return return None and no return at all

Question

Consider three functions   def my func1      print  Hello World    return None  def my func2      print  Hello World    return  def my func3      print  Hello World    They all appear to return None  Are there any differences between how the returned value of these functions behave   Are there any reasons to prefer one versus the other

User · Answer

In terms of functionality these are all the same  the difference between them is in code readability and style  which is important to consider

User · Answer

On the actual behavior  there is no difference  They all return None and that s it  However  there is a time and  place for all of these  The following instructions are basically how the different methods should be used  or at least how I was taught they should be used   but they are not absolute rules so you can mix them up if you feel necessary to   Using return None  This tells that the function is indeed meant to return a value for later use  and in this case it returns None  This value None can then be used elsewhere  return None is never used if there are no other possible return values from the function   In the following example  we return person s mother if the person given is a human  If it s not a human  we return None since the person doesn t have a mother  let s suppose it s not an animal or something    def get mother person       if is human person           return person mother     else          return None   Using return  This is used for the same reason as break in loops  The return value doesn t matter and you only want to exit the whole function  It s extremely useful in some places  even though you don t need it that often   We ve got 15 prisoners and we know one of them has a knife  We loop through each prisoner one by one to check if they have a knife  If we hit the person with a knife  we can just exit the function because we know there s only one knife and no reason the check rest of the prisoners  If we don t find the prisoner with a knife  we raise an alert  This could be done in many different ways and using return is probably not even the best way  but it s just an example to show how to use return for exiting a function   def find prisoner with knife prisoners       for prisoner in prisoners          if  knife  in prisoner items              prisoner move to inquisition               return   no need to check rest of the prisoners nor raise an alert     raise alert     Note  You should never do var   find prisoner with knife    since the return value is not meant to be caught   Using no return at all  This will also return None  but that value is not meant to be used or caught  It simply means that the function ended successfully  It s basically the same as return in void functions in languages such as C   or Java   In the following example  we set person s mother s name and then the function exits after completing successfully   def set mother person  mother       if is human person           person mother   mother   Note  You should never do var   set mother my person  my mother   since the return value is not meant to be caught

User · Answer

Yes  they are all the same    We can review the interpreted machine code to confirm that that they re all doing the exact same thing   import dis  def f1      print  Hello World    return None  def f2      print  Hello World    return  def f3      print  Hello World   dis dis f1      4   0 LOAD CONST    1   Hello World           3 PRINT ITEM         4 PRINT NEWLINE      5   5 LOAD CONST    0  None          8 RETURN VALUE  dis dis f2      9   0 LOAD CONST    1   Hello World           3 PRINT ITEM         4 PRINT NEWLINE      10  5 LOAD CONST    0  None          8 RETURN VALUE  dis dis f3      14  0 LOAD CONST    1   Hello World           3 PRINT ITEM         4 PRINT NEWLINE                     5 LOAD CONST    0  None          8 RETURN VALUE

User · Answer

They each return the same singleton None -- There is no functional difference    I think that it is reasonably idiomatic to leave off the return statement unless you need it to break out of the function early  in which case a bare return is more common   or return something other than None   It also makes sense and seems to be idiomatic to write return None when it is in a function that has another path that returns something other than None   Writing return None out explicitly is a visual cue to the reader that there s another branch which returns something more interesting  and that calling code will probably need to handle both types of return values    Often in Python  functions which return None are used like void functions in C -- Their purpose is generally to operate on the input arguments in place  unless you re using global data  shudders     Returning None usually makes it more explicit that the arguments were mutated   This makes it a little more clear why it makes sense to leave off the return statement from a  language conventions  standpoint   That said  if you re working in a code base that already has pre-set conventions around these things  I d definitely follow suit to help the code base stay uniform

User · Answer

As other have answered  the result is exactly the same  None is returned in all cases   The difference is stylistic  but please note that PEP8 requires the use to be consistent      Be consistent in return statements  Either all return statements in a   function should return an expression  or none of them should  If any   return statement returns an expression  any return statements where no   value is returned should explicitly state this as return None  and an   explicit return statement should be present at the end of the function    if reachable        Yes       def foo x       if x  gt   0          return math sqrt x      else          return None  def bar x       if x  lt  0          return None     return math sqrt x        No       def foo x       if x  gt   0          return math sqrt x   def bar x       if x  lt  0          return     return math sqrt x     https   www python org dev peps pep-0008  programming-recommendations    Basically  if you ever return non-None value in a function  it means the return value has meaning and is meant to be caught by callers  So when you return None  it must also be explicit  to convey None in this case has meaning  it is one of the possible return values   If you don t need return at all  you function basically works as a procedure instead of a function  so just don t include the return statement   If you are writing a procedure-like function and there is an opportunity to return earlier  i e  you are already done at that point and don t need to execute the remaining of the function  you may use empty an returns to signal for the reader it is just an early finish of execution and the None value returned implicitly doesn t have any meaning and is not meant to be caught  the procedure-like function always returns None anyway

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