Why doesn't the makeSix function change the value of the variable?

[Mogarrr]

I'm reading the online tutorial "How to Think Like a Computer Scientist", which is a Python tutorial. There is an exercise, which I'm trying to figure out.

What happens if you call a function and you don't do anything with the result (i.e., you don't assign it to a variable or use it as part of a larger expression)?

My intuition is that nothing happens. To show this, I typed the following in the command line...

def nada(something):
nil = something

As predicted, when I call the function nada, nothing happens. Is this what the author meant by doing nothing with the result?

Experimenting with functions, I also discovered something that disturbs me about Python. I made a function...

def makeSix(number):
number = 6

Now I would think that the makeSix function will change the value of a variable to 6. For instance, I typed this:

x = 9
makeSix(x)
print x
9

Which is frustrating, since I would think that the value of the variable would change to 6.

What's going on?

 

[Mark44]

What they mean, I believe, is a function that actually returns a value but you don't do anything with the returned value.

Here's an example in C, which I'm more familiar with than Python.

#include <stdio.h>
int fun(int arg)
{
   return arg * 2;
}

int main(void)
{
   int val = 3;
   fun(val);  // Here I call fun() but don't do anything with the returned value
   printf("val is %d", val);
   return 0;
}

 

[jbriggs444]

Experimenting with functions, I also discovered something that disturbs me about Python. I made a function...

def makeSix(number):
number = 6

Now I would think that the makeSix function will change the value of a variable to 6. For instance, I typed this:

x = 9
makeSix(x)
print x
9

Which is frustrating, since I would think that the value of the variable would change to 6.

What's going on?

This is a matter of the parameter passing strategy adopted by the language. It appears that you are expecting call-by-reference semantics and getting call-by-value semantics.

http://en.wikipedia.org/wiki/Evaluation_strategy

 

[Borek]

There are many functions that don't return any value, yet they are called to do something. If they do something, they apparently change the state of the system - so you can't say "nothing happens".

 

[Mark44]

The first post in this thread consists of two questions:
1. What happens when you call a function that returns a value, but don't do anything with the returned value? (I.e., don't store the value or print it or whatever.)
2. If a function changes the value of a passed parameter, does this affect the value of the actual argument?

For the first question, the OP says

As predicted, when I call the function nada, nothing happens.

However, the OP's example function doesn't return a value, so I don't believe he/she is really answering the question.
jbriggs444's post is in reponse to question #2. In call-by-value semantics, which are the default in C and probably Python, a function is given essentially a copy of the parameter, so any changes that the function makes don't affect the original parameter. Other languages, such as Fortran, use call-by-reference semantics, so a function can change the value of the actual argument. C, C++, and some other languages derived from C are able to simulate call-by-reference by using pointers to the arguments.

 

[mafagafo]

I am not a Python guru, but read the following code:

# SNIPPET 1

x = 9


def make_six():
  x = 6

make_six()

# here x == 9

# SNIPPET 2

x = 9


def make_six(x):
  x = 6

make_six()

# here x == 9

# SNIPPET 3

x = 9


def make_six(x):
  x = 6

x = make_six(x)

# here x == None

# SNIPPET 4
x = 9


def make_six(x):
  x = 6
  return x

x = make_six(x)

# here x == 6

# SNIPPET 5
x = 9

# the only one that actually messes with your original variable and makes it == 6
def make_six():
  global x
  x = 6

make_six()

# here x == 6

Comments:
SNIPPET 1: make_six initializes a local variable x and sets it to 6. Returns None.

SNIPPET 2: make_six has a parameter x, it sets the local variable x (that stored the value passed as an argument) to 6 and returns None.

SNIPPET 3: we assign the return value from the function to x. As it returns None, x becomes None.

SNIPPET 4: now our function gets the value x, overwrite it with 6 and returns it. (So, with an assignment, x becomes 6).

SNIPPET 5: finally, what you wanted: it reaches another scope, grab the reference to x, shove an integer with the value 6 in it and dies. No need for an assignment or parameter.

PLEASE, do note that make_six is ridiculous as you could just assign 6 to whatever you wanted in the first place.

 

[mafagafo]

As predicted, when I call the function nada, nothing happens. Is this what the author meant by doing nothing with the result?

Didn't see this one. Well, a lot can happen without an assignment.

import sys


def program_killer():
   sys.exit(0)

program_killer()

x = 4

x will never become four. x will never be defined. x will never be more than an instruction not to be followed by the interpreter. Poor x.

SO NO! Something happened. Indeed, as I said, a LOT can happen.


To do nothing with the result (that in Python is, unless specified otherwise, None) means not to hold it in a variable, list, tuple, (any other container), or pass it as an argument to something.

def get_none():
  pass

# does something with the result
print(get_none())
# does something too
woah_this_will_store_none = get_none()
# THIS IS DOING NOTHING WITH THE RESULT
get_none()

 

[Mogarrr]

Thanks for the help guys.

I'm curious, in a post how did you display code inside the white box?

 

[AlephZero]

Like this:
[NOPARSE]

... some code ...

[/NOPARSE]

... some code ...

 

[Mogarrr]

def give_thanks()
  print 'thanks'

give_thanks()

exit()