How is returning the output of a function different from printing it

Question

In my previous question  Andrew Jaffe writes       In addition to all of the other hints and tips  I think you re missing something crucial  your functions actually need to return something    When you create autoparts   or splittext    the idea is that this will be a function that you can call  and it can  and should  give something back    Once you figure out the output that you want your function to have  you need to put it in a return statement    def autoparts        parts dict          list of parts   open  list of parts txt    r        for line in list of parts          k  v   line split           parts dict k    v      print parts dict    gt  gt  gt  autoparts     part A   1   part B   2         This function creates a dictionary  but it does not return something   However  since I added the print  the output of the function is shown when I run the function   What is the difference between returning something and printing it

User · Answer

Unfortunately  there is a character limit so this will be in many parts  First thing to note is that return and print are statements  not functions  but that is just semantics  I   ll start with a basic explanation  print just shows the human user a string representing what is going on inside the computer  The computer cannot make use of that printing  return is how a function gives back a value  This value is often unseen by the human user  but it can be used by the computer in further functions  On a more expansive note  print will not in any way affect a function  It is simply there for the human user   s benefit  It is very useful for understanding how a program works and can be used in debugging to check various values in a program without interrupting the program  return is the main way that a function returns a value  All functions will return a value  and if there is no return statement  or yield but don   t worry about that yet   it will return None  The value that is returned by a function can then be further used as an argument passed to another function  stored as a variable  or just printed for the benefit of the human user  Consider these two programs  def function that prints        print  quot I printed quot   def function that returns        return  quot I returned quot   f1   function that prints   f2   function that returns    print  quot Now let us see what the values of f1 and f2 are quot  print f1 --- gt None print f2---- gt  quot I returned quot  When function that prints ran  it automatically printed to the console  quot I printed quot   However  the value stored in f1 is None because that function had no return statement  When function that returns ran  it did not print anything to the console  However  it did return a value  and that value was stored in f2  When we printed f2 at the end of the code  we saw  quot I returned quot

User · Answer

def add x  y       return x y   That way it can then become a variable   sum   add 3  5   print sum    But if the  add  function print the output  sum  would then be None as action would have already taken place after it being assigned

User · Answer

I think you re confused because you re running from the REPL  which automatically prints out the value returned when you call a function  In that case  you do get identical output whether you have a function that creates a value  prints it  and throws it away  or you have a function that creates a value and returns it  letting the REPL print it    However  these are very much not the same thing  as you will realize when you call autoparts with another function that wants to do something with the value that autoparts creates

User · Answer

A function is  at a basic level  a block of code that can executed  not when written  but when called  So let s say I have the following piece of code  which is a simple multiplication function  def multiply x y       return x   y  So if I called the function with multiply 2 3   it would return the value 6  If I modified the function so it looks like this  def multiply x y       print x y      return x y     then the output is as you would expect  the number 6 printed  However  the difference between these two statements is that print merely shows something on the console  but return  quot gives something back quot  to whatever called it  which is often a variable  The variable is then assigned the value of the return statement in the function that it called  Here is an example in the python shell   gt  gt  gt  def multiply x y           return x y   gt  gt  gt  multiply 2 3   no variable assignment 6  gt  gt  gt  answer   multiply 2 3   answer   whatever the function returns  gt  gt  gt  answer 6  So now the function has returned the result of calling the function to the place where it was called from  which is a variable called  answer  in this case  This does much more than simply printing the result  because you can then access it again  Here is an example of the function using return statements   gt  gt  gt  x   int input  quot Enter a number   quot    Enter a number  5  gt  gt  gt  y   int input  quot Enter another number   quot    Enter another number  6  gt  gt  gt  answer   multiply x y   gt  gt  gt  print  quot Your answer is    quot  format answer  Your answer is 30  So it basically stores the result of calling a function in a variable

User · Answer

Print simply prints out the structure to your output device  normally the console   Nothing more  To return it from your function  you would do   def autoparts      parts dict        list of parts   open  list of parts txt    r     for line in list of parts          k  v   line split           parts dict k    v   return parts dict   Why return  Well if you don t  that dictionary dies  gets garbage collected  and is no longer accessible as soon as this function call ends  If you return the value  you can do other stuff with it  Such as   my auto parts   autoparts    print my auto parts  engine       See what happened  autoparts   was called and it returned the parts dict and we stored it into the my auto parts variable  Now we can use this variable to access the dictionary object and it continues to live even though the function call is over  We then printed out the object in the dictionary with the key  engine    For a good tutorial  check out dive into python  It s free and very easy to follow

User · Answer

Major difference  Calling print will immediately make your program write out text for you to see  Use print when you want to show a value to a human  return is a keyword  When a return statement is reached  Python will stop the execution of the current function  sending a value out to where the function was called  Use return when you want to send a value from one point in your code to another  Using return changes the flow of the program  Using print does not

User · Answer

The below examples might help understand  def add nums1 x y       print x y   def add nums2 x y       return x y    ----Function output is usable for further processing add nums2 10 20  2 15 0   ----Function output can t be used further  gives TypeError  add nums1 10 20  2  30 --------------------------------------------------------------------------- TypeError                                 Traceback  most recent call last   lt ipython-input-124-e11302d7195e gt  in  lt module gt  ---- gt  1 add nums1 10 20  2  TypeError  unsupported operand type s  for     NoneType  and  int

User · Answer

The print statement will output an object to the user   A return statement will allow assigning the dictionary to a variable once the function is finished    gt  gt  gt  def foo            print  Hello  world         gt  gt  gt  a   foo   Hello  world   gt  gt  gt  a  gt  gt  gt  def foo            return  Hello  world         gt  gt  gt  a   foo    gt  gt  gt  a  Hello  world     Or in the context of returning a dictionary    gt  gt  gt  def foo            print   a    1   b    2        gt  gt  gt  a   foo     a   1   b   2   gt  gt  gt  a  gt  gt  gt  def foo            return   a    1   b    2        gt  gt  gt  a   foo    gt  gt  gt  a   a   1   b   2     The statements where nothing is printed out after a line is executed means the last statement returned None

User · Answer

you just add a return statement     def autoparts      parts dict      list of parts   open  list of parts txt    r     for line in list of parts          k  v   line split           parts dict k    v   return parts dict   printing out only prints out to the standard output  screen  of the application   You can also return multiple things by separating them with commas   return parts dict  list of parts   to use it   test dict      test dict   autoparts

[python] How is returning the output of a function different from printing it?

Examples related to python

Examples related to return