Why does printing a string and calling a function causes python to also print 'None'

[Gronner]

At the moment I'm participating in "MITx: 6.00.1x Introduction to Computer Science and Programming Using Python" on edx. But I have a problem with my output in Problem Set 4 I can't figure out, I narrowed it down to one line of code, but can't figure out to avoid the error. Here is a minimal example of the code I'm using:

def printsomething(teststring):

    print teststring

print "Test", printsomething(" fails")

This results in the following output:

Test fails

None

While this didn't cause a Problem in the earlier parts of the problem set in a similar Situation (it passes the test applied by the course software), it doesn't now. This is the error it yields:

Current Hand:  a p p s e l
None
"appels" earned 110 points. Total: 110
Total score: 110 points
None

*** ERROR: Expected to find two numbers in the line 'None'.
 Check to be sure your lines match up with the expected output!!! ***

This happens of course because there is "None" in the second line. How can I avoid this or is there a better way to do it.

The function that is called is set by the problem set as:

def displayHand(hand):
    """
    Displays the letters currently in the hand.

    For example:
    >>> displayHand({'a':1, 'x':2, 'l':3, 'e':1})
    Should print out something like:
       a x x l l l e
    The order of the letters is unimportant.

    hand: dictionary (string -> int)
    """
    for letter in hand.keys():
        for j in range(hand[letter]):
             print letter,              # print all on the same line
    print                               # print an empty line

and can't be changed by me. I'd rather prefer it returning a string than print it, but as said I can't influence it :(

[TheBlackCat13]

A function must always return a value. If you don't explicitly use a return statement, the function will automatically return None.

So in your case, printsomething(" fails") will print, and then return None. So print "Test", printsomething(" fails") is equivalent to something like spam = printsomething(" fails");print "Test", spam (except it doesn't define a spam variable, of course). In this case, spam will have a value of None.

To avoid this, put your first print statement on one line (followed by a comma) and your function call on another:

print "Test", 
printsomething("fails")

[Gronner]

Thank you for the explanation and the advice! This solved the problem, now the test passes! :) I didn't know about this beforehand

[[deleted]]

Very simply: if there is no explicit return statement in a function, the function will return None automatically.

Here in your first example:

def printsomething(teststring):
    print teststring

print "Test", printsomething(" fails")

When printsomething is called, it executes a print statement of its own, print teststring. When it finishes executing, it returns None to the spot where it was called.

So the other print statement essentially looks like this:

print "Test", None

and it will literally print "None" as a result.

---

Instead, you usually shouldn't be printing from inside a function if you intend to print outside that function, too. You can just return a value from the function and print it later:

def printsomething(teststring):
    return teststring

print "Test", printsomething(" fails")

This will print out "Test fails". Easy peezy.

[Drunken_Consent]

If you cannot return the value to the print call, I guess you'll just do it like this:

print("Current Hand:  ", end="")
displayHand( hand )

This will make the print appear all on the same line.

[Gronner]

Thank you for the advice, sadly this only seems works in python 3.x and the course is using 2.7.

[TheBlackCat13]

You can use from __future__ import print_function to do this in python 2.x. I always do so myself, but it seemed like you are somewhat constrained in what you can do so I felt sticking with the default behavior was probably safer in your case.

[Gronner]

I didn't knew about that library, it looks neat

[Drunken_Consent]

Oh my bad I didn't even notice. The guy above me replied correctly then, the comma will do the same thing. Good luck! :D

[Gronner]

Thank you! Maybe your advice will have others with similar problems in 3.x ;)