Keyboard Input Check

Python Help
1

I try to check input using:
try:
x = str(input("enter a string? "))
except:
print(“Invalid.”)

I enter a number at the prompt and it works. How do I detect the wrong input?

2

Hi.

First off, you don’t need x = str(input("enter a string? ")) because the input() function will return a string by default, so: x = input("enter a string? ")

Maybe something like this, if you want to find out if any of the characters is not ‘alpha’ or a space?

x = input("enter a string? ")

for char in x:
    if char.isalpha() or char == ' ':
        pass
    else:
        print("Error in input")
        break

You need to better define what your input will and will not accept as ‘valid’ or ‘invalid’.

Note: any character that is not ‘a’ to ‘z’ or ‘A’ to ‘Z’ will fail the above test.

edit to add:

Maybe a better way would be to use this, to filter just for numbers:

for char in x:
    if char.isdigit():
        print("Error in input")
        break
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3

Why is a string made up of digits “wrong input”? It is still a string of characters.

There is no point in calling str() on the result of input(), because input() already returns a string. So that’s just a waste of time which does nothing.

Nothing inside the try block can fail, so the except clause will never run. Which is good, because …

This sort of try...except block is one of the most diabolically awful things you can write, trust me, do not get into the habit of doing this!

# This is bad, don't do it!
try:
    code
except:
    print("Invalid")

The Python interpreter makes a huge effort to give the programmer a lot of information about the error, and why it happened, and you replace that with just “Invalid”. That’s terrible! As a programmer, there are very few things worse than such pointless and uninformative error messages as “Invalid” and “An error occurred”.

4

If you are only looking to reject numeric characters (digits) you can write this:

if any(c.isdigit() for c in x):
    print("Found a digit")
3 Likes