对变量使用round（）时出错

Question

I never coded before and started with Python-3.5 a few days ago. After some exercise i try to play around myself. Last time I wanted to create a script which stores the input as a variable and rounds it to three decimals. Unfortunately I get an error when I try to do that:

round (spam, 3)
TypeError: type str doesn't define __round__ method"

I tried to look this up in the Q&A but you guys seem to have more complex problems related to this error msg.

So this is what I entered in the file editor when I got the error msg:

print('Pls enter value')
spam = input()
#print(spam)
round(spam, 3)

when I enter the following in the interactive shell the rounding seems to work though:

>>> spam = 3.666666
>>> round (spam, 3)
3.667

So why is the same logic working in the shell but not in the File editor ? Thanks in advance!

Answer 1

The difference is that in the second case you supply the value of spam using a float literal (that is, spam = 3.666666 ) while, in the first case you get it from calling input() which isn't exactly the same.

The function input() returns a str instance in Python 3 and, for str types, the round function doesn't make much sense; you need to explicitly transform it to a float by wrapping the result of input() with float() :

spam = float(input())  # change input to 'float' type

Now, you can call round on it. You do need to be careful that the input you actually supply is indeed transformable to a float or else a ValueError will be raised.

In addition to that, no need to add the print call before input , input has a prompt argument that allows you to specify text before submitting input:

spam = input("Enter valid float number: ") 

You should now get similar results for both cases.