Use of True False and None as return values in Python functions

Question

I think that I fully understand this  but I just want to make sure since I keep seeing people say to never ever test against True  False  or None   They suggest that routines should raise an error rather than return False or None  Anyway  I have many situations where I simply want to know if a flag is set or not so my function returns True or False  There are other situations where I have a function return None if there was no useful result  From my thinking  neither is problematic so long as I realize that I should never use   if foo    True if foo    False if foo    None   and should instead use   if foo is True if foo is False if foo is None   since True  False  and None are all singletons and will always evaluate the way I expect when using  is  rather than       Am I wrong here   Along the same lines  would it be more Pythonic to modify the functions that sometimes return None so that they raise an error instead   Say I have an instance method called  get attr    that retrieves an attribute from some file  In the case where it finds that the attribute I requested does not exist  is it appropriate to return None  Would it be better to have them raise an error and catch it later

User · Answer

One thing to ensure is that nothing can reassign your variable. If it is not a Boolean in the end, relying on truthiness will lead to bugs. The beauty of conditional programming in dynamically typed languages :).

The following prints "no".

x = False
if x:
    print 'yes'
else:
    print 'no'

Now let's change x.

x = 'False'

Now the statement prints "yes", because the string is truthy.

if x:
    print 'yes'
else:
    print 'no'

This statement, however, correctly outputs "no".

if x == True:
    print 'yes'
else:
    print 'no'

User · Answer

The advice isn t that you should never use True  False  or None  It s just that you shouldn t use if x    True   if x    True is silly because    is just a binary operator  It has a return value of either True or False  depending on whether its arguments are equal or not  And if condition will proceed if condition is true  So when you write if x    True Python is going to first evaluate x    True  which will become True if x was True and False otherwise  and then proceed if the result of that is true  But if you re expecting x to be either True or False  why not just use if x directly   Likewise  x    False can usually be replaced by not x   There are some circumstances where you might want to use x    True  This is because an if statement condition is  evaluated in Boolean context  to see if it is  truthy  rather than testing exactly against True  For example  non-empty strings  lists  and dictionaries are all considered truthy by an if statement  as well as non-zero numeric values  but none of those are equal to True  So if you want to test whether an arbitrary value is exactly the value True  not just whether it is truthy  when you would use if x    True  But I almost never see a use for that  It s so rare that if you do ever need to write that  it s worth adding a comment so future developers  including possibly yourself  don t just assume the    True is superfluous and remove it     Using x is True instead is actually worse  You should never use is with basic built-in immutable types like Booleans  True  False   numbers  and strings  The reason is that for these types we care about values  not identity     tests that values are the same for these types  while is always tests identities   Testing identities rather than values is bad because an implementation could theoretically construct new Boolean values rather than go find existing ones  leading to you having two True values that have the same value  but they are stored in different places in memory and have different identities  In practice I m pretty sure True and False are always reused by the Python interpreter so this won t happen  but that s really an implementation detail  This issue trips people up all the time with strings  because short strings and literal strings that appear directly in the program source are recycled by Python so  foo  is  foo  always returns True  But it s easy to construct the same string 2 different ways and have Python give them different identities  Observe the following    gt  gt  gt  stars1      join     for   in xrange 100    gt  gt  gt  stars2         100  gt  gt  gt  stars1 is stars2 False  gt  gt  gt  stars1    stars2 True   EDIT  So it turns out that Python s equality on Booleans is a little unexpected  at least to me     gt  gt  gt  True is 1 False  gt  gt  gt  True    1 True  gt  gt  gt  True    2 False  gt  gt  gt  False is 0 False  gt  gt  gt  False    0 True  gt  gt  gt  False    0 0 True   The rationale for this  as explained in the notes when bools were introduced in Python 2 3 5  is that the old behaviour of using integers 1 and 0 to represent True and False was good  but we just wanted more descriptive names for numbers we intended to represent truth values   One way to achieve that would have been to simply have True   1 and False   0 in the builtins  then 1 and True really would be indistinguishable  including by is   But that would also mean a function returning True would show 1 in the interactive interpreter  so what s been done instead is to create bool as a subtype of int  The only thing that s different about bool is str and repr  bool instances still have the same data as int instances  and still compare equality the same way  so True    1   So it s wrong to use x is True when x might have been set by some code that expects that  True is just another way to spell 1   because there are lots of ways to construct values that are equal to True but do not have the same identity as it    gt  gt  gt  a   1L  gt  gt  gt  b   1L  gt  gt  gt  c   1  gt  gt  gt  d   1 0  gt  gt  gt  a    True  b    True  c    True  d    True  True  True  True  True   gt  gt  gt  a is b  a is c  a is d  c is d  False  False  False  False    And it s wrong to use x    True when x could be an arbitrary Python value and you only want to know whether it is the Boolean value True  The only certainty we have is that just using x is best when you just want to test  truthiness   Thankfully that is usually all that is required  at least in the code I write   A more sure way would be x    True and type x  is bool  But that s getting pretty verbose for a pretty obscure case  It also doesn t look very Pythonic by doing explicit type checking    but that really is what you re doing when you re trying to test precisely True rather than truthy  the duck typing way would be to accept truthy values and allow any user-defined class to declare itself to be truthy   If you re dealing with this extremely precise notion of truth where you not only don t consider non-empty collections to be true but also don t consider 1 to be true  then just using x is True is probably okay  because presumably then you know that x didn t come from code that considers 1 to be true  I don t think there s any pure-python way to come up with another True that lives at a different memory address  although you could probably do it from C   so this shouldn t ever break despite being theoretically the  wrong  thing to do   And I used to think Booleans were simple   End Edit    In the case of None  however  the idiom is to use if x is None  In many circumstances you can use if not x  because None is a  falsey  value to an if statement  But it s best to only do this if you re wanting to treat all falsey values  zero-valued numeric types  empty collections  and None  the same way  If you are dealing with a value that is either some possible other value or None to indicate  no value   such as when a function returns None on failure   then it s much better to use if x is None so that you don t accidentally assume the function failed when it just happened to return an empty list  or the number 0   My arguments for using    rather than is for immutable value types would suggest that you should use if x    None rather than if x is None  However  in the case of None Python does explicitly guarantee that there is exactly one None in the entire universe  and normal idiomatic Python code uses is     Regarding whether to return None or raise an exception  it depends on the context   For something like your get attr example I would expect it to raise an exception  because I m going to be calling it like do something with get attr file    The normal expectation of the callers is that they ll get the attribute value  and having them get None and assume that was the attribute value is a much worse danger than forgetting to handle the exception when you can actually continue if the attribute can t be found  Plus  returning None to indicate failure means that None is not a valid value for the attribute  This can be a problem in some cases   For an imaginary function like see if matching file exists  that we provide a pattern to and it checks several places to see if there s a match  it could return a match if it finds one or None if it doesn t  But alternatively it could return a list of matches  then no match is just the empty list  which is also  falsey   this is one of those situations where I d just use if x to see if I got anything back    So when choosing between exceptions and None to indicate failure  you have to decide whether None is an expected non-failure value  and then look at the expectations of code calling the function  If the  normal  expectation is that there will be a valid value returned  and only occasionally will a caller be able to work fine whether or not a valid value is returned  then you should use exceptions to indicate failure  If it will be quite common for there to be no valid value  so callers will be expecting to handle both possibilities  then you can use None

