Testing if user input is an int or a str, then failing if input is a str. I keep getting strs I enter as undefined

Question

I am trying to test if my user input is a string or an integer.

feet = input ("Enter your feet.")
inches = input ("Enter your inches.")

if type(eval(feet)) and type(eval(inches)) == int:
    print ("both are numbers!")
else:
    print ("That's not a Number!")

Here is my code, it works if I enter numbers for the values for feet and inches. However, if feet = a I get an error stating that a is undefined.

What am I doing wrong?

As an aside: if type(eval(feet)) and type(eval(inches)) == int means if bool(type(eval(feet))) and (type(eval(inches)) == int). It does not mean if type(eval(feet)) == int and type(eval(inches)) == int. — Steven Rumbalski
– Steven Rumbalski, Commented Feb 6, 2017 at 21:55
Or more succinctly: x and y == z means bool(x) and (y == z) not (x == z) and (y == z) If you need to test multiple items you can do all(item==z for item in (x, y)). If you need all items to be the same you can do {x, y} == {z}. — Steven Rumbalski
– Steven Rumbalski, Commented Feb 6, 2017 at 21:58

Daniel Roseman · Accepted Answer · 2017-02-06 21:51:04Z

3

What you're doing wrong is using eval. That's never a good way to do anything.

Instead, try to convert to int and catch the exception:

try:
    feet = int(feet)
    inches = int(inches)
except ValueError:
    print("not numbers!")
else:
    print("numbers!")

answered Feb 6, 2017 at 21:51

Daniel Roseman

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2 Comments

Wynn Boss Over a year ago

interesting! I have never heard of the Try command. Granted I am very new to python. Is there a good source to read about this method? I would like to understand what is going on more.

rlee827 Over a year ago

@WynnBoss See docs.python.org/3/reference/compound_stmts.html#try or docs.python.org/3/glossary.html#term-eafp

Andrew Jenkins · Accepted Answer · 2017-02-07 00:23:59Z

0

Don't use eval to test if user input is an integer. You get an error because the interpreter is trying to find a variable called a but there isn't one defined. Instead you can check if a string only contains digits.

def is_integer(s):
    for c in s:
        if not c.isdigit():
            return False
    return True

feet = input ("Enter your feet.")
inches = input ("Enter your inches.")

if is_integer(feet) and is_integer(inches):
    print ("both are numbers!")
else:
    print ("That's not a Number!")

This assumes that negative numbers are invalid.

edited Feb 7, 2017 at 0:23

answered Feb 6, 2017 at 21:52

Andrew Jenkins

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1 Comment

kjpc-tech Over a year ago

Isn't it isdigit not isDigit? docs.python.org/3/library/stdtypes.html#str.isdigit

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