函数应该在 Python 中返回打印吗？

Question

I'm writing function which prepare data in csv file. I wonder what should it return. I thought that information in string for user will be good idea, but how should I do it?

return print('Some info')

Or just

return 'Some info'

And how about exceptions, I mean like above. When exception will end work of function should I return print('Some info') or just 'Some info'?

Answer 1

No, return should return the result from the function, not the value of print (which is always None anyway).

Usually, a function should not print anything at all. In order to make programs modular and reusable, you want to keep any user interaction in the calling code.

For example,

def fibonacci(n):
    fib = some calculation ...
    print(fib)
    return fib

fibonacci(33)

This function has the side effect of printing the calculated value. But this means that you cannot calculate the value without also printing it. A common design principle from functional programming is that functions should not have side effects anyway. A better design is

def fibonacci(n):
    fib = some calculation ...
    return fib

print(fibonacci(33))

Exceptions are for situations where the code cannot perform the requested function. For example, you cannot calculate a negative Fibonacci number:

def fibonacci(n):
    if n < 0:
        raise ValueError('Cannot calculate negative Fibonacci number')
    fib = some calculation ...
    return fib

You could call this on arbitrary user input;

while True:
    number = input('Give me a number: ')
    try:
        print('Fibonacci: ', fibonacci(int(number))
    except ValueError as e:
        print('Oops, try again; ', e)

Notice how the except actually handles multiple error scenarios: if the use input isn't a number at all, int(number) will also raise a ValueError exception.

Answer 2

Sounds like you're learning Python for the first time. I recommend reading documentation to understand how I/O and function return statements work:

https://docs.python.org/3/tutorial/inputoutput.html

https://www.w3schools.com/python/python_functions.asp

If you want to return a string in a function, use return 'Some Info' . For printing something to the console for the user to view, simply use print('Some Info') . Combining the two seems pretty useless for your purposes.