Trying to understand in Python 3.7 how to get a number returned from a function to the main program-CodePudding

In general terms, I cannot get any function to return a value inside a variable (an integer, back to the main program. I have included the variable name as one of 2 arguments in the function (in this case mod_stone_value), and am using the return command to hopefully return that integer value to the main program, where that value is then added to a total value.

The key lines of code in the function would be:

def calc_real_stone_value(base_stone_value, mod_stone_value):

return mod_stone_value

and then, back in the main program:

total_stone_value = total_stone_value   mod_stone_value

The variable total_stone_value ends up being 0, yet non-zero values of mod_stone_value do print inside the function. I know I am doing something fundamentally wrong, but have no idea what it is.

CodePudding user response：

If I understand correctly, you want to use the number value returned by the function calc_real_stone_value and are confused as to why it is not changing after you call your function which supposedly "updates it's value".

The fundamental thing you are misunderstanding is that primitive data types (double, int, char, etc.) are passed by value, not by reference. Therefore, if you make changes to the local variable mod_stone_value within the function, the changes will not show in the variable that exists outside the function. If you want to use the value you are returning with the line return mod_stone_value you need to call the function with the variables you want to be used in the calculations and then assign that value to a variable (mod_stone_value in this case).

Example:

base_stone_value = 1
mod_stone_value = 4


def calc_real_stone_value(base_stone_value, mod_stone_value):
    # calculations
    # e.g. add the two integer arguments
    mod_stone_value  = base_stone_value
    return mod_stone_value

# print the values for mod_stone_value
print(mod_stone_value)
# call function and assign value to mod_stone_value
mod_stone_value = calc_real_stone_value(base_stone_value, mod_stone_value)
print(mod_stone_value)

Output:

4
5

Conclusion:

Primitive variables are passed by value and in order to retrieve the value from a function that returns an integer for example, it needs to be re-assigned.

To reduce confusion, you can avoid using variable names that shadow variable names from an outer scope.

CodePudding user response：

Using your code as a template, here is an example that illustrates passing a value from "inside" the function to the outer level (here, the outermost level of the program). I would recommend reading this part of the official Python tutorial (or from the beginning if it does not make sense) as it describes concepts (e.g. global vs. local variables) that are relevant to your problem.

c = 0
d = 5 
e = 2

def foo(a, b):
    # insert whatever code here
    d = 10 
    print(f"Inside foo(), d has value {d}")
    # to match your example
    return b

# c has the value 0, d has the value 5 
print(f"(c, d) has the value ({c}, {d})") 

# call function and pass return value to c 
c = foo(d, e)

# c has the value 2, d has the value 5 
print(f"(c, d) has the value ({c}, {d})")

By the way, regarding your code, statements inside the body of the function should be indented. This includes the return statement.