if A is not None    UPDATE  Further explanation Many times the two seem to do the same thing so a lot of people use them  interchangeably  The reason the two give the same results is many times by pure coincidence due to optimizations of the interpreter compiler like interning or something else  With those optimizations in mind  integers and strings of the same value end up using the same memory space  That probably explains why two separate strings act as if they are the same   gt  a    test   gt  b    test   gt  a is b True  gt  a    b True  Other things don t behave the same though    gt  a       gt  b       gt  a is b False  gt  a    b True  The two lists clearly have their own memory  Surprisingly tuples behave like strings   gt  a       gt  b       gt  a is b True  gt  a    b True  Probably this is because tuples are guaranteed to not change  thus it makes sense to reuse the same memory  This shows that you should be extra vigilant on what comparison operator you use  Use is and    depending on what you really want to check  These things can be hard to debug since is reads like prose that we often just skim over it

User · Answer

if A  will prove false if A is 0  False  empty string  empty list or None  which can lead to undesired results

User · Answer

None is a special value in Python which usually designates an uninitialized variable  To test whether A does not have this particular value you use    if A is not None   Falsey values are a special class of objects in Python  e g  false       To test whether A is falsey use    if not A   Thus  the two expressions are not the same And you d better not treat them as synonyms      P S  None is also falsey  so the first expression implies the second  But the second covers other falsey values besides None  Now    if you can be sure that you can t have other falsey values besides None in A  then you can replace the first expression with the second

User · Answer

python    2 6   if we write such as   if A    will generate warning as      FutureWarning  The behavior of this method will change in future   versions   Use specific  len elem   or  elem is not None  test   instead    So we can use   if A is not None

User · Answer

I created a file called test py and ran it on the interpreter  You may change what you want to  to test for sure how things is going on behind the scenes   import dis  def func1         matchesIterator   None      if matchesIterator           print   On if       def func2         matchesIterator   None      if matchesIterator is not None           print   On if       print    nFunction 1     dis dis func1   print    nFunction 2     dis dis func2    This is the assembler difference     Source    gt  gt  gt  import importlib  gt  gt  gt  reload  test    Function 1   6           0 LOAD CONST               0  None                3 STORE FAST               0  matchesIterator     8           6 LOAD FAST                0  matchesIterator                9 POP JUMP IF FALSE       20   10          12 LOAD CONST               1   On if                 15 PRINT ITEM              16 PRINT NEWLINE              17 JUMP FORWARD             0  to 20           gt  gt    20 LOAD CONST               0  None               23 RETURN VALUE  Function 2  14           0 LOAD CONST               0  None                3 STORE FAST               0  matchesIterator    16           6 LOAD FAST                0  matchesIterator                9 LOAD CONST               0  None               12 COMPARE OP               9  is not               15 POP JUMP IF FALSE       26   18          18 LOAD CONST               1   On if                 21 PRINT ITEM              22 PRINT NEWLINE              23 JUMP FORWARD             0  to 26           gt  gt    26 LOAD CONST               0  None               29 RETURN VALUE  lt module  test  from  test py  gt

User · Answer

Most guides I ve seen suggest that you should use       if A   unless you have a reason to be more specific    There are some slight differences  There are values other than None that return False  for example empty lists  or 0  so have a think about what it is you re really testing for

User · Answer

A lot of functions return None if there are no appropriate results  For example  an SQLAlchemy query s  first   method returns None if there were no rows in the result  Suppose you were selecting a value that might return 0 and need to know whether it s actually 0 or whether the query had no results at all   A common idiom is to give a function or method s optional argument the default value of None  and then to test that value being None to see if it was specified  For example   def spam eggs None       if eggs is None          eggs   retrievefromconfigfile     compare that to   def spam eggs None       if not eggs          eggs   retrievefromconfigfile     In the latter  what happens if you call spam 0  or spam      The function would  incorrectly  detect that you hadn t passed in a value for eggs and would compute a default value for you  That s probably not what you want   Or imagine a method like  return the list of transactions for a given account   If the account does not exist  it might return None  This is different than returning an empty list  which would mean  this account exists but has not recorded transactions    Finally  back to database stuff  There s a big difference between NULL and an empty string  An empty string typically says  there s a value here  and that value is nothing at all   NULL says  this value hasn t been entered    In each of those cases  you d want to use if A is None  You re checking for a specific value - None - not just  any value that happens to cast to False

User · Answer

The former is more Pythonic  better ideomatic code   but will not execute the block if A is False  not None

User · Answer

It depends on the context   I use if A  when I m expecting A to be some sort of collection  and I only want to execute the block if the collection isn t empty  This allows the caller to pass any well-behaved collection  empty or not  and have it do what I expect  It also allows None and False to suppress execution of the block  which is occasionally convenient to calling code   OTOH  if I expect A to be some completely arbitrary object but it could have been defaulted to None  then I always use if A is not None  as calling code could have deliberately passed a reference to an empty collection  empty string  or a 0-valued numeric type  or boolean False  or some class instance that happens to be false in boolean context   And on the other other hand  if I expect A to be some more-specific thing  e g  instance of a class I m going to call methods of   but it could have been defaulted to None  and I consider default boolean conversion to be a property of the class I don t mind enforcing on all subclasses  then I ll just use if A  to save my fingers the terrible burden of typing an extra 12 characters

User · Answer

The statement  if A    will call A   nonzero      see Special method names documentation  and use the return value of that function  Here s the summary      object   nonzero   self       Called to implement truth value testing and the built-in operation bool    should return False or True  or their integer equivalents 0 or 1  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   nonzero      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

User · Answer

As written in PEP8         Comparisons to singletons like None should always be done with    is  or  is not   never the equality operators       Also  beware of writing  if x  when you really mean  if x is not None    -- e g  when testing whether a variable or argument that defaults to   None was set to some other value   The other value might have a type    such as a container  that could be false in a boolean context

User · Answer

if x   x is treated True except for all empty data types             0 False  and None   so it is not same as   if x is not None   which works only on None

[python] if A vs if A is not None:

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