User · Answer

If checking for truth   if foo   For false   if not foo   For none   if foo is None   For non-none   if foo is not None   For getattr   the correct behaviour is not to return None  but raise an AttributError error instead - unless your class is something like defaultdict

User · Answer

Use if foo or if not foo  There isn t any need for either    or is for that   For checking against None  is None and is not None are recommended  This allows you to distinguish it from False  or things that evaluate to False  like    and       Whether get attr should return None would depend on the context  You might have an attribute where the value is None  and you wouldn t be able to do that  I would interpret None as meaning  unset   and a KeyError would mean the key does not exist in the file

User · Answer

You can directly check that a variable contains a value or not  like if var or not var

User · Answer

For True  not None      if foo    For false  None   if not foo

User · Answer

In the examples in PEP 8  Style Guide for Python Code  document  I have seen that foo is None or foo is not None are being used instead of foo    None or foo    None   Also using if boolean value is recommended in this document instead of if boolean value    True or if boolean value is True  So I think if this is the official Python way  We Python guys should go on this way  too

User · Answer

Concerning whether to raise an exception or return None  it depends on the use case  Either can be Pythonic   Look at Python s dict class for example  x y  hooks into dict   getitem    and it raises a KeyError if key is not present  But the dict get method returns the second argument  which is defaulted to None  if key is not present  They are both useful     The most important thing to consider is to document that behaviour in the docstring  and make sure that your get attr   method does what it says it does    To address your other questions  use these conventions   if foo        For testing truthiness  if not foo        For testing falsiness  if foo is None        Testing    Noneliness    if foo is not None        Check explicitly avoids common bugs caused by empty sequences being false   Functions that return True or False should probably have a name that makes this obvious to improve code readability   def is running on windows        return os name     nt    In Python nbsp 3 you can  type-hint  that    gt  gt  gt  def is running on windows   - gt  bool          return os name     nt        gt  gt  gt  is running on windows   annotations     return   bool

User · Answer

In the case of your fictional getattr function  if the requested attribute always should be available but isn t then throw an error  If the attribute is optional then return None

[python] Use of True, False, and None as return values in Python functions

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