What is the purpose of the return statement? How is it different from printing?

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Problem :

What is the simple basic explanation of what the return statement is, how to use it in Python?

And what is the difference between it and the print statement?

Solution :

The print() function writes, i.e., “prints”, a string in the console. The return statement causes your function to exit and hand back a value to its caller. The point of functions in general is to take in inputs and return something. The return statement is used when a function is ready to return a value to its caller.

For example, here’s a function utilizing both print() and return:

def foo():
    print("hello from inside of foo")
    return 1

Now you can run code that calls foo, like so:

if __name__ == '__main__':
    print("going to call foo")
    x = foo()
    print("called foo")
    print("foo returned " + str(x))

If you run this as a script (e.g. a .py file) as opposed to in the Python interpreter, you will get the following output:

going to call foo
hello from inside foo
called foo   
foo returned 1

I hope this makes it clearer. The interpreter writes return values to the console so I can see why somebody could be confused.

Here’s another example from the interpreter that demonstrates that:

>>> def foo():
...     print("hello from within foo")
...     return 1
...
>>> foo()
hello from within foo
1
>>> def bar():
...   return 10 * foo()
...
>>> bar()
hello from within foo
10

You can see that when foo() is called from bar(), 1 isn’t written to the console. Instead it is used to calculate the value returned from bar().

print() is a function that causes a side effect (it writes a string in the console), but execution resumes with the next statement. return causes the function to stop executing and hand a value back to whatever called it.

Think of the print statement as causing a side-effect, it makes your function write some text out to the user, but it can’t be used by another function.

I’ll attempt to explain this better with some examples, and a couple definitions from Wikipedia.

Here is the definition of a function from Wikipedia

A function, in mathematics, associates one quantity, the argument of the function, also known as the input, with another quantity, the value of the function, also known as the output..

Think about that for a second. What does it mean when you say the function has a value?

What it means is that you can actually substitute the value of a function with a normal value! (Assuming the two values are the same type of value)

Why would you want that you ask?

What about other functions that may accept the same type of value as an input?

def square(n):
    return n * n

def add_one(n):
    return n + 1

print square(12)

# square(12) is the same as writing 144

print add_one(square(12))
print add_one(144)
#These both have the same output

There is a fancy mathematical term for functions that only depend on their inputs to produce their outputs: Referential Transparency. Again, a definition from Wikipedia.

Referential transparency and referential opaqueness are properties of parts of computer programs. An expression is said to be referentially transparent if it can be replaced with its value without changing the behavior of a program

It might be a bit hard to grasp what this means if you’re just new to programming, but I think you will get it after some experimentation.
In general though, you can do things like print in a function, and you can also have a return statement at the end.

Just remember that when you use return you are basically saying “A call to this function is the same as writing the value that gets returned”

Python will actually insert a return value for you if you decline to put in your own, it’s called “None”, and it’s a special type that simply means nothing, or null.

I think the dictionary is your best reference here

Return and Print

In short:

return gives something back or replies to the caller of the function while print produces text

In python, we start defining a function with def, and generally – but not necessarily – end the function with return.

Suppose we want a function that adds 2 to the input value x. In mathematics, we might write something like f(x) = x + 2, describing that relationship: the value of the function, evaluated at x, is equal to x + 2.

In Python, it looks like this instead:

def f(x):
    return x + 2

That is: we define a function named f, which will be given an x value. When the code runs we figure out x + 2, and return that value. Instead of describing a relationship, we lay out steps that must be taken to calculate the result.

After defining the function, it can be called with whatever argument you like. It doesn’t have to be named x in the calling code, and it doesn’t even have to be a variable:

print f(2)
>>> 4

We could write the code for the function in some other ways. For example:

def f(x):
    y = x + 2
    return y

or even

def f(x):
    x = x + 2
    return x

Again, we are following steps in order – x = x + 2 changes what x refers to (now it means the result from the sum), and that is what gets returned by return x (because that’s the value *at the time that the return happens).

return means “output this value from this function”.

print means “send this value to (generally) stdout”

In the Python REPL, a function’s return value will be output to the screen by default (this isn’t the same as printing it). This output only happens at the REPL, not when running code from a .py file. It is the same as the output from any other expression at the REPL.

This is an example of print:

>>> n = "foonbar" #just assigning a variable. No output
>>> n #the value is output, but it is in a "raw form"
'foonbar'
>>> print(n) #the n is now a newline
foo
bar
>>>

This is an example of return:

>>> def getN():
...    return "foonbar"
...
>>> getN() #When this isn't assigned to something, it is just output
'foonbar'
>>> n = getN() # assigning a variable to the return value. No output
>>> n #the value is output, but it is in a "raw form"
'foonbar'
>>> print(n) #the n is now a newline
foo
bar
>>>

This answer goes over some of the cases that have not been discussed above.
The return statement allows you to terminate the execution of a function before you reach the end. This causes the flow of execution to immediately return to the caller.

