打印 void function 执行 function

Question

I wrote a function that takes in a number and prints its square. When I used the print function and passed the function-with 2 as its input- as the print function's argument, the output was 4 and 'None'.

'None' I can understand, but why did Python execute the function and output 4 here? I had only used a print statement.

Answer 1

Presumably you have something like this:

>>> def square(n):
...   print(n ** 2)
... 
>>> print(square(2))
4
None

You don't have a return statement so python returns None . Try this instead:

>>> def square(n):
...   return n ** 2
... 
>>> print(square(2))
4

The reason why the 4 got printed is becuase you called square() . Calling square had the side-effect of printing the square since that's what it's definition body says to do.

Answer 2

By placing the () next to the function, you called it. So it got executed. The print statement around it caused the return value None to be printed, while inside it printed the square.

If you don't want to call the function, remove the () (and any parameters).