In line number 4:

def ret(n):
    if n > 9:
         temp = "two digits"
         return temp     #Line 4        
    else:
         temp = "one digit"
         return temp     #Line 8
    print("return statement")
ret(10)

After the conditional statement gets executed the ret() function gets terminated due to return temp (line 4).
Thus the print("return statement") does not get executed.

Output:

two digits   

This code that appears after the conditional statements, or the place the flow of control cannot reach, is the dead code.

Returning Values
In lines number 4 and 8, the return statement is being used to return the value of a temporary variable after the condition has been executed.

To bring out the difference between print and return:

def ret(n):
    if n > 9:
        print("two digits")
        return "two digits"           
    else :
        print("one digit")
        return "one digit"        
ret(25)

Output:

two digits
'two digits'

Note that return can also be used for control flow. By putting one or more return statements in the middle of a function, we can say: “stop executing this function. We’ve either got what we wanted or something’s gone wrong!”

For example, imagine trying to implement str.find(sub) if we only had str.index(sub) available (index raises a ValueError if the substring isn’t found, whereas find returns -1).

We could use a try/except block:

def find(s: str, sub: str) -> int:
    try:
        return s.index(sub)
    except ValueError:
        return -1

This is fine, and it works, but it’s not very expressive. It’s not immediately clear what would cause str.index to raise a ValueError: a reader of this code must understand the workings of str.index in order to understand the logic of find.

Rather than add a doc-string, saying “…unless sub isn’t found, in which case return -1“, we could make the code document itself, like this:

def find(s: str, sub: str) -> int:
    if sub not in s:
        return -1
    return s.index(sub)

This makes the logic very clear.

The other nice thing about this is that once we get to return s.index(sub) we don’t need to wrap it in a try/except because we already know that the substring is present!

See the Code Style section of the Python Guide for more advice on this way of using return.

To put it as simply as possible:

return makes the value (a variable, often) available for use by the caller (for example, to be stored by a function that the function using return is within). Without return, your value or variable wouldn’t be available for the caller to store/re-use.

print, by contrast, prints to the screen – but does not make the value or variable available for use by the caller.

Difference between “return” and “print” can also be found in the following example:

RETURN:

def bigger(a, b):
    if a > b:
        return a
    elif a <b:
        return b
    else:
        return a

The above code will give correct results for all inputs.

PRINT:

def bigger(a, b):
    if a > b:
        print a
    elif a <b:
        print b
    else:
        print a

NOTE: This will fail for many test cases.

ERROR:

----  

FAILURE: Test case input: 3, 8.

            Expected result: 8  

FAILURE: Test case input: 4, 3.

            Expected result: 4  

FAILURE: Test case input: 3, 3.

            Expected result: 3  

You passed 0 out of 3 test cases

Here is my understanding. (hope it will help someone and it’s correct).

def count_number_of(x):
    count = 0
    for item in x:
        if item == "what_you_look_for":
        count = count + 1
    return count

So this simple piece of code counts number of occurrences of something. The placement of return is significant. It tells your program where do you need the value. So when you print, you send output to the screen. When you return you tell the value to go somewhere. In this case you can see that count = 0 is indented with return – we want the value (count + 1) to replace 0.
If you try to follow logic of the code when you indent the return command further the output will always be 1, because we would never tell the initial count to change.
I hope I got it right.
Oh, and return is always inside a function.

def fact(x):
    if x < 2:
        return 1
    return x * fact(x - 1)

print(fact(5))

return is part of a function definition, while print outputs text to the standard output (usually the console).

A function is a procedure accepting parameters and returning a value. return is for the latter, while the former is done with def.

Example:

def timestwo(x):
    return x*2

Best thing about return function is you can return a value from function but you can do same with print so whats the difference ?
Basically return not about just returning it gives output in object form so that we can save that return value from function to any variable but we can’t do with print because its same like stdout/cout in C Programming.

Follow below code for better understanding

CODE

def add(a, b):
    print "ADDING %d + %d" % (a, b)
    return a + b

def subtract(a, b):
    print "SUBTRACTING %d - %d" % (a, b)
    return a - b

def multiply(a, b):
    print "MULTIPLYING %d * %d" % (a, b)
    return a * b

def divide(a, b):
    print "DIVIDING %d / %d" % (a, b)
    return a / b


print "Let's do some math with just functions!"

age = add(30, 5)
height = subtract(78, 4)
weight = multiply(90, 2)
iq = divide(100, 2)

print "Age: %d, Height: %d, Weight: %d, IQ: %d" % (age, height, weight, iq)


# A puzzle for the extra credit, type it in anyway.
print "Here is a puzzle."

what = add(age, subtract(height, multiply(weight, divide(iq, 2))))

print "That becomes: ", what, "Can you do it by hand?"

We are now doing our own math functions for add, subtract, multiply, and divide. The important thing to notice is the last line where we say return a + b (in add). What this does is the following:

  1. Our function is called with two arguments: a and b.
  2. We print out what our function is doing, in this case “ADDING.”
  3. Then we tell Python to do something kind of backward: we return the addition of a + b. You might say this as, “I add a and b then return them.”
  4. Python adds the two numbers. Then when the function ends, any line that runs it will be able to assign this a + b result to a variable.

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